changeset 7639c17357dc in /z/repo/gem5
details: http://repo.gem5.org/gem5?cmd=changeset;node=7639c17357dc
description:
mem: Clarify express snoop behaviour
This patch adds a bit of documentation with insights around how
express snoops really work.
diffstat:
src/mem/cache/cache_impl.hh | 27 ++++++++++++++++++++-------
1 files changed, 20 insertions(+), 7 deletions(-)
diffs (59 lines):
diff -r 204a0f53035e -r 7639c17357dc src/mem/cache/cache_impl.hh
--- a/src/mem/cache/cache_impl.hh Tue Feb 03 14:25:59 2015 -0500
+++ b/src/mem/cache/cache_impl.hh Tue Feb 03 14:26:02 2015 -0500
@@ -465,13 +465,17 @@
promoteWholeLineWrites(pkt);
if (pkt->memInhibitAsserted()) {
+ // a cache above us (but not where the packet came from) is
+ // responding to the request
DPRINTF(Cache, "mem inhibited on 0x%x (%s): not responding\n",
pkt->getAddr(), pkt->isSecure() ? "s" : "ns");
assert(!pkt->req->isUncacheable());
- // Special tweak for multilevel coherence: snoop downward here
- // on invalidates since there may be other caches below here
- // that have shared copies. Not necessary if we know that
- // supplier had exclusive copy to begin with.
+
+ // if the packet needs exclusive, and the cache that has
+ // promised to respond (setting the inhibit flag) is not
+ // providing exclusive (it is in O vs M state), we know that
+ // there may be other shared copies in the system; go out and
+ // invalidate them all
if (pkt->needsExclusive() && !pkt->isSupplyExclusive()) {
// create a downstream express snoop with cleared packet
// flags, there is no need to allocate any data as the
@@ -484,21 +488,30 @@
// make this an instantaneous express snoop, and let the
// other caches in the system know that the packet is
- // inhibited
+ // inhibited, because we have found the authorative copy
+ // (O) that will supply the right data
snoop_pkt->setExpressSnoop();
snoop_pkt->assertMemInhibit();
+
+ // this express snoop travels towards the memory, and at
+ // every crossbar it is snooped upwards thus reaching
+ // every cache in the system
bool M5_VAR_USED success = memSidePort->sendTimingReq(snoop_pkt);
// express snoops always succeed
assert(success);
+
// main memory will delete the packet
}
- // since we're the official target but we aren't responding,
- // delete the packet now.
/// @todo nominally we should just delete the packet here,
/// however, until 4-phase stuff we can't because sending
/// cache is still relying on it
pendingDelete.push_back(pkt);
+
+ // no need to take any action in this particular cache as the
+ // caches along the path to memory are allowed to keep lines
+ // in a shared state, and a cache above us already committed
+ // to responding
return true;
}
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