cache: improve coherence handling of writebacks

If we write back an exclusive copy, we now mark it
as such, so the cache receiving the writeback can
mark its copy as exclusive.  This avoids some
unnecessary upgrade requests when a cache later
tries to re-acquire exclusive access to the block.

src/mem/cache/cache_impl.hh

@@ -360,6 +360,9 @@ Cache<TagStore>::access(PacketPtr pkt, BlkType *&blk,
         }
         std::memcpy(blk->data, pkt->getPtr<uint8_t>(), blkSize);
         blk->status |= BlkDirty;
+        if (pkt->isSupplyExclusive()) {
+            blk->status |= BlkWritable;
+        }
         // nothing else to do; writeback doesn't expect response
         assert(!pkt->needsResponse());
         incHitCount(pkt, id);
@@ -1007,6 +1010,9 @@ Cache<TagStore>::writebackBlk(BlkType *blk)
     Request *writebackReq =
         new Request(tags->regenerateBlkAddr(blk->tag, blk->set), blkSize, 0);
     PacketPtr writeback = new Packet(writebackReq, MemCmd::Writeback, -1);
+    if (blk->isWritable()) {
+        writeback->setSupplyExclusive();
+    }
     writeback->allocate();
     std::memcpy(writeback->getPtr<uint8_t>(), blk->data, blkSize);
 
@@ -1307,6 +1313,8 @@ Cache<TagStore>::snoopTiming(PacketPtr pkt)
             pkt->assertMemInhibit();
             if (!pkt->needsExclusive()) {
                 pkt->assertShared();
+                // the writeback is no longer the exclusive copy in the system
+                wb_pkt->clearSupplyExclusive();
             } else {
                 // if we're not asserting the shared line, we need to
                 // invalidate our copy.  we'll do that below as long as

src/mem/packet.hh

@@ -422,6 +422,7 @@ class Packet : public FastAlloc, public Printable
     void setExpressSnoop()      { flags.set(EXPRESS_SNOOP); }
     bool isExpressSnoop()       { return flags.isSet(EXPRESS_SNOOP); }
     void setSupplyExclusive()   { flags.set(SUPPLY_EXCLUSIVE); }
+    void clearSupplyExclusive() { flags.clear(SUPPLY_EXCLUSIVE); }
     bool isSupplyExclusive()    { return flags.isSet(SUPPLY_EXCLUSIVE); }
 
     // Network error conditions... encapsulate them as methods since