changeset 37d052b21555 in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=37d052b21555
description:
Mem: Fix issue with prefetches originating at non-L1 caches getting
stale data
Prefetch requests issued from the L2 or below wouldn't check if valid
data is
present higher in the system. If a prefetch into the L2 occured at the
same
time as writeback from a higher-level cache the dirty data could be
replaced
in by unmodified data in memory.
diffstat:
src/mem/cache/cache_impl.hh | 39 ++++++++++++++++++++++++++++++++++++++-
1 files changed, 38 insertions(+), 1 deletions(-)
diffs (56 lines):
diff -r 18e560ba1539 -r 37d052b21555 src/mem/cache/cache_impl.hh
--- a/src/mem/cache/cache_impl.hh Fri Jul 15 11:53:35 2011 -0500
+++ b/src/mem/cache/cache_impl.hh Fri Jul 15 11:53:35 2011 -0500
@@ -420,7 +420,17 @@
// must be cache-to-cache response from upper to lower level
ForwardResponseRecord *rec =
dynamic_cast<ForwardResponseRecord *>(pkt->senderState);
- assert(rec != NULL);
+
+ if (rec == NULL) {
+ assert(pkt->cmd == MemCmd::HardPFResp);
+ // Check if it's a prefetch response and handle it. We shouldn't
+ // get any other kinds of responses without FRRs.
+ DPRINTF(Cache, "Got prefetch response from above for addr %#x\n",
+ pkt->getAddr());
+ handleResponse(pkt);
+ return true;
+ }
+
rec->restore(pkt, this);
delete rec;
memSidePort->respond(pkt, time);
@@ -1472,6 +1482,33 @@
pkt = tgt_pkt;
} else {
BlkType *blk = tags->findBlock(mshr->addr);
+
+ if (tgt_pkt->cmd == MemCmd::HardPFReq) {
+ // It might be possible for a writeback to arrive between
+ // the time the prefetch is placed in the MSHRs and when
+ // it's selected to send... if so, this assert will catch
+ // that, and then we'll have to figure out what to do.
+ assert(blk == NULL);
+
+ // We need to check the caches above us to verify that they don't
have
+ // a copy of this block in the dirty state at the moment. Without
this
+ // check we could get a stale copy from memory that might get used
+ // in place of the dirty one.
+ PacketPtr snoop_pkt = new Packet(tgt_pkt, true);
+ snoop_pkt->setExpressSnoop();
+ snoop_pkt->senderState = mshr;
+ cpuSidePort->sendTiming(snoop_pkt);
+
+ if (snoop_pkt->memInhibitAsserted()) {
+ markInService(mshr, snoop_pkt);
+ DPRINTF(Cache, "Upward snoop of prefetch for addr %#x hit\n",
+ tgt_pkt->getAddr());
+ delete snoop_pkt;
+ return NULL;
+ }
+ delete snoop_pkt;
+ }
+
pkt = getBusPacket(tgt_pkt, blk, mshr->needsExclusive());
mshr->isForward = (pkt == NULL);
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