dev: add support for multi gem5 runs

Multi gem5 is an extension to gem5 to enable parallel simulation of a
distributed system (e.g. simulation of a pool of machines
connected by Ethernet links). A multi gem5 run consists of seperate gem5
processes running in parallel (potentially on different hosts/slots on
a cluster). Each gem5 process executes the simulation of a component of the
simulated distributed system (e.g. a multi-core board with an Ethernet NIC).

The patch implements the "distributed" Ethernet link device
(dev/src/multi_etherlink.[hh.cc]). This device will send/receive
(simulated) Ethernet packets to/from peer gem5 processes. The interface
to talk to the peer gem5 processes is defined in dev/src/multi_iface.hh and
in tcp_iface.hh.

There is also a central message server process (util/multi/tcp_server.[hh,cc])
which acts like an Ethernet switch and transfers messages among the gem5 peers.

A multi gem5 simulations can be kicked off by the util/multi/gem5-multi.sh
wrapper script.

Checkpoints are supported by multi-gem5. The checkpoint must be
initiated by a single gem5 process. E.g., the gem5 process with rank 0
can take a checkpoint from the bootscript just before it invokes
'mpirun' to launch an MPI test. The message server process will notify
all the other peer gem5 processes and make them take a checkpoint, too
(after completing a global synchronisation to ensure that there are no
inflight messages among gem5).

17 files changed, 3379 insertions, 0 deletions

diff --git a/src/dev/Ethernet.py b/src/dev/Ethernet.py
index 147ad156c..84fa0ae00 100644
--- a/src/dev/Ethernet.py
+++ b/src/dev/Ethernet.py
@@ -1,3 +1,15 @@
+# Copyright (c) 2015 ARM Limited
+# All rights reserved.
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
 # Copyright (c) 2005-2007 The Regents of The University of Michigan
 # All rights reserved.
 #
@@ -46,6 +58,20 @@ class EtherLink(EtherObject):
     speed = Param.NetworkBandwidth('1Gbps', "link speed")
     dump = Param.EtherDump(NULL, "dump object")
 
+class MultiEtherLink(EtherObject):
+    type = 'MultiEtherLink'
+    cxx_header = "dev/multi_etherlink.hh"
+    int0 = SlavePort("interface 0")
+    delay = Param.Latency('0us', "packet transmit delay")
+    delay_var = Param.Latency('0ns', "packet transmit delay variability")
+    speed = Param.NetworkBandwidth('1Gbps', "link speed")
+    dump = Param.EtherDump(NULL, "dump object")
+    multi_rank  =  Param.UInt32('0', "Rank of the this gem5 process (multi run)")
+    sync_start  = Param.Latency('5200000000000t', "first multi sync barrier")
+    sync_repeat = Param.Latency('10us', "multi sync barrier repeat")
+    server_name = Param.String('localhost', "Message server name")
+    server_port = Param.UInt32('2200', "Message server port")
+
 class EtherBus(EtherObject):
     type = 'EtherBus'
     cxx_header = "dev/etherbus.hh"
diff --git a/src/dev/SConscript b/src/dev/SConscript
index 416e5052f..3936210a6 100644
--- a/src/dev/SConscript
+++ b/src/dev/SConscript
@@ -60,6 +60,10 @@ Source('etherdevice.cc')
 Source('etherdump.cc')
 Source('etherint.cc')
 Source('etherlink.cc')
+Source('multi_packet.cc')
+Source('multi_iface.cc')
+Source('multi_etherlink.cc')
+Source('tcp_iface.cc')
 Source('etherpkt.cc')
 Source('ethertap.cc')
 Source('i2cbus.cc')
@@ -85,6 +89,8 @@ DebugFlag('DiskImageWrite')
 DebugFlag('DMA')
 DebugFlag('DMACopyEngine')
 DebugFlag('Ethernet')
+DebugFlag('MultiEthernet')
+DebugFlag('MultiEthernetPkt')
 DebugFlag('EthernetCksum')
 DebugFlag('EthernetDMA')
 DebugFlag('EthernetData')
diff --git a/src/dev/etherpkt.cc b/src/dev/etherpkt.cc
index 548a1f179..58b53b5e6 100644
--- a/src/dev/etherpkt.cc
+++ b/src/dev/etherpkt.cc
@@ -31,6 +31,7 @@
 #include <iostream>
 
 #include "base/misc.hh"
+#include "base/inet.hh"
 #include "dev/etherpkt.hh"
 #include "sim/serialize.hh"
 
@@ -50,3 +51,20 @@ EthPacketData::unserialize(const string &base, CheckpointIn &cp)
     if (length)
         arrayParamIn(cp, base + ".data", data, length);
 }
+
+void
+EthPacketData::packAddress(uint8_t *src_addr,
+                           uint8_t *dst_addr,
+                           unsigned &nbytes)
+{
+    Net::EthHdr *hdr = (Net::EthHdr *)data;
+    assert(hdr->src().size() == hdr->dst().size());
+    if (nbytes < hdr->src().size())
+        panic("EthPacketData::packAddress() Buffer overflow");
+
+    memcpy(dst_addr, hdr->dst().bytes(), hdr->dst().size());
+    memcpy(src_addr, hdr->src().bytes(), hdr->src().size());
+
+    nbytes = hdr->src().size();
+}
+
diff --git a/src/dev/etherpkt.hh b/src/dev/etherpkt.hh
index acda9fb47..0d8998b1d 100644
--- a/src/dev/etherpkt.hh
+++ b/src/dev/etherpkt.hh
@@ -71,8 +71,23 @@ class EthPacketData
     ~EthPacketData() { if (data) delete [] data; }
 
   public:
+    /**
+     * This function pulls out the MAC source and destination addresses from
+     * the packet data and stores them in the caller specified buffers.
+     *
+     * @param src_addr The buffer to store the source MAC address.
+     * @param dst_addr The buffer to store the destination MAC address.
+     * @param length This is an inout parameter. The caller stores in this
+     * the size of the address buffers. On return, this will contain the
+     * actual address size stored in the buffers. (We assume that source
+     * address size is equal to that of the destination address.)
+     */
+    void packAddress(uint8_t *src_addr, uint8_t *dst_addr, unsigned &length);
+
     void serialize(const std::string &base, CheckpointOut &cp) const;
     void unserialize(const std::string &base, CheckpointIn &cp);
+
+    unsigned size() const { return length; }
 };
 
 typedef std::shared_ptr<EthPacketData> EthPacketPtr;
diff --git a/src/dev/multi_etherlink.cc b/src/dev/multi_etherlink.cc
new file mode 100644
index 000000000..b7d411ac3
--- /dev/null
+++ b/src/dev/multi_etherlink.cc
@@ -0,0 +1,266 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * Device module for a full duplex ethernet link for multi gem5 simulations.
+ */
+
+#include "dev/multi_etherlink.hh"
+
+#include <arpa/inet.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+#include <cmath>
+#include <deque>
+#include <string>
+#include <vector>
+
+#include "base/random.hh"
+#include "base/trace.hh"
+#include "debug/EthernetData.hh"
+#include "debug/MultiEthernet.hh"
+#include "debug/MultiEthernetPkt.hh"
+#include "dev/etherdump.hh"
+#include "dev/etherint.hh"
+#include "dev/etherlink.hh"
+#include "dev/etherobject.hh"
+#include "dev/etherpkt.hh"
+#include "dev/multi_iface.hh"
+#include "dev/tcp_iface.hh"
+#include "params/EtherLink.hh"
+#include "sim/core.hh"
+#include "sim/serialize.hh"
+#include "sim/system.hh"
+
+using namespace std;
+
+MultiEtherLink::MultiEtherLink(const Params *p)
+    : EtherObject(p)
+{
+    DPRINTF(MultiEthernet,"MultiEtherLink::MultiEtherLink() "
+            "link delay:%llu\n", p->delay);
+
+    txLink = new TxLink(name() + ".link0", this, p->speed, p->delay_var,
+                        p->dump);
+    rxLink = new RxLink(name() + ".link1", this, p->delay, p->dump);
+
+    // create the multi (TCP) interface to talk to the peer gem5 processes.
+    multiIface = new TCPIface(p->server_name, p->server_port, p->multi_rank,
+                              p->sync_start, p->sync_repeat, this);
+
+    localIface = new LocalIface(name() + ".int0", txLink, rxLink, multiIface);
+}
+
+MultiEtherLink::~MultiEtherLink()
+{
+    delete txLink;
+    delete rxLink;
+    delete localIface;
+    delete multiIface;
+}
+
+EtherInt*
+MultiEtherLink::getEthPort(const std::string &if_name, int idx)
+{
+    if (if_name != "int0") {
+        return nullptr;
+    } else {
+        panic_if(localIface->getPeer(), "interface already connected to");
+    }
+    return localIface;
+}
+
+void MultiEtherLink::memWriteback()
+{
+    DPRINTF(MultiEthernet,"MultiEtherLink::memWriteback() called\n");
+    multiIface->drainDone();
+}
+
+void
+MultiEtherLink::serialize(CheckpointOut &cp) const
+{
+    multiIface->serialize("multiIface", cp);
+    txLink->serialize("txLink", cp);
+    rxLink->serialize("rxLink", cp);
+}
+
+void
+MultiEtherLink::unserialize(CheckpointIn &cp)
+{
+    multiIface->unserialize("multiIface", cp);
+    txLink->unserialize("txLink", cp);
+    rxLink->unserialize("rxLink", cp);
+}
+
+void
+MultiEtherLink::init()
+{
+    DPRINTF(MultiEthernet,"MultiEtherLink::init() called\n");
+    multiIface->initRandom();
+}
+
+void
+MultiEtherLink::startup()
+{
+    DPRINTF(MultiEthernet,"MultiEtherLink::startup() called\n");
+    multiIface->startPeriodicSync();
+}
+
+void
+MultiEtherLink::RxLink::setMultiInt(MultiIface *m)
+{
+    assert(!multiIface);
+    multiIface = m;
+    // Spawn a new receiver thread that will process messages
+    // coming in from peer gem5 processes.
+    // The receive thread will also schedule a (receive) doneEvent
+    // for each incoming data packet.
+    multiIface->spawnRecvThread(&doneEvent, linkDelay);
+}
+
+void
+MultiEtherLink::RxLink::rxDone()
+{
+    assert(!busy());
+
+    // retrieve the packet that triggered the receive done event
+    packet = multiIface->packetIn();
+
+    if (dump)
+        dump->dump(packet);
+
+    DPRINTF(MultiEthernetPkt, "MultiEtherLink::MultiLink::rxDone() "
+            "packet received: len=%d\n", packet->length);
+    DDUMP(EthernetData, packet->data, packet->length);
+
+    localIface->sendPacket(packet);
+
+    packet = nullptr;
+}
+
+void
+MultiEtherLink::TxLink::txDone()
+{
+    if (dump)
+        dump->dump(packet);
+
+    packet = nullptr;
+    assert(!busy());
+
+    localIface->sendDone();
+}
+
+bool
+MultiEtherLink::TxLink::transmit(EthPacketPtr pkt)
+{
+    if (busy()) {
+        DPRINTF(MultiEthernet, "packet not sent, link busy\n");
+        return false;
+    }
+
+    packet = pkt;
+    Tick delay = (Tick)ceil(((double)pkt->length * ticksPerByte) + 1.0);
+    if (delayVar != 0)
+        delay += random_mt.random<Tick>(0, delayVar);
+
+    // send the packet to the peers
+    assert(multiIface);
+    multiIface->packetOut(pkt, delay);
+
+    // schedule the send done event
+    parent->schedule(doneEvent, curTick() + delay);
+
+    return true;
+}
+
+void
+MultiEtherLink::Link::serialize(const string &base, CheckpointOut &cp) const
+{
+    bool packet_exists = (packet != nullptr);
+    paramOut(cp, base + ".packet_exists", packet_exists);
+    if (packet_exists)
+        packet->serialize(base + ".packet", cp);
+
+    bool event_scheduled = event->scheduled();
+    paramOut(cp, base + ".event_scheduled", event_scheduled);
+    if (event_scheduled) {
+        Tick event_time = event->when();
+        paramOut(cp, base + ".event_time", event_time);
+    }
+}
+
+void
+MultiEtherLink::Link::unserialize(const string &base, CheckpointIn &cp)
+{
+    bool packet_exists;
+    paramIn(cp, base + ".packet_exists", packet_exists);
+    if (packet_exists) {
+        packet = make_shared<EthPacketData>(16384);
+        packet->unserialize(base + ".packet", cp);
+    }
+
+    bool event_scheduled;
+    paramIn(cp, base + ".event_scheduled", event_scheduled);
+    if (event_scheduled) {
+        Tick event_time;
+        paramIn(cp, base + ".event_time", event_time);
+        parent->schedule(*event, event_time);
+    }
+}
+
+MultiEtherLink::LocalIface::LocalIface(const std::string &name,
+                                       TxLink *tx,
+                                       RxLink *rx,
+                                       MultiIface *m) :
+    EtherInt(name), txLink(tx)
+{
+    tx->setLocalInt(this);
+    rx->setLocalInt(this);
+    tx->setMultiInt(m);
+    rx->setMultiInt(m);
+}
+
+MultiEtherLink *
+MultiEtherLinkParams::create()
+{
+    return new MultiEtherLink(this);
+}
+
+
diff --git a/src/dev/multi_etherlink.hh b/src/dev/multi_etherlink.hh
new file mode 100644
index 000000000..7d1352d60
--- /dev/null
+++ b/src/dev/multi_etherlink.hh
@@ -0,0 +1,235 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * Device module for a full duplex ethernet link for multi gem5 simulations.
+ *
+ * See comments in dev/multi_iface.hh for a generic description of multi
+ * gem5 simulations.
+ *
+ * This class is meant to be a drop in replacement for the EtherLink class for
+ * multi gem5 runs.
+ *
+ */
+#ifndef __DEV_MULTIETHERLINK_HH__
+#define __DEV_MULTIETHERLINK_HH__
+
+#include <iostream>
+
+#include "dev/etherlink.hh"
+#include "params/MultiEtherLink.hh"
+
+class MultiIface;
+class EthPacketData;
+
+/**
+ * Model for a fixed bandwidth full duplex ethernet link.
+ */
+class MultiEtherLink : public EtherObject
+{
+  protected:
+    class LocalIface;
+
+    /**
+     * Model base class for a single uni-directional link.
+     *
+     * The link will encapsulate and transfer Ethernet packets to/from
+     * the message server.
+     */
+    class Link
+    {
+      protected:
+        std::string objName;
+        MultiEtherLink *parent;
+        LocalIface *localIface;
+        EtherDump *dump;
+        MultiIface *multiIface;
+        Event *event;
+        EthPacketPtr packet;
+
+      public:
+        Link(const std::string &name, MultiEtherLink *p,
+             EtherDump *d, Event *e) :
+            objName(name), parent(p), localIface(nullptr), dump(d),
+            multiIface(nullptr), event(e) {}
+
+        ~Link() {}
+
+        const std::string name() const { return objName; }
+        bool busy() const { return (bool)packet; }
+        void setLocalInt(LocalIface *i) { assert(!localIface); localIface=i; }
+
+        void serialize(const std::string &base, CheckpointOut &cp) const;
+        void unserialize(const std::string &base, CheckpointIn &cp);
+    };
+
+    /**
+     * Model for a send link.
+     */
+    class TxLink : public Link
+    {
+      protected:
+        /**
+         * Per byte send delay
+         */
+        double ticksPerByte;
+        /**
+         * Random component of the send delay
+         */
+        Tick delayVar;
+
+        /**
+         * Send done callback. Called from doneEvent.
+         */
+        void txDone();
+        typedef EventWrapper<TxLink, &TxLink::txDone> DoneEvent;
+        friend void DoneEvent::process();
+        DoneEvent doneEvent;
+
+      public:
+        TxLink(const std::string &name, MultiEtherLink *p,
+               double invBW, Tick delay_var, EtherDump *d) :
+            Link(name, p, d, &doneEvent), ticksPerByte(invBW),
+            delayVar(delay_var), doneEvent(this) {}
+        ~TxLink() {}
+
+        /**
+         * Register the multi interface to be used to talk to the
+         * peer gem5 processes.
+         */
+        void setMultiInt(MultiIface *m) { assert(!multiIface); multiIface=m; }
+
+        /**
+         * Initiate sending of a packet via this link.
+         *
+         * @param packet Ethernet packet to send
+         */
+        bool transmit(EthPacketPtr packet);
+    };
+
+    /**
+     * Model for a receive link.
+     */
+    class RxLink : public Link
+    {
+      protected:
+
+        /**
+         * Transmission delay for the simulated Ethernet link.
+         */
+        Tick linkDelay;
+
+        /**
+         * Receive done callback method. Called from doneEvent.
+         */
+        void rxDone() ;
+        typedef EventWrapper<RxLink, &RxLink::rxDone> DoneEvent;
+        friend void DoneEvent::process();
+        DoneEvent doneEvent;
+
+      public:
+
+        RxLink(const std::string &name, MultiEtherLink *p,
+               Tick delay, EtherDump *d) :
+            Link(name, p, d, &doneEvent),
+            linkDelay(delay), doneEvent(this) {}
+        ~RxLink() {}
+
+        /**
+         * Register our multi interface to talk to the peer gem5 processes.
+         */
+        void setMultiInt(MultiIface *m);
+    };
+
+    /**
+     * Interface to the local simulated system
+     */
+    class LocalIface : public EtherInt
+    {
+      private:
+        TxLink *txLink;
+
+      public:
+        LocalIface(const std::string &name, TxLink *tx, RxLink *rx,
+                   MultiIface *m);
+
+        bool recvPacket(EthPacketPtr pkt) { return txLink->transmit(pkt); }
+        void sendDone() { peer->sendDone(); }
+        bool isBusy() { return txLink->busy(); }
+    };
+
+
+  protected:
+    /**
+     * Interface to talk to the peer gem5 processes.
+     */
+    MultiIface *multiIface;
+    /**
+     * Send link
+     */
+    TxLink *txLink;
+    /**
+     * Receive link
+     */
+    RxLink *rxLink;
+    LocalIface *localIface;
+
+  public:
+    typedef MultiEtherLinkParams Params;
+    MultiEtherLink(const Params *p);
+    ~MultiEtherLink();
+
+    const Params *
+    params() const
+    {
+        return dynamic_cast<const Params *>(_params);
+    }
+
+    virtual EtherInt *getEthPort(const std::string &if_name,
+                                 int idx) M5_ATTR_OVERRIDE;
+
+    void memWriteback() M5_ATTR_OVERRIDE;
+    void init() M5_ATTR_OVERRIDE;
+    void startup() M5_ATTR_OVERRIDE;
+
+    void serialize(CheckpointOut &cp) const M5_ATTR_OVERRIDE;
+    void unserialize(CheckpointIn &cp) M5_ATTR_OVERRIDE;
+};
+
+#endif // __DEV_MULTIETHERLINK_HH__
diff --git a/src/dev/multi_iface.cc b/src/dev/multi_iface.cc
new file mode 100644
index 000000000..c924c1a17
--- /dev/null
+++ b/src/dev/multi_iface.cc
@@ -0,0 +1,622 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * The interface class for multi gem5 simulations.
+ */
+
+#include "dev/multi_iface.hh"
+
+#include <queue>
+#include <thread>
+
+#include "base/random.hh"
+#include "base/trace.hh"
+#include "debug/MultiEthernet.hh"
+#include "debug/MultiEthernetPkt.hh"
+#include "dev/etherpkt.hh"
+#include "sim/sim_exit.hh"
+#include "sim/sim_object.hh"
+
+
+MultiIface::Sync *MultiIface::sync = nullptr;
+MultiIface::SyncEvent *MultiIface::syncEvent = nullptr;
+unsigned MultiIface::recvThreadsNum = 0;
+MultiIface *MultiIface::master = nullptr;
+
+bool
+MultiIface::Sync::run(SyncTrigger t, Tick sync_tick)
+{
+    std::unique_lock<std::mutex> sync_lock(lock);
+
+    trigger = t;
+    if (trigger != SyncTrigger::periodic) {
+        DPRINTF(MultiEthernet,"MultiIface::Sync::run() trigger:%d\n",
+                (unsigned)trigger);
+    }
+
+    switch (state) {
+      case SyncState::asyncCkpt:
+        switch (trigger) {
+          case SyncTrigger::ckpt:
+            assert(MultiIface::syncEvent->interrupted == false);
+            state = SyncState::busy;
+            break;
+          case SyncTrigger::periodic:
+            if (waitNum == 0) {
+                // So all recv threads got an async checkpoint request already
+                // and a simExit is scheduled at the end of the current tick
+                // (i.e. it is a periodic sync scheduled at the same tick as
+                // the simExit).
+                state = SyncState::idle;
+                DPRINTF(MultiEthernet,"MultiIface::Sync::run() interrupted "
+                "due to async ckpt scheduled\n");
+                return false;
+            } else {
+                // we still need to wait for some receiver thread to get the
+                // aysnc ckpt request. We are going to proceed as 'interrupted'
+                // periodic sync.
+                state = SyncState::interrupted;
+                DPRINTF(MultiEthernet,"MultiIface::Sync::run() interrupted "
+                "due to ckpt request is coming in\n");
+            }
+            break;
+          case SyncTrigger::atomic:
+            assert(trigger != SyncTrigger::atomic);
+        }
+        break;
+      case SyncState::idle:
+        state = SyncState::busy;
+        break;
+        // Only one sync can be active at any time
+      case SyncState::interrupted:
+      case SyncState::busy:
+        assert(state != SyncState::interrupted);
+        assert(state != SyncState::busy);
+        break;
+    }
+    // Kick-off the sync unless we are in the middle of an interrupted
+    // periodic sync
+    if (state != SyncState::interrupted) {
+        assert(waitNum == 0);
+        waitNum = MultiIface::recvThreadsNum;
+        // initiate the global synchronisation
+        assert(MultiIface::master != nullptr);
+        MultiIface::master->syncRaw(triggerToMsg[(unsigned)trigger], sync_tick);
+    }
+    // now wait until all receiver threads complete the synchronisation
+    auto lf = [this]{ return waitNum == 0; };
+    cv.wait(sync_lock, lf);
+
+    // we are done
+    assert(state == SyncState::busy || state == SyncState::interrupted);
+    bool ret = (state != SyncState::interrupted);
+    state = SyncState::idle;
+    return ret;
+}
+
+void
+MultiIface::Sync::progress(MsgType msg)
+{
+    std::unique_lock<std::mutex> sync_lock(lock);
+
+    switch (msg) {
+      case MsgType::cmdAtomicSyncAck:
+        assert(state == SyncState::busy && trigger == SyncTrigger::atomic);
+        break;
+      case MsgType::cmdPeriodicSyncAck:
+        assert(state == SyncState::busy && trigger == SyncTrigger::periodic);
+        break;
+      case MsgType::cmdCkptSyncAck:
+        assert(state == SyncState::busy && trigger == SyncTrigger::ckpt);
+        break;
+      case MsgType::cmdCkptSyncReq:
+        switch (state) {
+          case SyncState::busy:
+            if (trigger == SyncTrigger::ckpt) {
+                // We are already in a checkpoint sync but got another ckpt
+                // sync request. This may happen if two (or more) peer gem5
+                // processes try to start a ckpt nearly at the same time.
+                // Incrementing waitNum here (before decrementing it below)
+                // effectively results in ignoring this new ckpt sync request.
+                waitNum++;
+                break;
+            }
+            assert (waitNum == recvThreadsNum);
+            state = SyncState::interrupted;
+            // we need to fall over here to handle "recvThreadsNum == 1" case
+          case SyncState::interrupted:
+            assert(trigger == SyncTrigger::periodic);
+            assert(waitNum >= 1);
+            if (waitNum == 1) {
+                exitSimLoop("checkpoint");
+            }
+            break;
+          case SyncState::idle:
+            // There is no on-going sync so we got an async ckpt request. If we
+            // are the only receiver thread then we need to schedule the
+            // checkpoint. Otherwise, only change the state to 'asyncCkpt' and
+            // let the last receiver thread to schedule the checkpoint at the
+            // 'asyncCkpt' case.
+            // Note that a periodic or resume sync may start later and that can
+            // trigger a state change to 'interrupted' (so the checkpoint may
+            // get scheduled at 'interrupted' case finally).
+            assert(waitNum == 0);
+            state = SyncState::asyncCkpt;
+            waitNum = MultiIface::recvThreadsNum;
+            // we need to fall over here to handle "recvThreadsNum == 1" case
+          case SyncState::asyncCkpt:
+            assert(waitNum >= 1);
+            if (waitNum == 1)
+                exitSimLoop("checkpoint");
+            break;
+          default:
+            panic("Unexpected state for checkpoint request message");
+            break;
+        }
+        break;
+      default:
+        panic("Unknown msg type");
+        break;
+    }
+    waitNum--;
+    assert(state != SyncState::idle);
+    // Notify the simultaion thread if there is an on-going sync.
+    if (state != SyncState::asyncCkpt) {
+        sync_lock.unlock();
+        cv.notify_one();
+    }
+}
+
+void MultiIface::SyncEvent::start(Tick start, Tick interval)
+{
+    assert(!scheduled());
+    if (interval == 0)
+        panic("Multi synchronisation period must be greater than zero");
+    repeat = interval;
+    schedule(start);
+}
+
+void
+MultiIface::SyncEvent::adjust(Tick start_tick, Tick repeat_tick)
+{
+    // The new multi interface may require earlier start of the
+    // synchronisation.
+    assert(scheduled() == true);
+    if (start_tick < when())
+        reschedule(start_tick);
+    // The new multi interface may require more frequent synchronisation.
+    if (repeat == 0)
+        panic("Multi synchronisation period must be greater than zero");
+    if (repeat < repeat_tick)
+        repeat = repeat_tick;
+}
+
+void
+MultiIface::SyncEvent::process()
+{
+    /*
+     * Note that this is a global event so this process method will be called
+     * by only exactly one thread.
+     */
+    // if we are draining the system then we must not start a periodic sync (as
+    // it is not sure that all peer gem5 will reach this tick before taking
+    // the checkpoint).
+    if (isDraining == true) {
+        assert(interrupted == false);
+        interrupted = true;
+        DPRINTF(MultiEthernet,"MultiIface::SyncEvent::process() interrupted "
+                "due to draining\n");
+        return;
+    }
+    if (interrupted == false)
+        scheduledAt = curTick();
+    /*
+     * We hold the eventq lock at this point but the receiver thread may
+     * need the lock to schedule new recv events while waiting for the
+     * multi sync to complete.
+     * Note that the other simulation threads also release their eventq
+     * locks while waiting for us due to the global event semantics.
+     */
+    curEventQueue()->unlock();
+    // we do a global sync here
+    interrupted = !MultiIface::sync->run(SyncTrigger::periodic, scheduledAt);
+    // Global sync completed or got interrupted.
+    // we are expected to exit with the eventq lock held
+    curEventQueue()->lock();
+    // schedule the next global sync event if this one completed. Otherwise
+    // (i.e. this one was interrupted by a checkpoint request), we will 
+    // reschedule this one after the draining is complete.
+    if (!interrupted)
+        schedule(scheduledAt + repeat);
+}
+
+void MultiIface::SyncEvent::resume()
+{
+    Tick sync_tick;
+    assert(!scheduled());
+    if (interrupted) {
+        assert(curTick() >= scheduledAt);
+        // We have to complete the interrupted periodic sync asap.
+        // Note that this sync might be interrupted now again with a checkpoint
+        // request from a peer gem5...
+        sync_tick = curTick();
+        schedule(sync_tick);
+    } else {
+        // So we completed the last periodic sync, let's find  out the tick for
+        // next one
+        assert(curTick() > scheduledAt);
+        sync_tick = scheduledAt + repeat;
+        if (sync_tick < curTick())
+            panic("Cannot resume periodic synchronisation");
+        schedule(sync_tick);
+    }
+    DPRINTF(MultiEthernet,
+            "MultiIface::SyncEvent periodic sync resumed at %lld "
+            "(curTick:%lld)\n", sync_tick, curTick());
+}
+
+void MultiIface::SyncEvent::serialize(const std::string &base,
+                                      CheckpointOut &cp) const
+{
+    // Save the periodic multi sync schedule information
+    paramOut(cp, base + ".periodicSyncRepeat", repeat);
+    paramOut(cp, base + ".periodicSyncInterrupted", interrupted);
+    paramOut(cp, base + ".periodicSyncAt", scheduledAt);
+}
+
+void MultiIface::SyncEvent::unserialize(const std::string &base,
+                                        CheckpointIn &cp)
+{
+    paramIn(cp, base + ".periodicSyncRepeat", repeat);
+    paramIn(cp, base + ".periodicSyncInterrupted", interrupted);
+    paramIn(cp, base + ".periodicSyncAt", scheduledAt);
+}
+
+MultiIface::MultiIface(unsigned multi_rank,
+                       Tick sync_start,
+                       Tick sync_repeat,
+                       EventManager *em) :
+    syncStart(sync_start), syncRepeat(sync_repeat),
+    recvThread(nullptr), eventManager(em), recvDone(nullptr),
+    scheduledRecvPacket(nullptr), linkDelay(0), rank(multi_rank)
+{
+    DPRINTF(MultiEthernet, "MultiIface() ctor rank:%d\n",multi_rank);
+    if (master == nullptr) {
+        assert(sync == nullptr);
+        assert(syncEvent == nullptr);
+        sync = new Sync();
+        syncEvent = new SyncEvent();
+        master = this;
+    }
+}
+
+MultiIface::~MultiIface()
+{
+    assert(recvThread);
+    delete recvThread;
+    if (this == master) {
+        assert(syncEvent);
+        delete syncEvent;
+        assert(sync);
+        delete sync;
+    }
+}
+
+void
+MultiIface::packetOut(EthPacketPtr pkt, Tick send_delay)
+{
+    MultiHeaderPkt::Header header_pkt;
+    unsigned address_length = MultiHeaderPkt::maxAddressLength();
+
+    // Prepare a multi header packet for the Ethernet packet we want to
+    // send out.
+    header_pkt.msgType = MsgType::dataDescriptor;
+    header_pkt.sendTick  = curTick();
+    header_pkt.sendDelay = send_delay;
+
+    // Store also the source and destination addresses.
+    pkt->packAddress(header_pkt.srcAddress, header_pkt.dstAddress,
+                     address_length);
+
+    header_pkt.dataPacketLength = pkt->size();
+
+    // Send out the multi hedare packet followed by the Ethernet packet.
+    sendRaw(&header_pkt, sizeof(header_pkt), header_pkt.dstAddress);
+    sendRaw(pkt->data, pkt->size(), header_pkt.dstAddress);
+    DPRINTF(MultiEthernetPkt,
+            "MultiIface::sendDataPacket() done size:%d send_delay:%llu "
+            "src:0x%02x%02x%02x%02x%02x%02x "
+            "dst:0x%02x%02x%02x%02x%02x%02x\n",
+            pkt->size(), send_delay,
+            header_pkt.srcAddress[0], header_pkt.srcAddress[1],
+            header_pkt.srcAddress[2], header_pkt.srcAddress[3],
+            header_pkt.srcAddress[4], header_pkt.srcAddress[5],
+            header_pkt.dstAddress[0], header_pkt.dstAddress[1],
+            header_pkt.dstAddress[2], header_pkt.dstAddress[3],
+            header_pkt.dstAddress[4], header_pkt.dstAddress[5]);
+}
+
+bool
+MultiIface::recvHeader(MultiHeaderPkt::Header &header_pkt)
+{
+    // Blocking receive of an incoming multi header packet.
+    return recvRaw((void *)&header_pkt, sizeof(header_pkt));
+}
+
+void
+MultiIface::recvData(const MultiHeaderPkt::Header &header_pkt)
+{
+    // We are here beacuse a header packet has been received implying
+    // that an Ethernet (data) packet is coming in next.
+    assert(header_pkt.msgType == MsgType::dataDescriptor);
+    // Allocate storage for the incoming Ethernet packet.
+    EthPacketPtr new_packet(new EthPacketData(header_pkt.dataPacketLength));
+    // Now execute the blocking receive and store the incoming data directly
+    // in the new EthPacketData object.
+    if (! recvRaw((void *)(new_packet->data), header_pkt.dataPacketLength))
+        panic("Missing data packet");
+
+    new_packet->length = header_pkt.dataPacketLength;
+    // Grab the event queue lock to schedule a new receive event for the
+    // data packet.
+    curEventQueue()->lock();
+    // Compute the receive tick. It includes the send delay and the
+    // simulated link delay.
+    Tick recv_tick = header_pkt.sendTick + header_pkt.sendDelay + linkDelay;
+    DPRINTF(MultiEthernetPkt, "MultiIface::recvThread() packet receive, "
+            "send_tick:%llu send_delay:%llu link_delay:%llu recv_tick:%llu\n",
+            header_pkt.sendTick, header_pkt.sendDelay, linkDelay, recv_tick);
+
+    if (recv_tick <= curTick()) {
+        panic("Simulators out of sync - missed packet receive by %llu ticks",
+              curTick() - recv_tick);
+    }
+    // Now we are about to schedule a recvDone event for the new data packet.
+    // We use the same recvDone object for all incoming data packets. If
+    // that is already scheduled - i.e. a receive event for a previous
+    // data packet is already pending - then we have to check whether the
+    // receive tick for the new packet is earlier than that of the currently
+    // pending event. Packets may arrive out-of-order with respect to
+    // simulated receive time. If that is the case, we need to re-schedule the
+    // recvDone event for the new packet. Otherwise, we save the packet
+    // pointer and the recv tick for the new packet in the recvQueue. See
+    // the implementation of the packetIn() method for comments on how this
+    // information is retrieved from the recvQueue by the simulation thread.
+    if (!recvDone->scheduled()) {
+        assert(recvQueue.size() == 0);
+        assert(scheduledRecvPacket == nullptr);
+        scheduledRecvPacket = new_packet;
+        eventManager->schedule(recvDone, recv_tick);
+    } else if (recvDone->when() > recv_tick) {
+        recvQueue.emplace(scheduledRecvPacket, recvDone->when());
+        eventManager->reschedule(recvDone, recv_tick);
+        scheduledRecvPacket = new_packet;
+    } else {
+        recvQueue.emplace(new_packet, recv_tick);
+    }
+    curEventQueue()->unlock();
+}
+
+void
+MultiIface::recvThreadFunc()
+{
+    EthPacketPtr new_packet;
+    MultiHeaderPkt::Header header;
+
+    // The new receiver thread shares the event queue with the simulation
+    // thread (associated with the simulated Ethernet link).
+    curEventQueue(eventManager->eventQueue());
+    // Main loop to wait for and process any incoming message.
+    for (;;) {
+        // recvHeader() blocks until the next multi header packet comes in.
+        if (!recvHeader(header)) {
+            // We lost connection to the peer gem5 processes most likely
+            // because one of them called m5 exit. So we stop here.
+            exit_message("info", 0, "Message server closed connection, "
+                         "simulation is exiting");
+        }
+        // We got a valid multi header packet, let's process it
+        if (header.msgType == MsgType::dataDescriptor) {
+            recvData(header);
+        } else {
+            // everything else must be synchronisation related command
+            sync->progress(header.msgType);
+        }
+    }
+}
+
+EthPacketPtr
+MultiIface::packetIn()
+{
+    // We are called within the process() method of the recvDone event. We
+    // return the packet that triggered the current receive event.
+    // If there is further packets in the recvQueue, we also have to schedule
+    // the recvEvent for the next packet with the smallest receive tick.
+    // The priority queue container ensures that smallest receive tick is
+    // always on the top of the queue.
+    assert(scheduledRecvPacket != nullptr);
+    EthPacketPtr next_packet = scheduledRecvPacket;
+
+    if (! recvQueue.empty()) {
+        eventManager->schedule(recvDone, recvQueue.top().second);
+        scheduledRecvPacket = recvQueue.top().first;
+        recvQueue.pop();
+    } else {
+        scheduledRecvPacket = nullptr;
+    }
+
+    return next_packet;
+}
+
+void
+MultiIface::spawnRecvThread(Event *recv_done, Tick link_delay)
+{
+    assert(recvThread == nullptr);
+    // all receive thread must be spawned before simulation starts
+    assert(eventManager->eventQueue()->getCurTick() == 0);
+
+    recvDone = recv_done;
+    linkDelay = link_delay;
+
+    recvThread = new std::thread(&MultiIface::recvThreadFunc, this);
+
+    recvThreadsNum++;
+}
+
+DrainState
+MultiIface::drain()
+{
+    DPRINTF(MultiEthernet,"MultiIFace::drain() called\n");
+
+    // This can be called multiple times in the same drain cycle.
+    if (master == this) {
+        syncEvent->isDraining = true;
+    }
+
+    return DrainState::Drained;
+}
+
+void MultiIface::drainDone() {
+    if (master == this) {
+        assert(syncEvent->isDraining == true);
+        syncEvent->isDraining = false;
+        // We need to resume the interrupted periodic sync here now that the
+        // draining is done. If the last periodic sync completed before the
+        // checkpoint then the next one is already scheduled.
+        if (syncEvent->interrupted)
+            syncEvent->resume();
+    }
+}
+
+void MultiIface::serialize(const std::string &base, CheckpointOut &cp) const
+{
+    // Drain the multi interface before the checkpoint is taken. We cannot call
+    // this as part of the normal drain cycle because this multi sync has to be
+    // called exactly once after the system is fully drained.
+    // Note that every peer will take a checkpoint but they may take it at
+    // different ticks.
+    // This sync request may interrupt an on-going periodic sync in some peers.
+    sync->run(SyncTrigger::ckpt, curTick());
+
+    // Save the periodic multi sync status
+    syncEvent->serialize(base, cp);
+
+    unsigned n_rx_packets = recvQueue.size();
+    if (scheduledRecvPacket != nullptr)
+        n_rx_packets++;
+
+    paramOut(cp, base + ".nRxPackets", n_rx_packets);
+
+    if (n_rx_packets > 0) {
+        assert(recvDone->scheduled());
+        scheduledRecvPacket->serialize(base + ".rxPacket[0]", cp);
+    }
+
+    for (unsigned i=1; i < n_rx_packets; i++)  {
+        const RecvInfo recv_info = recvQueue.impl().at(i-1);
+        recv_info.first->serialize(base + csprintf(".rxPacket[%d]", i), cp);
+        Tick rx_tick = recv_info.second;
+        paramOut(cp, base + csprintf(".rxTick[%d]", i), rx_tick);
+    }
+}
+
+void MultiIface::unserialize(const std::string &base, CheckpointIn &cp)
+{
+    assert(recvQueue.size() == 0);
+    assert(scheduledRecvPacket == nullptr);
+    assert(recvDone->scheduled() == false);
+
+    // restore periodic sync info
+    syncEvent->unserialize(base, cp);
+
+    unsigned n_rx_packets;
+    paramIn(cp, base + ".nRxPackets", n_rx_packets);
+
+    if (n_rx_packets > 0) {
+        scheduledRecvPacket = std::make_shared<EthPacketData>(16384);
+        scheduledRecvPacket->unserialize(base + ".rxPacket[0]", cp);
+        // Note: receive event will be scheduled when the link is unserialized
+    }
+
+    for (unsigned i=1; i < n_rx_packets; i++) {
+        EthPacketPtr rx_packet = std::make_shared<EthPacketData>(16384);
+        rx_packet->unserialize(base + csprintf(".rxPacket[%d]", i), cp);
+        Tick rx_tick = 0;
+        paramIn(cp, base + csprintf(".rxTick[%d]", i), rx_tick);
+        assert(rx_tick > 0);
+        recvQueue.emplace(rx_packet,rx_tick);
+    }
+}
+
+void MultiIface::initRandom()
+{
+    // Initialize the seed for random generator to avoid the same sequence
+    // in all gem5 peer processes
+    assert(master != nullptr);
+    if (this == master)
+        random_mt.init(5489 * (rank+1) + 257);
+}
+
+void MultiIface::startPeriodicSync()
+{
+    DPRINTF(MultiEthernet, "MultiIface:::initPeriodicSync started\n");
+    // Do a global sync here to ensure that peer gem5 processes are around
+    // (actually this may not be needed...)
+    sync->run(SyncTrigger::atomic, curTick());
+
+    // Start the periodic sync if it is a fresh simulation from scratch
+    if (curTick() == 0) {
+        if (this == master) {
+        syncEvent->start(syncStart, syncRepeat);
+        inform("Multi synchronisation activated: start at %lld, "
+               "repeat at every %lld ticks.\n",
+               syncStart, syncRepeat);
+        } else {
+            // In case another multiIface object requires different schedule
+            // for periodic sync than the master does.
+            syncEvent->adjust(syncStart, syncRepeat);
+        }
+    } else {
+        // Schedule the next periodic sync if resuming from a checkpoint
+        if (this == master)
+            syncEvent->resume();
+    }
+    DPRINTF(MultiEthernet, "MultiIface::initPeriodicSync done\n");
+}
diff --git a/src/dev/multi_iface.hh b/src/dev/multi_iface.hh
new file mode 100644
index 000000000..0e4859ecd
--- /dev/null
+++ b/src/dev/multi_iface.hh
@@ -0,0 +1,492 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * The interface class for multi gem5 simulations.
+ *
+ * Multi gem5 is an extension to gem5 to enable parallel simulation of a
+ * distributed system (e.g. simulation of a pool of machines
+ * connected by Ethernet links). A multi gem5 run consists of seperate gem5
+ * processes running in parallel. Each gem5 process executes
+ * the simulation of a component of the simulated distributed system.
+ * (An example component can be a multi-core board with an Ethernet NIC.)
+ * The MultiIface class below provides services to transfer data and
+ * control messages among the gem5 processes. The main such services are
+ * as follows.
+ *
+ * 1. Send a data packet coming from a simulated Ethernet link. The packet
+ * will be transferred to (all) the target(s) gem5 processes. The send
+ * operation is always performed by the simulation thread, i.e. the gem5
+ * thread that is processing the event queue associated with the simulated
+ * Ethernet link.
+ *
+ * 2. Spawn a receiver thread to process messages coming in from the
+ * from other gem5 processes. Each simulated Ethernet link has its own
+ * associated receiver thread. The receiver thread saves the incoming packet
+ * and schedule an appropriate receive event in the event queue.
+ *
+ * 3. Schedule a global barrier event periodically to keep the gem5
+ * processes in sync.
+ * Periodic barrier event to keep peer gem5 processes in sync. The basic idea
+ * is that no gem5 process can go ahead further than the simulated link
+ * transmission delay to ensure that a corresponding receive event can always
+ * be scheduled for any message coming in from a peer gem5 process.
+ *
+ *
+ *
+ * This interface is an abstract class (sendRaw() and recvRaw()
+ * methods are pure virtual). It can work with various low level
+ * send/receive service implementations (e.g. TCP/IP, MPI,...). A TCP
+ * stream socket version is implemented in dev/src/tcp_iface.[hh,cc].
+ */
+#ifndef __DEV_MULTI_IFACE_HH__
+#define __DEV_MULTI_IFACE_HH__
+
+#include <array>
+#include <mutex>
+#include <queue>
+#include <thread>
+#include <utility>
+
+#include "dev/etherpkt.hh"
+#include "dev/multi_packet.hh"
+#include "sim/core.hh"
+#include "sim/drain.hh"
+#include "sim/global_event.hh"
+
+class EventManager;
+
+/**
+ * The interface class to talk to peer gem5 processes.
+ */
+class MultiIface : public Drainable
+{
+  public:
+    /*!
+     * The possible reasons a multi sync among gem5 peers is needed for.
+     */
+    enum
+    class SyncTrigger {
+        periodic, /*!< Regular periodic sync. This can be interrupted by a
+                   checkpoint sync request */
+        ckpt,     /*!< sync before taking a checkpoint */
+        atomic    /*!< sync that cannot be interrupted (e.g. sync at startup) */
+    };
+
+  private:
+    typedef MultiHeaderPkt::MsgType MsgType;
+
+    /** Sync State-Machine
+     \dot
+     digraph Sync {
+     node [shape=box, fontsize=10];
+     idle -> busy
+     [ label="new trigger\n by run()" fontsize=8 ];
+     busy -> busy
+     [ label="new message by progress():\n(msg == SyncAck &&\nwaitNum > 1) || \n(msg==CkptSyncReq &&\ntrigger == ckpt)" fontsize=8 ];
+     busy -> idle
+     [ label="new message by progress():\n(msg == SyncAck &&\nwaitNum == 1)" fontsize=8 ];
+     busy -> interrupted
+     [ label="new message by progress():\n(msg == CkptSyncReq &&\ntrigger == periodic)" fontsize=8 ];
+     idle -> asyncCkpt
+     [ label="new message by progress():\nmsg == CkptSyncReq" fontsize=8 ];
+     asyncCkpt -> asyncCkpt
+     [ label="new message by progress():\nmsg == CkptSyncReq" fontsize=8 ];
+     asyncCkpt -> busy
+     [ label="new trigger by run():\ntrigger == ckpt" fontsize=8 ];
+     asyncCkpt -> idle
+     [ label="new trigger by run():\n(trigger == periodic &&\nwaitNum == 0) " fontsize=8 ];
+     asyncCkpt -> interrupted
+     [ label="new trigger by run():\n(trigger == periodic &&\nwaitNum > 0) " fontsize=8 ];
+     interrupted -> interrupted
+     [ label="new message by progress():\n(msg == CkptSyncReq &&\nwaitNum > 1)" fontsize=8 ];
+     interrupted -> idle
+     [ label="new message by progress():\n(msg == CkptSyncReq &&\nwaitNum == 1)" fontsize=8 ];
+     }
+     \enddot
+     */
+    /** @class Sync
+     * This class implements global sync operations among gem5 peer processes.
+     *
+     * @note This class is used as a singleton object (shared by all MultiIface
+     * objects).
+     */
+    class Sync
+    {
+      private:
+        /*!
+         * Internal state of the sync singleton object.
+         */
+        enum class SyncState {
+            busy,        /*!< There is an on-going sync. */
+            interrupted, /*!< An on-going periodic sync was interrupted. */
+            asyncCkpt,   /*!< A checkpoint (sim_exit) is already scheduled */
+            idle         /*!< There is no active sync. */
+        };
+        /**
+         * The lock to protect access to the MultiSync object.
+         */
+        std::mutex lock;
+        /**
+         * Condition variable for the simulation thread to wait on
+         * until all receiver threads completes the current global
+         * synchronisation.
+         */
+        std::condition_variable cv;
+        /**
+         * Number of receiver threads that not yet completed the current global
+         * synchronisation.
+         */
+        unsigned waitNum;
+        /**
+         * The trigger for the most recent sync.
+         */
+        SyncTrigger trigger;
+        /**
+         * Map sync triggers to request messages.
+         */
+        std::array<MsgType, 3> triggerToMsg = {{
+                MsgType::cmdPeriodicSyncReq,
+                MsgType::cmdCkptSyncReq,
+                MsgType::cmdAtomicSyncReq
+            }};
+
+        /**
+         * Current sync state.
+         */
+        SyncState state;
+
+      public:
+        /**
+         *  Core method to perform a full multi sync.
+         *
+         * @param t Sync trigger.
+         * @param sync_tick The tick the sync was expected to happen at.
+         * @return true if the sync completed, false if it was interrupted.
+         *
+         * @note In case of an interrupted periodic sync, sync_tick can be less
+         * than curTick() when we resume (i.e. re-run) it
+         */
+        bool run(SyncTrigger t, Tick sync_tick);
+        /**
+         * Callback when the receiver thread gets a sync message.
+         */
+        void progress(MsgType m);
+
+        Sync() : waitNum(0), state(SyncState::idle) {}
+        ~Sync() {}
+    };
+
+
+    /**
+     * The global event to schedule peridic multi sync. It is used as a
+     * singleton object.
+     *
+     * The periodic synchronisation works as follows.
+     * 1. A MultisyncEvent is scheduled as a global event when startup() is
+     * called.
+     * 2. The progress() method of the MultisyncEvent initiates a new barrier
+     * for each simulated Ethernet links.
+     * 3. Simulation thread(s) then waits until all receiver threads
+     * completes the ongoing barrier. The global sync event is done.
+     */
+    class SyncEvent : public GlobalSyncEvent
+    {
+      public:
+        /**
+         * Flag to indicate that the most recent periodic sync was interrupted
+         * (by a checkpoint request).
+         */
+        bool interrupted;
+        /**
+         * The tick when the most recent periodic synchronisation was scheduled
+         * at.
+         */
+        Tick scheduledAt;
+        /**
+         * Flag to indicate an on-going drain cycle.
+         */
+         bool isDraining;
+
+      public:
+        /**
+         * Only the firstly instanstiated MultiIface object will
+         * call this constructor.
+         */
+        SyncEvent() : GlobalSyncEvent(Default_Pri, 0), interrupted(false),
+                      scheduledAt(0), isDraining(false) {}
+
+        ~SyncEvent() { assert (scheduled() == false); }
+        /**
+         * Schedule the first periodic sync event.
+         *
+         * @param start Start tick for multi synchronisation
+         * @param repeat Frequency of multi synchronisation
+         *
+         */
+        void start(Tick start, Tick repeat);
+        /**
+         * Reschedule (if necessary) the periodic sync event.
+         *
+         * @param start Start tick for multi synchronisation
+         * @param repeat Frequency of multi synchronisation
+         *
+         * @note Useful if we have multiple MultiIface objects with
+         * different 'start' and 'repeat' values for global sync.
+         */
+        void adjust(Tick start, Tick repeat);
+        /**
+         * This is a global event so process() will be called by each
+         * simulation threads. (See further comments in the .cc file.)
+         */
+        void process() M5_ATTR_OVERRIDE;
+        /**
+         * Schedule periodic sync when resuming from a checkpoint.
+         */
+        void resume();
+
+        void serialize(const std::string &base, CheckpointOut &cp) const;
+        void unserialize(const std::string &base, CheckpointIn &cp);
+    };
+
+    /**
+     * The receive thread needs to store the packet pointer and the computed
+     * receive tick for each incoming data packet. This information is used
+     * by the simulation thread when it processes the corresponding receive
+     * event. (See more comments at the implemetation of the recvThreadFunc()
+     * and RecvPacketIn() methods.)
+     */
+    typedef std::pair<EthPacketPtr, Tick> RecvInfo;
+
+    /**
+     * Comparison predicate for RecvInfo, needed by the recvQueue.
+     */
+    struct RecvInfoCompare {
+        bool operator()(const RecvInfo &lhs, const RecvInfo &rhs)
+        {
+            return lhs.second > rhs.second;
+        }
+    };
+
+    /**
+     * Customized priority queue used to store incoming data packets info by
+     * the receiver thread. We need to expose the underlying container to
+     * enable iterator access for serializing.
+     */
+    class RecvQueue : public std::priority_queue<RecvInfo,
+                                                 std::vector<RecvInfo>,
+                                                 RecvInfoCompare>
+    {
+      public:
+        std::vector<RecvInfo> &impl() { return c; }
+        const std::vector<RecvInfo> &impl() const { return c; }
+    };
+
+    /*
+     * The priority queue to store RecvInfo items ordered by receive ticks.
+     */
+    RecvQueue recvQueue;
+    /**
+     * The singleton Sync object to perform multi synchronisation.
+     */
+    static Sync *sync;
+    /**
+     * The singleton SyncEvent object to schedule periodic multi sync.
+     */
+    static SyncEvent *syncEvent;
+    /**
+     * Tick to schedule the first multi sync event.
+     * This is just as optimization : we do not need any multi sync
+     * event until the simulated NIC is brought up by the OS.
+     */
+    Tick syncStart;
+    /**
+     * Frequency of multi sync events in ticks.
+     */
+    Tick syncRepeat;
+    /**
+     * Receiver thread pointer.
+     * Each MultiIface object must have exactly one receiver thread.
+     */
+    std::thread *recvThread;
+    /**
+     * The event manager associated with the MultiIface object.
+     */
+    EventManager *eventManager;
+
+    /**
+     * The receive done event for the simulated Ethernet link.
+     * It is scheduled by the receiver thread for each incoming data
+     * packet.
+     */
+    Event *recvDone;
+
+    /**
+     * The packet that belongs to the currently scheduled recvDone event.
+     */
+    EthPacketPtr scheduledRecvPacket;
+
+    /**
+     * The link delay in ticks for the simulated Ethernet link.
+     */
+    Tick linkDelay;
+
+    /**
+     * The rank of this process among the gem5 peers.
+     */
+    unsigned rank;
+    /**
+     * Total number of receiver threads (in this gem5 process).
+     * During the simulation it should be constant and equal to the
+     * number of MultiIface objects (i.e. simulated Ethernet
+     * links).
+     */
+    static unsigned recvThreadsNum;
+    /**
+     * The very first MultiIface object created becomes the master. We need
+     * a master to co-ordinate the global synchronisation.
+     */
+    static MultiIface *master;
+
+  protected:
+    /**
+     * Low level generic send routine.
+     * @param buf buffer that holds the data to send out
+     * @param length number of bytes to send
+     * @param dest_addr address of the target (simulated NIC). This may be
+     * used by a subclass for optimization (e.g. optimize broadcast)
+     */
+    virtual void sendRaw(void *buf,
+                         unsigned length,
+                         const MultiHeaderPkt::AddressType dest_addr) = 0;
+    /**
+     * Low level generic receive routine.
+     * @param buf the buffer to store the incoming message
+     * @param length buffer size (in bytes)
+     */
+    virtual bool recvRaw(void *buf, unsigned length) = 0;
+    /**
+     * Low level request for synchronisation among gem5 processes. Only one
+     * MultiIface object needs to call this (in each gem5 process) to trigger
+     * a multi sync.
+     *
+     * @param sync_req Sync request command.
+     * @param sync_tick The tick when sync is expected to happen in the sender.
+     */
+    virtual void syncRaw(MsgType sync_req, Tick sync_tick) = 0;
+    /**
+     * The function executed by a receiver thread.
+     */
+    void recvThreadFunc();
+    /**
+     * Receive a multi header packet. Called by the receiver thread.
+     * @param header the structure to store the incoming header packet.
+     * @return false if any error occured during the receive, true otherwise
+     *
+     * A header packet can carry a control command (e.g. 'barrier leave') or
+     * information about a data packet that is following the header packet
+     * back to back.
+     */
+    bool recvHeader(MultiHeaderPkt::Header &header);
+    /**
+     * Receive a data packet. Called by the receiver thread.
+     * @param data_header The packet descriptor for the expected incoming data
+     * packet.
+     */
+    void recvData(const MultiHeaderPkt::Header &data_header);
+
+  public:
+
+    /**
+     * ctor
+     * @param multi_rank Rank of this gem5 process within the multi run
+     * @param sync_start Start tick for multi synchronisation
+     * @param sync_repeat Frequency for multi synchronisation
+     * @param em The event manager associated with the simulated Ethernet link
+     */
+    MultiIface(unsigned multi_rank,
+               Tick sync_start,
+               Tick sync_repeat,
+               EventManager *em);
+
+    virtual ~MultiIface();
+    /**
+     * Send out an Ethernet packet.
+     * @param pkt The Ethernet packet to send.
+     * @param send_delay The delay in ticks for the send completion event.
+     */
+    void packetOut(EthPacketPtr pkt, Tick send_delay);
+    /**
+     * Fetch the next packet from the receive queue.
+     */
+    EthPacketPtr packetIn();
+
+    /**
+     * spawn the receiver thread.
+     * @param recv_done The receive done event associated with the simulated
+     * Ethernet link.
+     * @param link_delay The link delay for the simulated Ethernet link.
+     */
+    void spawnRecvThread(Event *recv_done,
+                         Tick link_delay);
+    /**
+     * Initialize the random number generator with a different seed in each
+     * peer gem5 process.
+     */
+    void initRandom();
+
+    DrainState drain() M5_ATTR_OVERRIDE;
+
+    /**
+     * Callback when draining is complete.
+     */
+    void drainDone();
+
+    /**
+     * Initialize the periodic synchronisation among peer gem5 processes.
+     */
+    void startPeriodicSync();
+
+    void serialize(const std::string &base, CheckpointOut &cp) const;
+    void unserialize(const std::string &base, CheckpointIn &cp);
+
+};
+
+
+#endif
diff --git a/src/dev/multi_packet.cc b/src/dev/multi_packet.cc
new file mode 100644
index 000000000..1fd0c0e81
--- /dev/null
+++ b/src/dev/multi_packet.cc
@@ -0,0 +1,100 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * MultiHeaderPkt class to encapsulate multi-gem5 header packets
+ *
+ */
+
+#include "dev/multi_packet.hh"
+
+#include <cstdint>
+#include <cstring>
+
+#include "base/inet.hh"
+
+unsigned
+MultiHeaderPkt::maxAddressLength()
+{
+    return sizeof(AddressType);
+}
+
+void
+MultiHeaderPkt::clearAddress(AddressType &addr)
+{
+    std::memset(addr, 0, sizeof(addr));
+}
+
+bool
+MultiHeaderPkt::isAddressEqual(const AddressType &addr1,
+                               const AddressType &addr2)
+{
+    return (std::memcmp(addr1, addr2, sizeof(addr1)) == 0);
+}
+
+bool
+MultiHeaderPkt::isAddressLess(const AddressType &addr1,
+                              const AddressType &addr2)
+{
+    return (std::memcmp(addr1, addr2, sizeof(addr1)) < 0);
+}
+
+void
+MultiHeaderPkt::copyAddress(AddressType &dest, const AddressType &src)
+{
+    std::memcpy(dest, src, sizeof(dest));
+}
+
+bool
+MultiHeaderPkt::isBroadcastAddress(const AddressType &addr)
+{
+    return ((Net::EthAddr *)&addr)->broadcast();
+}
+
+bool
+MultiHeaderPkt::isMulticastAddress(const AddressType &addr)
+{
+    return ((Net::EthAddr *)&addr)->multicast();
+}
+
+bool
+MultiHeaderPkt::isUnicastAddress(const AddressType &addr)
+{
+    return ((Net::EthAddr *)&addr)->unicast();
+}
diff --git a/src/dev/multi_packet.hh b/src/dev/multi_packet.hh
new file mode 100644
index 000000000..4b270645d
--- /dev/null
+++ b/src/dev/multi_packet.hh
@@ -0,0 +1,130 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * Header packet class for multi gem5 runs.
+ *
+ * For a high level description about multi gem5 see comments in
+ * header file multi_iface.hh.
+ *
+ * The MultiHeaderPkt class defines the format of message headers
+ * sent among gem5 processes during a multi gem5 simulation. A header packet
+ * can either carry the description of data packet (i.e. a simulated Ethernet
+ * packet) or a synchronisation related control command. In case of
+ * data packet description, the corresponding data packet always follows
+ * the header packet back-to-back.
+ */
+#ifndef __DEV_MULTI_PACKET_HH__
+#define __DEV_MULTI_PACKET_HH__
+
+#include <cstring>
+
+#include "base/types.hh"
+
+class MultiHeaderPkt
+{
+  private:
+    MultiHeaderPkt() {}
+    ~MultiHeaderPkt() {}
+
+  public:
+    /**
+     * Simply type to help with calculating space requirements for
+     * the corresponding header field.
+     */
+    typedef uint8_t AddressType[6];
+
+    /**
+     *  The msg type defines what informarion a multi header packet carries.
+     */
+    enum class MsgType
+    {
+        dataDescriptor,
+        cmdPeriodicSyncReq,
+        cmdPeriodicSyncAck,
+        cmdCkptSyncReq,
+        cmdCkptSyncAck,
+        cmdAtomicSyncReq,
+        cmdAtomicSyncAck,
+        unknown
+    };
+
+    struct Header
+    {
+        /**
+         * The msg type field is valid for all header packets. In case of
+         * a synchronisation control command this is the only valid field.
+         */
+        MsgType msgType;
+        Tick sendTick;
+        Tick sendDelay;
+        /**
+         * Actual length of the simulated Ethernet packet.
+         */
+        unsigned dataPacketLength;
+        /**
+         * Source MAC address.
+         */
+        AddressType srcAddress;
+        /**
+         * Destination MAC address.
+         */
+        AddressType dstAddress;
+    };
+
+    static unsigned maxAddressLength();
+
+    /**
+     * Static functions for manipulating and comparing MAC addresses.
+     */
+    static void clearAddress(AddressType &addr);
+    static bool isAddressEqual(const AddressType &addr1,
+                               const AddressType &addr2);
+    static bool isAddressLess(const AddressType &addr1,
+                              const AddressType &addr2);
+
+    static void copyAddress(AddressType &dest,
+                            const AddressType &src);
+
+    static bool isUnicastAddress(const AddressType &addr);
+    static bool isMulticastAddress(const AddressType &addr);
+    static bool isBroadcastAddress(const AddressType &addr);
+};
+
+#endif
diff --git a/src/dev/tcp_iface.cc b/src/dev/tcp_iface.cc
new file mode 100644
index 000000000..f18486fa3
--- /dev/null
+++ b/src/dev/tcp_iface.cc
@@ -0,0 +1,158 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * TCP stream socket based interface class implementation for multi gem5 runs.
+ */
+
+#include "dev/tcp_iface.hh"
+
+#include <arpa/inet.h>
+#include <netdb.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <cerrno>
+#include <cstring>
+
+#include "base/types.hh"
+#include "debug/MultiEthernet.hh"
+
+// MSG_NOSIGNAL does not exists on OS X
+#if defined(__APPLE__) || defined(__MACH__)
+#ifndef MSG_NOSIGNAL
+#define MSG_NOSIGNAL SO_NOSIGPIPE
+#endif
+#endif
+
+using namespace std;
+
+vector<int> TCPIface::sockRegistry;
+
+TCPIface::TCPIface(string server_name, unsigned server_port,
+                   unsigned multi_rank, Tick sync_start, Tick sync_repeat,
+                   EventManager *em) :
+    MultiIface(multi_rank, sync_start, sync_repeat, em)
+{
+    struct addrinfo addr_hint, *addr_results;
+    int ret;
+
+    string port_str = to_string(server_port);
+
+    sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
+    panic_if(sock < 0, "socket() failed: %s", strerror(errno));
+
+    bzero(&addr_hint, sizeof(addr_hint));
+    addr_hint.ai_family = AF_INET;
+    addr_hint.ai_socktype = SOCK_STREAM;
+    addr_hint.ai_protocol = IPPROTO_TCP;
+
+    ret = getaddrinfo(server_name.c_str(), port_str.c_str(),
+                      &addr_hint, &addr_results);
+    panic_if(ret < 0, "getaddrinf() failed: %s", strerror(errno));
+
+    DPRINTF(MultiEthernet, "Connecting to %s:%u\n",
+            server_name.c_str(), port_str.c_str());
+
+    ret = ::connect(sock, (struct sockaddr *)(addr_results->ai_addr),
+                    addr_results->ai_addrlen);
+    panic_if(ret < 0, "connect() failed: %s", strerror(errno));
+
+    freeaddrinfo(addr_results);
+    // add our socket to the static registry
+    sockRegistry.push_back(sock);
+    // let the server know who we are
+    sendTCP(sock, &multi_rank, sizeof(multi_rank));
+}
+
+TCPIface::~TCPIface()
+{
+    int M5_VAR_USED ret;
+
+    ret = close(sock);
+    assert(ret == 0);
+}
+
+void
+TCPIface::sendTCP(int sock, void *buf, unsigned length)
+{
+    ssize_t ret;
+
+    ret = ::send(sock, buf, length, MSG_NOSIGNAL);
+    panic_if(ret < 0, "send() failed: %s", strerror(errno));
+    panic_if(ret != length, "send() failed");
+}
+
+bool
+TCPIface::recvTCP(int sock, void *buf, unsigned length)
+{
+    ssize_t ret;
+
+    ret = ::recv(sock, buf, length,  MSG_WAITALL );
+    if (ret < 0) {
+        if (errno == ECONNRESET || errno == EPIPE)
+            inform("recv(): %s", strerror(errno));
+        else if (ret < 0)
+            panic("recv() failed: %s", strerror(errno));
+    } else if (ret == 0) {
+        inform("recv(): Connection closed");
+    } else if (ret != length)
+        panic("recv() failed");
+
+    return (ret == length);
+}
+
+void
+TCPIface::syncRaw(MultiHeaderPkt::MsgType sync_req, Tick sync_tick)
+{
+    /*
+     * Barrier is simply implemented by point-to-point messages to the server
+     * for now. This method is called by only one TCPIface object.
+     * The server will send back an 'ack' message when it gets the
+     * sync request from all clients.
+     */
+    MultiHeaderPkt::Header header_pkt;
+    header_pkt.msgType = sync_req;
+    header_pkt.sendTick = sync_tick;
+
+    for (auto s : sockRegistry)
+        sendTCP(s, (void *)&header_pkt, sizeof(header_pkt));
+}
+
diff --git a/src/dev/tcp_iface.hh b/src/dev/tcp_iface.hh
new file mode 100644
index 000000000..d34d3d002
--- /dev/null
+++ b/src/dev/tcp_iface.hh
@@ -0,0 +1,134 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ * TCP stream socket based interface class for multi gem5 runs.
+ *
+ * For a high level description about multi gem5 see comments in
+ * header file multi_iface.hh.
+ *
+ * The TCP subclass of MultiIface uses a separate server process
+ * (see tcp_server.[hh,cc] under directory gem5/util/multi). Each gem5
+ * process connects to the server via a stream socket. The server process
+ * transfers messages and co-ordinates the synchronisation among the gem5
+ * peers.
+ */
+#ifndef __DEV_TCP_IFACE_HH__
+#define __DEV_TCP_IFACE_HH__
+
+
+#include <string>
+
+#include "dev/multi_iface.hh"
+
+class EventManager;
+
+class TCPIface : public MultiIface
+{
+  private:
+    /**
+     * The stream socket to connect to the server.
+     */
+    int sock;
+
+    /**
+     * Registry for all sockets to the server opened by this gem5 process.
+     */
+    static std::vector<int> sockRegistry;
+
+  private:
+
+    /**
+     * Send out a message through a TCP stream socket.
+     *
+     * @param sock TCP stream socket.
+     * @param buf Start address of the message.
+     * @param length Size of the message in bytes.
+     */
+    void
+    sendTCP(int sock, void *buf, unsigned length);
+
+    /**
+     * Receive the next incoming message through a TCP stream socket.
+     *
+     * @param sock TCP stream socket.
+     * @param buf Start address of buffer to store the message.
+     * @param length Exact size of the expected message in bytes.
+     */
+    bool recvTCP(int sock, void *buf, unsigned length);
+
+
+  protected:
+
+    virtual void
+    sendRaw(void *buf, unsigned length,
+            const MultiHeaderPkt::AddressType dest_addr=nullptr)
+        M5_ATTR_OVERRIDE
+    {
+        sendTCP(sock, buf, length);
+    }
+
+    virtual bool recvRaw(void *buf, unsigned length) M5_ATTR_OVERRIDE
+    {
+        return recvTCP(sock, buf, length);
+    }
+
+    virtual void syncRaw(MultiHeaderPkt::MsgType sync_req,
+                         Tick sync_tick) M5_ATTR_OVERRIDE;
+
+  public:
+    /**
+     * The ctor creates and connects the stream socket to the server.
+     * @param server_name The name (or IP address) of the host running the
+     * server process.
+     * @param server_port The port number the server listening for new
+     * connections.
+     * @param sync_start The tick for the first multi synchronisation.
+     * @param sync_repeat The frequency of multi synchronisation.
+     * @param em The EventManager object associated with the simulated
+     * Ethernet link.
+     */
+    TCPIface(std::string server_name, unsigned server_port,
+             unsigned multi_rank, Tick sync_start, Tick sync_repeat,
+             EventManager *em);
+
+    ~TCPIface() M5_ATTR_OVERRIDE;
+};
+
+#endif
diff --git a/util/multi/Makefile b/util/multi/Makefile
new file mode 100644
index 000000000..a58dd3307
--- /dev/null
+++ b/util/multi/Makefile
@@ -0,0 +1,63 @@
+#
+# Copyright (c) 2015 ARM Limited
+# All rights reserved
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+# Authors: Gabor Dozsa
+
+CXX= g++
+
+DEBUG= -DDEBUG
+
+M5_ARCH?= ARM
+M5_DIR?= ../..
+
+vpath % $(M5_DIR)/build/$(M5_ARCH)/dev
+
+INCDIRS= -I. -I$(M5_DIR)/build/$(M5_ARCH) -I$(M5_DIR)/ext
+CCFLAGS= -g -Wall -O3 $(DEBUG) -std=c++11 -MMD $(INCDIRS)
+
+default: tcp_server
+
+clean:
+	@rm -f tcp_server *.o *.d *~
+
+tcp_server: tcp_server.o multi_packet.o
+	$(CXX) $(LFLAGS) -o $@ $^
+
+%.o: %.cc
+	@echo '$(CXX) $(CCFLAGS) -c $(notdir $<) -o $@'
+	@$(CXX) $(CCFLAGS) -c $< -o $@
+
+-include *.d
diff --git a/util/multi/bootscript.rcS b/util/multi/bootscript.rcS
new file mode 100644
index 000000000..95736f4b7
--- /dev/null
+++ b/util/multi/bootscript.rcS
@@ -0,0 +1,122 @@
+#!/bin/bash
+
+
+#
+# Copyright (c) 2015 ARM Limited
+# All rights reserved
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+# Authors: Gabor Dozsa
+#
+#
+# This is an example boot script to use for muti gem5 runs. The important
+# task here is to extract the rank and size information from the kernel
+# boot args and use those to configure MAC/IP addresses and hostname.
+# Then we can kick off our (parallel) workload ...
+#
+# You are expected to costumize this scipt for your needs (e.g. change
+# the command at the end of the scipt to run your tests/workloads.
+
+source /root/.bashrc
+echo "bootscript.rcS is running"
+
+m='GEM5\_RANK=([0-9]+) GEM5\_SIZE=([0-9]+)'
+if [[ $(cat /proc/cmdline) =~ $m ]]
+then
+    MY_RANK=${BASH_REMATCH[1]}
+    MY_SIZE=${BASH_REMATCH[2]}
+else
+    echo "(E) GEM5_RANK/GEM5_SIZE was not defined in bootargs, exiting ..."
+    /sbin/m5 abort
+fi
+
+/bin/hostname node${MY_RANK}
+
+# Keep MAC address assignment simple for now ...
+(($MY_RANK>97)) && { echo "(E) Rank must be less than 98"; /sbin/m5 abort; }
+((MY_ADDR=MY_RANK+2))
+if (($MY_ADDR<10))
+then
+    MY_ADDR_PADDED=0${MY_ADDR}
+else
+    MY_ADDR_PADDED=${MY_ADDR}
+fi
+
+/sbin/ifconfig eth0 hw ether 00:90:00:00:00:${MY_ADDR_PADDED}
+/sbin/ifconfig eth0 192.168.0.${MY_ADDR} netmask 255.255.255.0 up
+
+/sbin/ifconfig -a
+
+# Prepare host lists for mpirun
+MY_MPI_HOSTS="192.168.0.2"
+for ((i=1; i<MY_SIZE; i++))
+do
+    MY_MPI_HOSTS+=",192.168.0.$((i+2))"
+done
+
+# Check that Ethernet links work, then take a checkpoint
+if  [ "$MY_RANK" == "0" ]
+then
+    OLDIFS=$IFS
+    IFS=","
+    for i in $MY_MPI_HOSTS
+    do
+        ping -c 1  $i || { echo "ping $i failed, exiting ..."; exit -1; }
+        ssh $i hostname || { echo "ssh $i failed, exiting ..."; exit -1; }
+    done
+    IFS=$OLDIFS
+    /sbin/m5 checkpoint
+fi
+
+# --------------------------------------------
+#  ------ Start your tests below ... ---------
+# --------------------------------------------
+
+if [ "$MY_RANK" == "0" ]
+then
+    echo "MPI test"
+    #mpirun -H 192.168.0.3,192.168.0.2 hostname
+    cd /benchmarks
+    mpirun -H $MY_MPI_HOSTS lulesh/lulesh2.0-mpi -s 5
+else
+    # This is to avoid other (rank!=0) gem5 processes exiting
+    # before the test (started by rank 0) completes. When rank 0 completes the
+    # test it will exit and that will trigger a notification to all the peer 
+    # gem5 peocesses to stop the simulation.
+    echo "sleep forever..."
+    while /bin/true
+    do
+        sleep 5
+    done
+fi
diff --git a/util/multi/gem5-multi.sh b/util/multi/gem5-multi.sh
new file mode 100755
index 000000000..4b4937c90
--- /dev/null
+++ b/util/multi/gem5-multi.sh
@@ -0,0 +1,275 @@
+#! /bin/bash
+
+#
+# Copyright (c) 2015 ARM Limited
+# All rights reserved
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are
+# met: redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer;
+# redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution;
+# neither the name of the copyright holders nor the names of its
+# contributors may be used to endorse or promote products derived from
+# this software without specific prior written permission.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#
+# Authors: Gabor Dozsa
+
+
+# This is a wrapper script to run a multi gem5 simulations.
+# See the usage_func() below for hints on how to use it. Also,
+# there are some examples in the util/multi directory (e.g.
+# see util/multi/test-2nodes-AArch64.sh)
+#
+#
+# Allocated hosts/cores are assumed to be listed in the LSB_MCPU_HOSTS
+# environment variable (which is what LSF does by default).
+# E.g. LSB_MCPU_HOSTS=\"hname1 2 hname2 4\" means we have altogether 6 slots
+# allocated to launch the gem5 processes, 2 of them are on host hname1
+# and 4 of them are on host hname2.
+# If LSB_MCPU_HOSTS environment variable is not defined then we launch all
+# processes on the localhost.
+#
+# Each gem5 process are passed in a unique rank ID [0..N-1] via the kernel
+# boot params. The total number of gem5 processes is also passed in.
+# These values can be used in the boot script to configure the MAC/IP
+# addresses - among other things (see util/multi/bootscript.rcS).
+#
+# Each gem5 process will create an m5out.$GEM5_RANK directory for
+# the usual output files. Furthermore, there will be a separate log file
+# for each ssh session (we use ssh to start gem5 processes) and one for
+# the server. These are called log.$GEM5_RANK and log.server.
+#
+
+
+# print help
+usage_func ()
+{
+    echo "Usage:$0 [-debug] [-n nnodes] [-s server] [-p port] gem5_exe gem5_args"
+    echo "     -debug   : debug mode (start gem5 in gdb)"
+    echo "     nnodes   : number of gem5 processes"
+    echo "     server   : message server executable"
+    echo "     port     : message server listen port"
+    echo "     gem5_exe : gem5 executable (full path required)"
+    echo "     gem5_args: usual gem5 arguments ( m5 options, config script options)"
+    echo "Note: if no LSF slots allocation is found all proceses are launched on the localhost."
+}
+
+
+# Process (optional) command line options
+
+while true
+do
+    case "x$1" in
+        x-n|x-nodes)
+            NNODES=$2
+            shift 2
+            ;;
+        x-s|x-server)
+            TCP_SERVER=$2
+            shift 2
+            ;;
+        x-p|x-port)
+            SERVER_PORT=$2
+            shift 2
+            ;;
+        x-debug)
+            GEM5_DEBUG="-debug"
+            shift 1
+            ;;
+        *)
+            break
+            ;;
+    esac
+done
+
+# The remaining command line args must be the usual gem5 command
+(($# < 2)) && { usage_func; exit -1; }
+GEM5_EXE=$1
+shift
+GEM5_ARGS="$*"
+
+# Default values to use (in case they are not defined as command line options)
+DEFAULT_TCP_SERVER=$(dirname $0)/../../util/multi/tcp_server
+DEFAULT_SERVER_PORT=2200
+
+[ -z "$TCP_SERVER" ]  && TCP_SERVER=$DEFAULT_TCP_SERVER
+[ -z "$SERVER_PORT" ] && SERVER_PORT=$DEFAULT_SERVER_PORT
+[ -z "$NNODES" ]      && NNODES=2
+
+
+#  Check if all the executables we need exist
+[ -x "$TCP_SERVER" ] || { echo "Executable ${TCP_SERVER} not found"; exit 1; }
+[ -x "$GEM5_EXE" ]   || { echo "Executable ${GEM5_EXE} not found"; exit 1; }
+
+
+declare -a SSH_PIDS
+declare -a HOSTS
+declare -a NCORES
+
+# Find out which cluster hosts/slots are allocated or
+# use localhost if there is no LSF allocation.
+# We assume that allocated slots are listed in the LSB_MCPU_HOSTS
+# environment variable in the form:
+# host1 nslots1 host2 nslots2 ...
+# (This is what LSF does by default.)
+NH=0
+[ "x$LSB_MCPU_HOSTS" != "x" ] || LSB_MCPU_HOSTS="localhost $NNODES"
+host=""
+for hc in $LSB_MCPU_HOSTS
+do
+    if [ "x$host" == "x" ]
+    then
+        host=$hc
+        HOSTS+=($hc)
+    else
+        NCORES+=($hc)
+        ((NH+=hc))
+        host=""
+    fi
+done
+((NNODES==NH)) || { echo "(E) Number of cluster slots ($NH) and gem5 instances ($N) differ"; exit -1; }
+#echo "hosts: ${HOSTS[@]}"
+#echo "hosts: ${NCORES[@]}"
+#echo ${#HOSTS[@]}
+
+
+# function to clean up and abort if something goes wrong
+abort_func ()
+{
+    echo
+    echo "KILLED $(date)"
+    # (try to) kill all gem5 processes on all hosts
+    bname=$(basename $GEM5_EXE)
+    killall -q $bname
+    for h in ${HOSTS[@]}
+    do
+        ssh $h killall -q $bname
+    done
+    sleep 3
+    # kill the message server and the watchdog
+    [ "x$SERVER_PID" != "x" ] && kill $SERVER_PID 2>/dev/null
+    [ "x$WATCHDOG_PID" != "x" ] && kill $WATCHDOG_PID 2>/dev/null
+    exit -1
+}
+
+
+# We need a watchdog to trigger full clean up if a gem5 process dies
+watchdog_func ()
+{
+    while true
+    do
+        sleep 30
+        ((NDEAD=0))
+        for p in ${SSH_PIDS[*]}
+        do
+            kill -0 $p 2>/dev/null || ((NDEAD+=1))
+        done
+        kill -0 $SERVER_PID || ((NDEAD+=1))
+        if ((NDEAD>0))
+        then
+            # we may be in the middle of an orderly termination,
+            # give it some time to complete before reporting abort
+            sleep 60
+            echo -n "(I) (some) gem5 process(es) exited"
+            abort_func
+        fi
+    done
+}
+
+# This function launches the gem5 processes. We use it only to allow launching
+# gem5 processes under gdb control (in the foreground) for debugging
+start_func ()
+{
+    local N=$1
+    local HOST=$2
+    local ENV_ARGS=$3
+    shift 3
+    if [ "x$GEM5_DEBUG" != "x" ]
+    then
+        gdb --args "$@"
+    else
+        ssh $HOST $ENV_ARGS "$@" &>log.$N & 
+    fi
+}
+
+
+# Trigger full clean up in case we are being killed by external signal
+trap 'abort_func' INT TERM
+
+# env args to be passed explicitly to gem5 processes started via ssh
+ENV_ARGS="LD_LIBRARY_PATH=$LD_LIBRARY_PATH M5_PATH=$M5_PATH"
+
+# launch the mesage server and check if it has started okay
+$TCP_SERVER $GEM5_DEBUG $NNODES $SERVER_PORT &>log.server &
+SERVER_PID=$!
+sleep 2
+kill -0 $SERVER_PID || { echo "Failed to start message server"; exit -1; }
+
+# Now launch all the gem5 processes with ssh.
+echo "START $(date)"
+n=0
+for ((i=0; i < ${#HOSTS[@]}; i++))
+do
+    h=${HOSTS[$i]}
+    for ((j=0; j < ${NCORES[i]}; j++))
+    do
+        echo "starting gem5 on $h ..."
+        start_func $n $h "$ENV_ARGS" $GEM5_EXE -d $(pwd)/m5out.$n $GEM5_ARGS \
+        --multi                                                              \
+        --multi-rank=$n                                                      \
+        --multi-server-name=${HOSTS[0]}                                      \
+        --multi-server-port=$SERVER_PORT                                     \
+        --testsys-toplevel-LinuxArmSystem.boot_osflags="\"GEM5_RANK=$n GEM5_SIZE=$NNODES\""
+        SSH_PIDS[$n]=$!
+        ((n+=1))
+    done
+done
+
+[ "x$GEM5_DEBUG" == "x" ] || {  kill $SERVER_PID; echo "DEBUG exit"; exit -1; }
+
+# start watchdog to trigger complete abort (after a grace period) if any
+# gem5 process dies
+watchdog_func &
+WATCHDOG_PID=$!
+
+# wait for exit statuses
+((NFAIL=0))
+for p in ${SSH_PIDS[*]}
+do
+    wait $p || ((NFAIL+=1))
+done
+wait $SERVER_PID || ((NFAIL+=1))
+
+# all done, let's terminate the watchdog
+kill $WATCHDOG_PID 2>/dev/null
+
+if ((NFAIL==0))
+then
+    echo "EXIT $(date)"
+else
+    echo "ABORT $(date)"
+fi
diff --git a/util/multi/tcp_server.cc b/util/multi/tcp_server.cc
new file mode 100644
index 000000000..1ec4303d3
--- /dev/null
+++ b/util/multi/tcp_server.cc
@@ -0,0 +1,463 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Copyright (c) 2008 The Regents of The University of Michigan
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+
+/* @file
+ *  Message server implementation using TCP stream sockets for parallel gem5
+ * runs.
+ */
+#include <arpa/inet.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <cstdio>
+#include <cstdlib>
+
+#include "tcp_server.hh"
+
+using namespace std;
+
+// Some basic macros for information and error reporting.
+#define PRINTF(...) fprintf(stderr, __VA_ARGS__)
+
+#ifdef DEBUG
+static bool debugSetup = true;
+static bool debugPeriodic = false;
+static bool debugSync = true;
+static bool debugPkt = false;
+#define DPRINTF(v,...) if (v) PRINTF(__VA_ARGS__)
+#else
+#define DPRINTF(v,...)
+#endif
+
+#define inform(...) do { PRINTF("info: ");      \
+        PRINTF(__VA_ARGS__); } while(0)
+
+#define panic(...)  do { PRINTF("panic: ");     \
+        PRINTF(__VA_ARGS__);                    \
+        PRINTF("\n[%s:%s], line %d\n",          \
+               __FUNCTION__, __FILE__, __LINE__); \
+        exit(-1); } while(0)
+
+TCPServer *TCPServer::instance = nullptr;
+
+TCPServer::Channel::Channel() : fd(-1), isAlive(false), state(SyncState::idle)
+{
+    MultiHeaderPkt::clearAddress(address);
+}
+
+unsigned
+TCPServer::Channel::recvRaw(void *buf, unsigned size) const
+{
+    ssize_t n;
+    // This is a blocking receive.
+    n = recv(fd, buf, size, MSG_WAITALL);
+
+    if (n < 0)
+        panic("read() failed:%s", strerror(errno));
+    else if (n > 0 && n < size)
+        // the recv() call should wait for the full message
+        panic("read() failed");
+
+    return n;
+}
+
+void
+TCPServer::Channel::sendRaw(const void *buf, unsigned size) const
+{
+    ssize_t n;
+    n = send(fd, buf, size, MSG_NOSIGNAL);
+    if (n < 0)
+        panic("write() failed:%s", strerror(errno));
+    else if (n != size)
+        panic("write() failed");
+}
+
+void TCPServer::Channel::updateAddress(const AddressType &new_address)
+{
+    // check if the known address has changed (e.g. the client reconfigured
+    // its Ethernet NIC)
+    if (MultiHeaderPkt::isAddressEqual(address, new_address))
+        return;
+
+    // So we have to update the address. Note that we always
+    // store the same address as key in the map but the ordering
+    // may change so we need to erase and re-insert it again.
+    auto info = TCPServer::instance->addressMap.find(&address);
+    if (info != TCPServer::instance->addressMap.end()) {
+        TCPServer::instance->addressMap.erase(info);
+    }
+
+    MultiHeaderPkt::copyAddress(address, new_address);
+    TCPServer::instance->addressMap[&address] = this;
+}
+
+void
+TCPServer::Channel::headerPktIn()
+{
+    ssize_t n;
+    Header hdr_pkt;
+
+    n = recvRaw(&hdr_pkt, sizeof(hdr_pkt));
+
+    if (n == 0) {
+        // EOF - nothing to do here, we will handle this as a POLLRDHUP event
+        // in the main loop.
+        return;
+    }
+
+    if (hdr_pkt.msgType == MsgType::dataDescriptor) {
+        updateAddress(hdr_pkt.srcAddress);
+        TCPServer::instance->xferData(hdr_pkt, *this);
+    } else {
+        processCmd(hdr_pkt.msgType, hdr_pkt.sendTick);
+    }
+}
+
+void TCPServer::Channel::processCmd(MsgType cmd, Tick send_tick)
+{
+    switch (cmd) {
+      case MsgType::cmdAtomicSyncReq:
+        DPRINTF(debugSync,"Atomic sync request (rank:%d)\n",rank);
+        assert(state == SyncState::idle);
+        state = SyncState::atomic;
+        TCPServer::instance->syncTryComplete(SyncState::atomic,
+                                             MsgType::cmdAtomicSyncAck);
+        break;
+      case MsgType::cmdPeriodicSyncReq:
+        DPRINTF(debugPeriodic,"PERIODIC sync request (at %ld)\n",send_tick);
+        // sanity check
+        if (TCPServer::instance->periodicSyncTick() == 0) {
+            TCPServer::instance->periodicSyncTick(send_tick);
+        } else if ( TCPServer::instance->periodicSyncTick() != send_tick) {
+            panic("Out of order periodic sync request - rank:%d "
+                  "(send_tick:%ld ongoing:%ld)", rank, send_tick,
+                  TCPServer::instance->periodicSyncTick());
+        }
+        switch (state) {
+          case SyncState::idle:
+            state = SyncState::periodic;
+            TCPServer::instance->syncTryComplete(SyncState::periodic,
+                                                 MsgType::cmdPeriodicSyncAck);
+            break;
+          case SyncState::asyncCkpt:
+            // An async ckpt request has already been sent to this client and
+            // that will interrupt this periodic sync. We can simply drop this
+            // message.
+            break;
+          default:
+            panic("Unexpected state for periodic sync request (rank:%d)",
+                rank);
+            break;
+        }
+        break;
+      case MsgType::cmdCkptSyncReq:
+        DPRINTF(debugSync, "CKPT sync request (rank:%d)\n",rank);
+        switch (state) {
+          case SyncState::idle:
+            TCPServer::instance->ckptPropagate(*this);
+            // we fall through here to complete #clients==1 case
+          case SyncState::asyncCkpt:
+            state = SyncState::ckpt;
+            TCPServer::instance->syncTryComplete(SyncState::ckpt,
+                                                 MsgType::cmdCkptSyncAck);
+            break;
+          default:
+            panic("Unexpected state for ckpt sync request (rank:%d)", rank);
+            break;
+        }
+        break;
+      default:
+        panic("Unexpected header packet (rank:%d)",rank);
+        break;
+    }
+}
+
+TCPServer::TCPServer(unsigned clients_num,
+                     unsigned listen_port,
+                     int timeout_in_sec)
+{
+    assert(instance == nullptr);
+    construct(clients_num, listen_port, timeout_in_sec);
+    instance = this;
+}
+
+TCPServer::~TCPServer()
+{
+    for (auto &c : clientsPollFd)
+        close(c.fd);
+}
+
+void
+TCPServer::construct(unsigned clients_num, unsigned port, int timeout_in_sec)
+{
+    int listen_sock, new_sock, ret;
+    unsigned client_len;
+    struct sockaddr_in server_addr, client_addr;
+    struct pollfd new_pollfd;
+    Channel new_channel;
+
+    DPRINTF(debugSetup, "Start listening on port %u ...\n", port);
+
+    listen_sock = socket(AF_INET, SOCK_STREAM, 0);
+    if (listen_sock < 0)
+        panic("socket() failed:%s", strerror(errno));
+
+    bzero(&server_addr, sizeof(server_addr));
+    server_addr.sin_family = AF_INET;
+    server_addr.sin_addr.s_addr = INADDR_ANY;
+    server_addr.sin_port = htons(port);
+    if (bind(listen_sock, (struct sockaddr *) &server_addr,
+             sizeof(server_addr)) < 0)
+        panic("bind() failed:%s", strerror(errno));
+    listen(listen_sock, 10);
+
+    clientsPollFd.reserve(clients_num);
+    clientsChannel.reserve(clients_num);
+
+    new_pollfd.events = POLLIN | POLLRDHUP;
+    new_pollfd.revents = 0;
+    while (clientsPollFd.size() < clients_num) {
+        new_pollfd.fd = listen_sock;
+        ret = poll(&new_pollfd, 1, timeout_in_sec*1000);
+        if (ret == 0)
+            panic("Timeout while waiting for clients to connect");
+        assert(ret == 1 && new_pollfd.revents == POLLIN);
+        client_len = sizeof(client_addr);
+        new_sock = accept(listen_sock,
+                          (struct sockaddr *) &client_addr,
+                          &client_len);
+        if (new_sock < 0)
+            panic("accept() failed:%s", strerror(errno));
+        new_pollfd.fd = new_sock;
+        new_pollfd.revents = 0;
+        clientsPollFd.push_back(new_pollfd);
+        new_channel.fd = new_sock;
+        new_channel.isAlive = true;
+        new_channel.recvRaw(&new_channel.rank, sizeof(new_channel.rank));
+        clientsChannel.push_back(new_channel);
+
+        DPRINTF(debugSetup, "New client connection addr:%u port:%hu rank:%d\n",
+                client_addr.sin_addr.s_addr, client_addr.sin_port,
+                new_channel.rank);
+    }
+    ret = close(listen_sock);
+    assert(ret == 0);
+
+    DPRINTF(debugSetup, "Setup complete\n");
+}
+
+void
+TCPServer::run()
+{
+    int nfd;
+    unsigned num_active_clients = clientsPollFd.size();
+
+    DPRINTF(debugSetup, "Entering run() loop\n");
+    while (num_active_clients ==  clientsPollFd.size()) {
+        nfd = poll(&clientsPollFd[0], clientsPollFd.size(), -1);
+        if (nfd == -1)
+            panic("poll() failed:%s", strerror(errno));
+
+        for (unsigned i = 0, n = 0;
+             i < clientsPollFd.size() && (signed)n < nfd;
+             i++) {
+            struct pollfd &pfd = clientsPollFd[i];
+            if (pfd.revents) {
+                if (pfd.revents & POLLERR)
+                    panic("poll() returned POLLERR");
+                if (pfd.revents & POLLIN) {
+                    clientsChannel[i].headerPktIn();
+                }
+                if (pfd.revents & POLLRDHUP) {
+                    // One gem5 process exited or aborted. Either way, we
+                    // assume the full simulation should stop now (either
+                    // because m5 exit was called or a serious error
+                    // occurred.) So we quit the run loop here and close all
+                    // sockets to notify the remaining peer gem5 processes.
+                    pfd.events = 0;
+                    clientsChannel[i].isAlive = false;
+                    num_active_clients--;
+                    DPRINTF(debugSetup, "POLLRDHUP event");
+                }
+                n++;
+                if ((signed)n == nfd)
+                    break;
+            }
+        }
+    }
+    DPRINTF(debugSetup, "Exiting run() loop\n");
+}
+
+void
+TCPServer::xferData(const Header &hdr_pkt, const Channel &src)
+{
+    unsigned n;
+    assert(hdr_pkt.dataPacketLength <= sizeof(packetBuffer));
+    n = src.recvRaw(packetBuffer, hdr_pkt.dataPacketLength);
+
+    if (n == 0)
+        panic("recvRaw() failed");
+    DPRINTF(debugPkt, "Incoming data packet (from rank %d) "
+            "src:0x%02x%02x%02x%02x%02x%02x "
+            "dst:0x%02x%02x%02x%02x%02x%02x\n",
+            src.rank,
+            hdr_pkt.srcAddress[0],
+            hdr_pkt.srcAddress[1],
+            hdr_pkt.srcAddress[2],
+            hdr_pkt.srcAddress[3],
+            hdr_pkt.srcAddress[4],
+            hdr_pkt.srcAddress[5],
+            hdr_pkt.dstAddress[0],
+            hdr_pkt.dstAddress[1],
+            hdr_pkt.dstAddress[2],
+            hdr_pkt.dstAddress[3],
+            hdr_pkt.dstAddress[4],
+            hdr_pkt.dstAddress[5]);
+    // Now try to figure out the destination client(s).
+    auto dst_info = addressMap.find(&hdr_pkt.dstAddress);
+
+    // First handle the multicast/broadcast or unknonw destination case. These
+    // all trigger a broadcast of the packet to all clients.
+    if (MultiHeaderPkt::isUnicastAddress(hdr_pkt.dstAddress) == false ||
+        dst_info == addressMap.end()) {
+        unsigned n = 0;
+        for (auto const &c: clientsChannel) {
+            if (c.isAlive && &c!=&src) {
+                c.sendRaw(&hdr_pkt, sizeof(hdr_pkt));
+                c.sendRaw(packetBuffer, hdr_pkt.dataPacketLength);
+                n++;
+            }
+        }
+        if (n == 0) {
+            inform("Broadcast/multicast packet dropped\n");
+        }
+    } else {
+        // It is a unicast address with a known destination
+        Channel *dst = dst_info->second;
+        if (dst->isAlive) {
+            dst->sendRaw(&hdr_pkt, sizeof(hdr_pkt));
+            dst->sendRaw(packetBuffer, hdr_pkt.dataPacketLength);
+            DPRINTF(debugPkt, "Unicast packet sent (to rank %d)\n",dst->rank);
+        } else {
+            inform("Unicast packet dropped (destination exited)\n");
+        }
+    }
+}
+
+void
+TCPServer::syncTryComplete(SyncState st, MsgType ack)
+{
+    // Check if the barrieris complete. If so then notify all the clients.
+    for (auto &c : clientsChannel) {
+        if (c.isAlive && (c.state != st)) {
+            // sync not complete yet, stop here
+            return;
+        }
+    }
+    // Sync complete, send out the acks
+    MultiHeaderPkt::Header hdr_pkt;
+    hdr_pkt.msgType = ack;
+    for (auto &c : clientsChannel) {
+        if (c.isAlive) {
+            c.sendRaw(&hdr_pkt, sizeof(hdr_pkt));
+            c.state = SyncState::idle;
+        }
+    }
+    // Reset periodic send tick
+    _periodicSyncTick = 0;
+    DPRINTF(st == SyncState::periodic ? debugPeriodic : debugSync,
+            "Sync COMPLETE\n");
+}
+
+void
+TCPServer::ckptPropagate(Channel &ch)
+{
+    // Channel ch got a ckpt request that needs to be propagated to the other
+    // clients
+    MultiHeaderPkt::Header hdr_pkt;
+    hdr_pkt.msgType = MsgType::cmdCkptSyncReq;
+    for (auto &c : clientsChannel) {
+        if (c.isAlive && (&c != &ch)) {
+            switch (c.state) {
+              case SyncState::idle:
+              case SyncState::periodic:
+                c.sendRaw(&hdr_pkt, sizeof(hdr_pkt));
+                c.state = SyncState::asyncCkpt;
+                break;
+              default:
+                panic("Unexpected state for ckpt sync request propagation "
+                      "(rank:%d)\n",c.rank);
+                break;
+            }
+        }
+    }
+}
+
+int main(int argc, char *argv[])
+{
+    TCPServer *server;
+    int clients_num = -1, listen_port = -1;
+    int first_arg = 1, timeout_in_sec = 60;
+
+    if (argc > 1 && string(argv[1]).compare("-debug") == 0) {
+        timeout_in_sec = -1;
+        first_arg++;
+        argc--;
+    }
+
+    if (argc != 3)
+        panic("We need two command line args (number of clients and tcp listen"
+              " port");
+
+    clients_num = atoi(argv[first_arg]);
+    listen_port = atoi(argv[first_arg + 1]);
+
+    server = new TCPServer(clients_num, listen_port, timeout_in_sec);
+
+    server->run();
+
+    delete server;
+
+    return 0;
+}
diff --git a/util/multi/tcp_server.hh b/util/multi/tcp_server.hh
new file mode 100644
index 000000000..c5f2a9ce8
--- /dev/null
+++ b/util/multi/tcp_server.hh
@@ -0,0 +1,254 @@
+/*
+ * Copyright (c) 2015 ARM Limited
+ * All rights reserved
+ *
+ * The license below extends only to copyright in the software and shall
+ * not be construed as granting a license to any other intellectual
+ * property including but not limited to intellectual property relating
+ * to a hardware implementation of the functionality of the software
+ * licensed hereunder.  You may use the software subject to the license
+ * terms below provided that you ensure that this notice is replicated
+ * unmodified and in its entirety in all distributions of the software,
+ * modified or unmodified, in source code or in binary form.
+ *
+ * Copyright (c) 2008 The Regents of The University of Michigan
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * Authors: Gabor Dozsa
+ */
+
+/* @file
+ *  Message server using TCP stream sockets for parallel gem5 runs.
+ *
+ * For a high level description about multi gem5 see comments in
+ * header files src/dev/multi_iface.hh and src/dev/tcp_iface.hh.
+ *
+ * This file implements the central message server process for multi gem5.
+ * The server is responsible the following tasks.
+ * 1. Establishing a TCP socket connection for each gem5 process (clients).
+ *
+ * 2. Process data messages coming in from clients. The server checks
+ * the MAC addresses in the header message and transfers the message
+ * to the target(s) client(s).
+ *
+ * 3. Processing synchronisation related control messages. Synchronisation
+ * is performed as follows. The server waits for a 'barrier enter' message
+ * from all the clients. When the last such control message arrives, the
+ * server sends out a 'barrier leave' control message to all the clients.
+ *
+ * 4. Triggers complete termination in case a client exits. A client may
+ * exit either by calling 'm5 exit' pseudo instruction or due to a fatal
+ * error. In either case, we assume that the entire multi simulation needs to
+ * terminate. The server triggers full termination by tearing down the
+ * open TCP sockets.
+ *
+ * The TCPServer class is instantiated as a singleton object.
+ *
+ * The server can be built independently from the rest of gem5 (and it is
+ * architecture agnostic). See the Makefile in the same directory.
+ *
+ */
+
+#include <poll.h>
+
+#include <map>
+#include <vector>
+
+#include "dev/etherpkt.hh"
+#include "dev/multi_packet.hh"
+
+/**
+ * The maximum length of an Ethernet packet (allowing Jumbo frames).
+ */
+#define MAX_ETH_PACKET_LENGTH 9014
+
+class TCPServer
+{
+  public:
+    typedef MultiHeaderPkt::AddressType AddressType;
+    typedef MultiHeaderPkt::Header Header;
+    typedef MultiHeaderPkt::MsgType MsgType;
+
+  private:
+
+    enum
+    class SyncState { periodic, ckpt, asyncCkpt, atomic, idle };
+    /**
+     * The Channel class encapsulates all the information about a client
+     * and its current status.
+     */
+    class Channel
+    {
+      private:
+        /**
+         * The MAC address of the client.
+         */
+        AddressType address;
+
+        /**
+         * Update the client MAC address. It is called every time a new data
+         * packet is to come in.
+         */
+        void updateAddress(const AddressType &new_addr);
+        /**
+         * Process an incoming command message.
+         */
+        void processCmd(MultiHeaderPkt::MsgType cmd, Tick send_tick);
+
+
+      public:
+        /**
+         * TCP stream socket.
+         */
+        int fd;
+        /**
+         * Is client connected?
+         */
+        bool isAlive;
+        /**
+         * Current state of the channel wrt. multi synchronisation.
+         */
+        SyncState state;
+        /**
+         * Multi rank of the client
+         */
+        unsigned rank;
+
+      public:
+        Channel();
+        ~Channel () {}
+
+
+        /**
+         * Receive and process the next incoming header packet.
+         */
+        void headerPktIn();
+        /**
+         * Send raw data to the connected client.
+         *
+         * @param data The data to send.
+         * @param size Size of the data (in bytes).
+         */
+        void sendRaw(const void *data, unsigned size) const;
+        /**
+         * Receive raw data from the connected client.
+         *
+         * @param buf The buffer to store the incoming data into.
+         * @param size Size of data to receive (in bytes).
+         * @return In case of success, it returns size. Zero is returned
+         * if the socket is already closed by the client.
+         */
+        unsigned recvRaw(void *buf, unsigned size) const;
+    };
+
+    /**
+     * The array of socket descriptors needed by the poll() system call.
+     */
+    std::vector<struct pollfd> clientsPollFd;
+    /**
+     * Array holding all clients info.
+     */
+    std::vector<Channel> clientsChannel;
+
+
+    /**
+     * We use a map to select the target client based on the destination
+     * MAC address.
+     */
+    struct AddressCompare
+    {
+        bool operator()(const AddressType *a1, const AddressType *a2)
+        {
+            return MultiHeaderPkt::isAddressLess(*a1, *a2);
+        }
+    };
+    std::map<const AddressType *, Channel *, AddressCompare> addressMap;
+
+    /**
+     * As we dealt with only one message at a time, we can allocate and re-use
+     * a single packet buffer (to hold any incoming data packet).
+     */
+    uint8_t packetBuffer[MAX_ETH_PACKET_LENGTH];
+    /**
+     * Send tick of the current periodic sync. It is used for sanity check.
+     */
+    Tick _periodicSyncTick;
+    /**
+     * The singleton server object.
+     */
+    static TCPServer *instance;
+
+    /**
+     * Set up the socket connections to all the clients.
+     *
+     * @param listen_port The port we are listening on for new client
+     * connection requests.
+     * @param nclients The number of clients to connect to.
+     * @param timeout Timeout in sec to complete the setup phase
+     * (i.e. all gem5 establish socket connections)
+     */
+    void construct(unsigned listen_port, unsigned nclients, int timeout);
+    /**
+     * Transfer the header and the follow up data packet to the target(s)
+     * clients.
+     *
+     * @param hdr The header message structure.
+     * @param ch The source channel for the message.
+     */
+    void xferData(const Header &hdr, const Channel &ch);
+    /**
+     * Check if the current round of a synchronisation is completed and notify
+     * the clients if it is so.
+     *
+     * @param st The state all channels should have if sync is complete.
+     * @param ack The type of ack message to send out if the sync is compete.
+     */
+    void syncTryComplete(SyncState st, MultiHeaderPkt::MsgType ack);
+    /**
+     * Broadcast a request for checkpoint sync.
+     *
+     * @param ch The source channel of the checkpoint sync request.
+     */
+    void ckptPropagate(Channel &ch);
+    /**
+     * Setter for current periodic send tick.
+     */
+    void periodicSyncTick(Tick t) { _periodicSyncTick = t; }
+    /**
+     * Getter for current periodic send tick.
+     */
+    Tick periodicSyncTick() { return _periodicSyncTick; }
+
+  public:
+
+    TCPServer(unsigned clients_num, unsigned listen_port, int timeout_in_sec);
+    ~TCPServer();
+
+    /**
+     * The main server loop that waits for and processes incoming messages.
+     */
+    void run();
+};