From e9185363804489ce2b84d50fe77ed94f3a5f1e01 Mon Sep 17 00:00:00 2001
From: Steve Reinhardt <steve.reinhardt@amd.com>
Date: Tue, 21 Sep 2010 23:07:34 -0700
Subject: [PATCH] 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 | 8 ++++++++
 src/mem/packet.hh           | 1 +
 2 files changed, 9 insertions(+)

diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index 4f4d720b6..86bf79b7b 100644
--- a/src/mem/cache/cache_impl.hh
+++ b/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
diff --git a/src/mem/packet.hh b/src/mem/packet.hh
index cefb7c2ed..48b80879e 100644
--- a/src/mem/packet.hh
+++ b/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
-- 
2.30.2