mem: Transition away from isSupplyExclusive for writebacks

This patch changes how writebacks communicate whether the line is
passed as modified or owned. Previously we relied on the
isSupplyExclusive mechanism, which was originally designed to avoid
unecessary snoops.

For normal cache requests we use the sharedAsserted mechanism to
determine if a block should be marked writeable or not, and with this
patch we transition the writebacks to also use this
mechanism. Conceptually this is cleaner and more consistent.

2 files changed, 14 insertions, 6 deletions

diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index 03593554a..040b49464 100644
--- a/src/mem/cache/cache_impl.hh
+++ b/src/mem/cache/cache_impl.hh
@@ -400,7 +400,9 @@ Cache::access(PacketPtr pkt, CacheBlk *&blk, Cycles &lat,
             }
         }
         blk->status |= BlkDirty;
-        if (pkt->isSupplyExclusive()) {
+        // if shared is not asserted we got the writeback in modified
+        // state, if it is asserted we are in the owned state
+        if (!pkt->sharedAsserted()) {
             blk->status |= BlkWritable;
         }
         // nothing else to do; writeback doesn't expect response
@@ -1475,8 +1477,14 @@ Cache::writebackBlk(CacheBlk *blk)
 
     PacketPtr writeback = new Packet(writebackReq, MemCmd::Writeback);
     if (blk->isWritable()) {
-        writeback->setSupplyExclusive();
+        // not asserting shared means we pass the block in modified
+        // state, mark our own block non-writeable
+        blk->status &= ~BlkWritable;
+    } else {
+        // we are in the owned state, tell the receiver
+        writeback->assertShared();
     }
+
     writeback->allocate();
     std::memcpy(writeback->getPtr<uint8_t>(), blk->data, blkSize);
 
@@ -1998,9 +2006,10 @@ Cache::recvTimingSnoopReq(PacketPtr pkt)
             pkt->assertMemInhibit();
             if (!pkt->needsExclusive()) {
                 pkt->assertShared();
-                // the writeback is no longer the exclusive copy in
-                // the system
-                wb_pkt->clearSupplyExclusive();
+                // the writeback is no longer passing exclusivity (the
+                // receiving cache should consider the block owned
+                // rather than modified)
+                wb_pkt->assertShared();
             } 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 fe5e10d0a..d1774e5a8 100644
--- a/src/mem/packet.hh
+++ b/src/mem/packet.hh
@@ -500,7 +500,6 @@ class Packet : public Printable
     void setExpressSnoop()          { flags.set(EXPRESS_SNOOP); }
     bool isExpressSnoop() const     { return flags.isSet(EXPRESS_SNOOP); }
     void setSupplyExclusive()       { flags.set(SUPPLY_EXCLUSIVE); }
-    void clearSupplyExclusive()     { flags.clear(SUPPLY_EXCLUSIVE); }
     bool isSupplyExclusive() const  { return flags.isSet(SUPPLY_EXCLUSIVE); }
     void setSuppressFuncError()     { flags.set(SUPPRESS_FUNC_ERROR); }
     bool suppressFuncError() const  { return flags.isSet(SUPPRESS_FUNC_ERROR); }