mem: Clarify cache MSHR handling on fill

This patch addresses the upgrading of deferred targets in the MSHR,
and makes it clearer by explicitly calling out what is happening
(deferred targets are promoted if we get exclusivity without asking
for it).

src/mem/cache/cache.cc

@@ -1190,7 +1190,6 @@ Cache::recvTimingResp(PacketPtr pkt)
 
     // Initial target is used just for stats
     MSHR::Target *initial_tgt = mshr->getTarget();
-    CacheBlk *blk = tags->findBlock(pkt->getAddr(), pkt->isSecure());
     int stats_cmd_idx = initial_tgt->pkt->cmdToIndex();
     Tick miss_latency = curTick() - initial_tgt->recvTime;
     PacketList writebacks;
@@ -1212,16 +1211,20 @@ Cache::recvTimingResp(PacketPtr pkt)
             miss_latency;
     }
 
+    // upgrade deferred targets if we got exclusive
+    if (!pkt->sharedAsserted()) {
+        mshr->promoteExclusive();
+    }
+
     bool is_fill = !mshr->isForward &&
         (pkt->isRead() || pkt->cmd == MemCmd::UpgradeResp);
 
+    CacheBlk *blk = tags->findBlock(pkt->getAddr(), pkt->isSecure());
+
     if (is_fill && !is_error) {
         DPRINTF(Cache, "Block for addr %#llx being updated in Cache\n",
                 pkt->getAddr());
 
-        // give mshr a chance to do some dirty work
-        mshr->handleFill(pkt, blk);
-
         blk = handleFill(pkt, blk, writebacks);
         assert(blk != NULL);
     }
@@ -1262,9 +1265,10 @@ Cache::recvTimingResp(PacketPtr pkt)
             // from above.
             if (tgt_pkt->cmd == MemCmd::WriteLineReq) {
                 assert(!is_error);
-
+                // we got the block in exclusive state, so promote any
+                // deferred targets if possible
+                mshr->promoteExclusive();
                 // NB: we use the original packet here and not the response!
-                mshr->handleFill(tgt_pkt, blk);
                 blk = handleFill(tgt_pkt, blk, writebacks);
                 assert(blk != NULL);
 

src/mem/cache/mshr.cc

@@ -430,11 +430,10 @@ MSHR::promoteDeferredTargets()
 
 
 void
-MSHR::handleFill(PacketPtr pkt, CacheBlk *blk)
+MSHR::promoteExclusive()
 {
-    if (!pkt->sharedAsserted()
-        && !(hasPostInvalidate() || hasPostDowngrade())
-        && deferredTargets.needsExclusive) {
+    if (deferredTargets.needsExclusive &&
+        !(hasPostInvalidate() || hasPostDowngrade())) {
         // We got an exclusive response, but we have deferred targets
         // which are waiting to request an exclusive copy (not because
         // of a pending invalidate).  This can happen if the original

src/mem/cache/mshr.hh

@@ -282,7 +282,7 @@ class MSHR : public Packet::SenderState, public Printable
 
     bool promoteDeferredTargets();
 
-    void handleFill(PacketPtr pkt, CacheBlk *blk);
+    void promoteExclusive();
 
     bool checkFunctional(PacketPtr pkt);