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authorAli Jafri <ali.jafri@arm.com>2015-03-02 04:00:34 -0500
committerAli Jafri <ali.jafri@arm.com>2015-03-02 04:00:34 -0500
commit6ebe8d863ae0c5a7799e9421da32593ac35e1cc7 (patch)
tree4cec332a59408c6a2e7bb031f528cdf894949a5e /src
parentde46eeade74746aa4939ce9522b3c50a609f3be5 (diff)
downloadgem5-6ebe8d863ae0c5a7799e9421da32593ac35e1cc7.tar.xz
mem: Fix prefetchSquash + memInhibitAsserted bug
This patch resolves a bug with hardware prefetches. Before a hardware prefetch is sent towards the memory, the system generates a snoop request to check all caches above the prefetch generating cache for the presence of the prefetth target. If the prefetch target is found in the tags or the MSHRs of the upper caches, the cache sets the prefetchSquashed flag in the snoop packet. When the snoop packet returns with the prefetchSquashed flag set, the prefetch generating cache deallocates the MSHR reserved for the prefetch. If the prefetch target is found in the writeback buffer of the upper cache, the cache sets the memInhibit flag, which signals the prefetch generating cache to expect the data from the writeback. When the snoop packet returns with the memInhibitAsserted flag set, it marks the allocated MSHR as inService and waits for the data from the writeback. If the prefetch target is found in multiple upper level caches, specifically in the tags or MSHRs of one upper level cache and the writeback buffer of another, the snoop packet will return with both prefetchSquashed and memInhibitAsserted set, while the current code is not written to handle such an outcome. Current code checks for the prefetchSquashed flag first, if it finds the flag, it deallocates the reserved MSHR. This leads to assert failure when the data from the writeback appears at cache. In this fix, we simply switch the order of checks. We first check for memInhibitAsserted and then for prefetch squashed.
Diffstat (limited to 'src')
-rw-r--r--src/mem/cache/cache_impl.hh38
1 files changed, 22 insertions, 16 deletions
diff --git a/src/mem/cache/cache_impl.hh b/src/mem/cache/cache_impl.hh
index 29285abce..14e49e1f7 100644
--- a/src/mem/cache/cache_impl.hh
+++ b/src/mem/cache/cache_impl.hh
@@ -1966,10 +1966,28 @@ Cache<TagStore>::getTimingPacket()
snoop_pkt.senderState = mshr;
cpuSidePort->sendTimingSnoopReq(&snoop_pkt);
- // Check to see if the prefetch was squashed by an upper
- // cache (to prevent us from grabbing the line) or if a
- // writeback arrived between the time the prefetch was
- // placed in the MSHRs and when it was selected to be sent.
+ // Check to see if the prefetch was squashed by an upper cache (to
+ // prevent us from grabbing the line) or if a Check to see if a
+ // writeback arrived between the time the prefetch was placed in
+ // the MSHRs and when it was selected to be sent or if the
+ // prefetch was squashed by an upper cache.
+
+ // It is important to check msmInhibitAsserted before
+ // prefetchSquashed. If another cache has asserted MEM_INGIBIT, it
+ // will be sending a response which will arrive at the MSHR
+ // allocated ofr this request. Checking the prefetchSquash first
+ // may result in the MSHR being prematurely deallocated.
+
+ if (snoop_pkt.memInhibitAsserted()) {
+ // If we are getting a non-shared response it is dirty
+ bool pending_dirty_resp = !snoop_pkt.sharedAsserted();
+ markInService(mshr, pending_dirty_resp);
+ DPRINTF(Cache, "Upward snoop of prefetch for addr"
+ " %#x (%s) hit\n",
+ tgt_pkt->getAddr(), tgt_pkt->isSecure()? "s": "ns");
+ return NULL;
+ }
+
if (snoop_pkt.prefetchSquashed() || blk != NULL) {
DPRINTF(Cache, "Prefetch squashed by cache. "
"Deallocating mshr target %#x.\n", mshr->addr);
@@ -1983,18 +2001,6 @@ Cache<TagStore>::getTimingPacket()
return NULL;
}
- // Check if the prefetch hit a writeback in an upper cache
- // and if so we will eventually get a HardPFResp from
- // above
- if (snoop_pkt.memInhibitAsserted()) {
- // If we are getting a non-shared response it is dirty
- bool pending_dirty_resp = !snoop_pkt.sharedAsserted();
- markInService(mshr, pending_dirty_resp);
- DPRINTF(Cache, "Upward snoop of prefetch for addr"
- " %#x (%s) hit\n",
- tgt_pkt->getAddr(), tgt_pkt->isSecure()? "s": "ns");
- return NULL;
- }
}
pkt = getBusPacket(tgt_pkt, blk, mshr->needsExclusive());