mem: Simple Snoop Filter

This is a first cut at a simple snoop filter that tracks presence of lines in
the caches "above" it. The snoop filter can be applied at any given cache
hierarchy and will then handle the caches above it appropriately; there is no
need to use this only in the last-level bus.

This design currently has some limitations: missing stats, no notion of clean
evictions (these will not update the underlying snoop filter, because they are
not sent from the evicting cache down), no notion of capacity for the snoop
filter and thus no need for invalidations caused by capacity pressure in the
snoop filter. These are planned to be added on top with future change sets.

1 files changed, 274 insertions, 0 deletions

diff --git a/src/mem/snoop_filter.cc b/src/mem/snoop_filter.cc
new file mode 100755
index 000000000..1acfbef6c
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+++ b/src/mem/snoop_filter.cc
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+/*
+ * Copyright (c) 2013 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: Stephan Diestelhorst <stephan.diestelhorst@arm.com>
+ */
+
+/**
+ * @file
+ * Definition of a snoop filter.
+ */
+
+#include "base/misc.hh"
+#include "base/trace.hh"
+#include "debug/SnoopFilter.hh"
+#include "mem/snoop_filter.hh"
+#include "sim/system.hh"
+
+std::pair<SnoopFilter::SnoopList, Cycles>
+SnoopFilter::lookupRequest(const Packet* cpkt, const SlavePort& slave_port)
+{
+    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
+            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopMask req_port = portToMask(slave_port);
+    SnoopItem& sf_item  = cachedLocations[line_addr];
+
+    DPRINTF(SnoopFilter, "%s:   SF value %x.%x\n",
+            __func__, sf_item.requested, sf_item.holder);
+
+    if (cpkt->needsResponse()) {
+        if (!cpkt->memInhibitAsserted()) {
+            // Max one request per address per port
+            panic_if(sf_item.requested & req_port, "double request :( "\
+                     "SF value %x.%x\n", sf_item.requested, sf_item.holder);
+
+            // Mark in-flight requests to distinguish later on
+            sf_item.requested |= req_port;
+        } else {
+            // NOTE: The memInhibit might have been asserted by a cache closer
+            // to the CPU, already -> the response will not be seen by this
+            // filter -> we do not need to keep the in-flight request, but make
+            // sure that we know that that cluster has a copy
+            panic_if(!(sf_item.holder & req_port), "Need to hold the value!");
+            DPRINTF(SnoopFilter, "%s:   not marking request. SF value %x.%x\n",
+                    __func__,  sf_item.requested, sf_item.holder);
+        }
+        DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
+                __func__,  sf_item.requested, sf_item.holder);
+    }
+    SnoopMask interested = (sf_item.holder | sf_item.requested) & ~req_port;
+    return snoopSelected(maskToPortList(interested), lookupLatency);
+}
+
+void
+SnoopFilter::updateRequest(const Packet* cpkt, const SlavePort& slave_port,
+                           bool will_retry)
+{
+    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
+            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopMask req_port = portToMask(slave_port);
+    SnoopItem& sf_item  = cachedLocations[line_addr];
+
+    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x retry: %i\n",
+            __func__, sf_item.requested, sf_item.holder, will_retry);
+
+    if (will_retry) {
+        // Unmark a request that will come again.
+        sf_item.requested &= ~req_port;
+        return;
+    }
+
+    // will_retry == false
+    if (!cpkt->needsResponse()) {
+        // Packets that will not evoke a response but still need updates of the
+        // snoop filter; WRITEBACKs for now only
+        if (cpkt->cmd == MemCmd::Writeback) {
+            // make sure that the sender actually had the line
+            panic_if(sf_item.requested & req_port, "double request :( "\
+                     "SF value %x.%x\n", sf_item.requested, sf_item.holder);
+            panic_if(!(sf_item.holder & req_port), "requester %x is not a "\
+                     "holder :( SF value %x.%x\n", req_port,
+                     sf_item.requested, sf_item.holder);
+            // Writebacks -> the sender does not have the line anymore
+            sf_item.holder &= ~req_port;
+        } else {
+            assert(0 == "Handle non-writeback, here");
+        }
+        DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
+                __func__,  sf_item.requested, sf_item.holder);
+    }
+}
+
+std::pair<SnoopFilter::SnoopList, Cycles>
+SnoopFilter::lookupSnoop(const Packet* cpkt)
+{
+    DPRINTF(SnoopFilter, "%s: packet addr 0x%x cmd %s\n",
+            __func__, cpkt->getAddr(), cpkt->cmdString());
+
+    assert(cpkt->isRequest());
+
+    // Broadcast / filter upward snoops
+    const bool filter_upward = true;  // @todo: Make configurable
+
+    if (!filter_upward)
+        return snoopAll(lookupLatency);
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopItem& sf_item = cachedLocations[line_addr];
+
+    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
+            __func__, sf_item.requested, sf_item.holder);
+
+    SnoopMask interested = (sf_item.holder | sf_item.requested);
+    assert(cpkt->isInvalidate() == cpkt->needsExclusive());
+    if (cpkt->isInvalidate() && !sf_item.requested) {
+        // Early clear of the holder, if no other request is currently going on
+        // @todo: This should possibly be updated even though we do not filter
+        // upward snoops
+        sf_item.holder = 0;
+    }
+
+    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x interest: %x \n",
+            __func__, sf_item.requested, sf_item.holder, interested);
+
+    return snoopSelected(maskToPortList(interested), lookupLatency);
+}
+
+void
+SnoopFilter::updateSnoopResponse(const Packet* cpkt,
+                                 const SlavePort& rsp_port,
+                                 const SlavePort& req_port)
+{
+    DPRINTF(SnoopFilter, "%s: packet rsp %s req %s addr 0x%x cmd %s\n",
+            __func__, rsp_port.name(), req_port.name(), cpkt->getAddr(),
+            cpkt->cmdString());
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopMask rsp_mask = portToMask(rsp_port);
+    SnoopMask req_mask = portToMask(req_port);
+    SnoopItem& sf_item = cachedLocations[line_addr];
+
+    assert(cpkt->isResponse());
+    assert(cpkt->memInhibitAsserted());
+
+    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
+            __func__,  sf_item.requested, sf_item.holder);
+
+    // The source should have the line
+    panic_if(!(sf_item.holder & rsp_mask), "SF value %x.%x does not have "\
+             "the line\n", sf_item.requested, sf_item.holder);
+
+    // The destination should have had a request in
+    panic_if(!(sf_item.requested & req_mask), "SF value %x.%x missing "\
+             "the original request\n",  sf_item.requested, sf_item.holder);
+
+    // Update the residency of the cache line.
+    if (cpkt->needsExclusive() || !cpkt->sharedAsserted()) {
+        DPRINTF(SnoopFilter, "%s:  dropping %x because needs: %i shared: %i "\
+                "SF val: %x.%x\n", __func__,  rsp_mask,
+                cpkt->needsExclusive(), cpkt->sharedAsserted(),
+                sf_item.requested, sf_item.holder);
+
+        sf_item.holder &= ~rsp_mask;
+        // The snoop filter does not see any ACKs from non-responding sharers
+        // that have been invalidated :(  So below assert would be nice, but..
+        //assert(sf_item.holder == 0);
+        sf_item.holder = 0;
+    }
+    assert(cpkt->cmd != MemCmd::Writeback);
+    sf_item.holder |=  req_mask;
+    sf_item.requested &= ~req_mask;
+    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
+            __func__, sf_item.requested, sf_item.holder);
+}
+
+void
+SnoopFilter::updateSnoopForward(const Packet* cpkt,
+        const SlavePort& rsp_port, const MasterPort& req_port)
+{
+    DPRINTF(SnoopFilter, "%s: packet rsp %s req %s addr 0x%x cmd %s\n",
+            __func__, rsp_port.name(), req_port.name(), cpkt->getAddr(),
+            cpkt->cmdString());
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopItem& sf_item = cachedLocations[line_addr];
+    SnoopMask rsp_mask M5_VAR_USED = portToMask(rsp_port);
+
+    assert(cpkt->isResponse());
+    assert(cpkt->memInhibitAsserted());
+
+    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
+            __func__,  sf_item.requested, sf_item.holder);
+
+    // Remote (to this snoop filter) snoops update the filter already when they
+    // arrive from below, because we may not see any response.
+    if (cpkt->needsExclusive()) {
+        // If the request to this snoop response hit an in-flight transaction,
+        // the holder was not reset -> no assertion & do that here, now!
+        //assert(sf_item.holder == 0);
+        sf_item.holder = 0;
+    }
+    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
+            __func__, sf_item.requested, sf_item.holder);
+}
+
+void
+SnoopFilter::updateResponse(const Packet* cpkt, const SlavePort& slave_port)
+{
+    DPRINTF(SnoopFilter, "%s: packet src %s addr 0x%x cmd %s\n",
+            __func__, slave_port.name(), cpkt->getAddr(), cpkt->cmdString());
+
+    Addr line_addr = cpkt->getAddr() & ~(linesize - 1);
+    SnoopMask slave_mask = portToMask(slave_port);
+    SnoopItem& sf_item = cachedLocations[line_addr];
+
+    assert(cpkt->isResponse());
+
+    DPRINTF(SnoopFilter, "%s:   old SF value %x.%x\n",
+            __func__,  sf_item.requested, sf_item.holder);
+
+    // Make sure we have seen the actual request, too
+    panic_if(!(sf_item.requested & slave_mask), "SF value %x.%x missing "\
+             "request bit\n", sf_item.requested, sf_item.holder);
+
+    // Update the residency of the cache line.
+    if (cpkt->needsExclusive() || !cpkt->sharedAsserted())
+        sf_item.holder = 0;
+    sf_item.holder |=  slave_mask;
+    sf_item.requested &= ~slave_mask;
+    DPRINTF(SnoopFilter, "%s:   new SF value %x.%x\n",
+            __func__, sf_item.requested, sf_item.holder);
+}
+
+SnoopFilter *
+SnoopFilterParams::create()
+{
+    return new SnoopFilter(this);
+}