mem: Make the XBar responsible for tracking response routing

This patch removes the need for a source and destination field in the
packet by shifting the onus of the tracking to the crossbar, much like
a real implementation. This change in behaviour also means we no
longer need a SenderState to remember the source/dest when ever we
have multiple crossbars in the system. Thus, the stack that was
created by the SenderState is not needed, and each crossbar locally
tracks the response routing.

The fields in the packet are still left behind as the RubyPort (which
also acts as a crossbar) does routing based on them. In the succeeding
patches the uses of the src and dest field will be removed. Combined,
these patches improve the simulation performance by roughly 2%.

1 files changed, 78 insertions, 64 deletions

diff --git a/src/mem/coherent_xbar.cc b/src/mem/coherent_xbar.cc
index 6ad7b1785..02580702b 100644
--- a/src/mem/coherent_xbar.cc
+++ b/src/mem/coherent_xbar.cc
@@ -142,6 +142,10 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
 
     // remember if the packet is an express snoop
     bool is_express_snoop = pkt->isExpressSnoop();
+    bool is_inhibited = pkt->memInhibitAsserted();
+    // for normal requests, going downstream, the express snoop flag
+    // and the inhibited flag should always be the same
+    assert(is_express_snoop == is_inhibited);
 
     // determine the destination based on the address
     PortID master_port_id = findPort(pkt->getAddr());
@@ -163,9 +167,6 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
     unsigned int pkt_size = pkt->hasData() ? pkt->getSize() : 0;
     unsigned int pkt_cmd = pkt->cmdToIndex();
 
-    // set the source port for routing of the response
-    pkt->setSrc(slave_port_id);
-
     calcPacketTiming(pkt);
     Tick packetFinishTime = pkt->lastWordDelay + curTick();
 
@@ -187,21 +188,10 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
         }
     }
 
-    // remember if we add an outstanding req so we can undo it if
-    // necessary, if the packet needs a response, we should add it
-    // as outstanding and express snoops never fail so there is
-    // not need to worry about them
-    bool add_outstanding = !is_express_snoop && pkt->needsResponse();
-
-    // keep track that we have an outstanding request packet
-    // matching this request, this is used by the coherency
-    // mechanism in determining what to do with snoop responses
-    // (in recvTimingSnoop)
-    if (add_outstanding) {
-        // we should never have an exsiting request outstanding
-        assert(outstandingReq.find(pkt->req) == outstandingReq.end());
-        outstandingReq.insert(pkt->req);
-    }
+    // remember if the packet will generate a snoop response
+    const bool expect_snoop_resp = !is_inhibited && pkt->memInhibitAsserted();
+    const bool expect_response = pkt->needsResponse() &&
+        !pkt->memInhibitAsserted();
 
     // Note: Cannot create a copy of the full packet, here.
     MemCmd orig_cmd(pkt->cmd);
@@ -224,41 +214,58 @@ CoherentXBar::recvTimingReq(PacketPtr pkt, PortID slave_port_id)
         pkt->cmd = tmp_cmd;
     }
 
-    // if this is an express snoop, we are done at this point
-    if (is_express_snoop) {
-        assert(success);
-        snoops++;
-    } else {
-        // for normal requests, check if successful
-        if (!success)  {
-            // inhibited packets should never be forced to retry
-            assert(!pkt->memInhibitAsserted());
+    // check if we were successful in sending the packet onwards
+    if (!success)  {
+        // express snoops and inhibited packets should never be forced
+        // to retry
+        assert(!is_express_snoop);
+        assert(!pkt->memInhibitAsserted());
 
-            // if it was added as outstanding and the send failed, then
-            // erase it again
-            if (add_outstanding)
-                outstandingReq.erase(pkt->req);
+        // undo the calculation so we can check for 0 again
+        pkt->firstWordDelay = pkt->lastWordDelay = 0;
 
-            // undo the calculation so we can check for 0 again
-            pkt->firstWordDelay = pkt->lastWordDelay = 0;
+        DPRINTF(CoherentXBar, "recvTimingReq: src %s %s 0x%x RETRY\n",
+                src_port->name(), pkt->cmdString(), pkt->getAddr());
 
-            DPRINTF(CoherentXBar, "recvTimingReq: src %s %s 0x%x RETRY\n",
-                    src_port->name(), pkt->cmdString(), pkt->getAddr());
+        // update the layer state and schedule an idle event
+        reqLayers[master_port_id]->failedTiming(src_port,
+                                                clockEdge(headerCycles));
+    } else {
+        // express snoops currently bypass the crossbar state entirely
+        if (!is_express_snoop) {
+            // if this particular request will generate a snoop
+            // response
+            if (expect_snoop_resp) {
+                // we should never have an exsiting request outstanding
+                assert(outstandingSnoop.find(pkt->req) ==
+                       outstandingSnoop.end());
+                outstandingSnoop.insert(pkt->req);
+
+                // basic sanity check on the outstanding snoops
+                panic_if(outstandingSnoop.size() > 512,
+                         "Outstanding snoop requests exceeded 512\n");
+            }
+
+            // remember where to route the normal response to
+            if (expect_response || expect_snoop_resp) {
+                assert(routeTo.find(pkt->req) == routeTo.end());
+                routeTo[pkt->req] = slave_port_id;
+
+                panic_if(routeTo.size() > 512,
+                         "Routing table exceeds 512 packets\n");
+            }
 
-            // update the layer state and schedule an idle event
-            reqLayers[master_port_id]->failedTiming(src_port,
-                                                    clockEdge(headerCycles));
-        } else {
             // update the layer state and schedule an idle event
             reqLayers[master_port_id]->succeededTiming(packetFinishTime);
         }
-    }
 
-    // stats updates only consider packets that were successfully sent
-    if (success) {
+        // stats updates only consider packets that were successfully sent
         pktCount[slave_port_id][master_port_id]++;
         pktSize[slave_port_id][master_port_id] += pkt_size;
         transDist[pkt_cmd]++;
+
+        if (is_express_snoop)
+            snoops++;
     }
 
     return success;
@@ -270,8 +277,10 @@ CoherentXBar::recvTimingResp(PacketPtr pkt, PortID master_port_id)
     // determine the source port based on the id
     MasterPort *src_port = masterPorts[master_port_id];
 
-    // determine the destination based on what is stored in the packet
-    PortID slave_port_id = pkt->getDest();
+    // determine the destination
+    const auto route_lookup = routeTo.find(pkt->req);
+    assert(route_lookup != routeTo.end());
+    const PortID slave_port_id = route_lookup->second;
     assert(slave_port_id != InvalidPortID);
     assert(slave_port_id < respLayers.size());
 
@@ -294,18 +303,11 @@ CoherentXBar::recvTimingResp(PacketPtr pkt, PortID master_port_id)
     calcPacketTiming(pkt);
     Tick packetFinishTime = pkt->lastWordDelay + curTick();
 
-    // the packet is a normal response to a request that we should
-    // have seen passing through the crossbar
-    assert(outstandingReq.find(pkt->req) != outstandingReq.end());
-
     if (snoopFilter && !pkt->req->isUncacheable() && !system->bypassCaches()) {
         // let the snoop filter inspect the response and update its state
         snoopFilter->updateResponse(pkt, *slavePorts[slave_port_id]);
     }
 
-    // remove it as outstanding
-    outstandingReq.erase(pkt->req);
-
     // send the packet through the destination slave port
     bool success M5_VAR_USED = slavePorts[slave_port_id]->sendTimingResp(pkt);
 
@@ -313,6 +315,9 @@ CoherentXBar::recvTimingResp(PacketPtr pkt, PortID master_port_id)
     // deadlock
     assert(success);
 
+    // remove the request from the routing table
+    routeTo.erase(route_lookup);
+
     respLayers[slave_port_id]->succeededTiming(packetFinishTime);
 
     // stats updates
@@ -337,8 +342,8 @@ CoherentXBar::recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id)
     // we should only see express snoops from caches
     assert(pkt->isExpressSnoop());
 
-    // set the source port for routing of the response
-    pkt->setSrc(master_port_id);
+    // remeber if the packet is inhibited so we can see if it changes
+    const bool is_inhibited = pkt->memInhibitAsserted();
 
     if (snoopFilter) {
         // let the Snoop Filter work its magic and guide probing
@@ -355,6 +360,12 @@ CoherentXBar::recvTimingSnoopReq(PacketPtr pkt, PortID master_port_id)
         forwardTiming(pkt, InvalidPortID);
     }
 
+    // if we can expect a response, remember how to route it
+    if (!is_inhibited && pkt->memInhibitAsserted()) {
+        assert(routeTo.find(pkt->req) == routeTo.end());
+        routeTo[pkt->req] = master_port_id;
+    }
+
     // a snoop request came from a connected slave device (one of
     // our master ports), and if it is not coming from the slave
     // device responsible for the address range something is
@@ -369,16 +380,18 @@ CoherentXBar::recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id)
     // determine the source port based on the id
     SlavePort* src_port = slavePorts[slave_port_id];
 
-    // get the destination from the packet
-    PortID dest_port_id = pkt->getDest();
+    // get the destination
+    const auto route_lookup = routeTo.find(pkt->req);
+    assert(route_lookup != routeTo.end());
+    const PortID dest_port_id = route_lookup->second;
     assert(dest_port_id != InvalidPortID);
 
     // determine if the response is from a snoop request we
     // created as the result of a normal request (in which case it
-    // should be in the outstandingReq), or if we merely forwarded
+    // should be in the outstandingSnoop), or if we merely forwarded
     // someone else's snoop request
-    bool forwardAsSnoop = outstandingReq.find(pkt->req) ==
-        outstandingReq.end();
+    const bool forwardAsSnoop = outstandingSnoop.find(pkt->req) ==
+        outstandingSnoop.end();
 
     // test if the crossbar should be considered occupied for the
     // current port, note that the check is bypassed if the response
@@ -440,13 +453,11 @@ CoherentXBar::recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id)
         // i.e. from a coherent master connected to the crossbar, and
         // since we created the snoop request as part of recvTiming,
         // this should now be a normal response again
-        outstandingReq.erase(pkt->req);
+        outstandingSnoop.erase(pkt->req);
 
-        // this is a snoop response from a coherent master, with a
-        // destination field set on its way through the crossbar as
-        // request, hence it should never go back to where the snoop
-        // response came from, but instead to where the original
-        // request came from
+        // this is a snoop response from a coherent master, hence it
+        // should never go back to where the snoop response came from,
+        // but instead to where the original request came from
         assert(slave_port_id != dest_port_id);
 
         if (snoopFilter) {
@@ -476,6 +487,9 @@ CoherentXBar::recvTimingSnoopResp(PacketPtr pkt, PortID slave_port_id)
         respLayers[dest_port_id]->succeededTiming(packetFinishTime);
     }
 
+    // remove the request from the routing table
+    routeTo.erase(route_lookup);
+
     // stats updates
     transDist[pkt_cmd]++;
     snoops++;