commit 612f242359dd28a3fe48628f766f03dabf03f3ab
author Tiago Mück <tiago.muck@arm.com> Mon May 16 17:50:18 2022 -0500
committer Tiago Muck <tiago.muck@arm.com> Wed Jun 01 15:23:47 2022 +0000
tree 6018f7a8180681e147a8354c4b871dad303098ff
parent 1dfd319d9891dc65c59d786c22cfe28864ef9ad5

mem-ruby: fix CHI snoops clearing WU data

When just forwarding a WU request, the controller waits until the WU is
acked from downstream before sending the ack upstream. This
prevents snoops clearing valid WU data.

JIRA: https://gem5.atlassian.net/browse/GEM5-1195

This was more likely to happen with shared exclusive caches, e.g:
assume core C1 and C2 with private L1/L2 and a shared exclusive L3.
C1 has as dirty copy of the line while C2 issues a WriteUnique request
to that line. The line state is RU in the L3, so the L3 will just
forward the request to the HNF, so:
- C2 issues WU to L3 cache
- L3 acks the WU, allowing C2 to send the data, while concurrently
  forwarding the WU to the HNF.
- L3 receives data from C2
- HNF sends invalidating snoops upstream because line is RU
- The snoop hazards with the pending WU at the L3 and invalidates
  the data previously received. This causes an assertion to fail when
  we resume handling the WU.

Change-Id: I51e457e0bdb648c0fff3f702b7d2c95dcf431dc5
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/59990
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/mem/ruby/protocol/chi/CHI-cache-actions.sm

diff --git a/src/mem/ruby/protocol/chi/CHI-cache-actions.sm b/src/mem/ruby/protocol/chi/CHI-cache-actions.sm
index 7208fc2..9c59bf2 100644
--- a/src/mem/ruby/protocol/chi/CHI-cache-actions.sm
+++ b/src/mem/ruby/protocol/chi/CHI-cache-actions.sm
@@ -842,16 +842,9 @@
 }
 
 action(Initiate_WriteUnique_Forward, desc="") {
-  if (comp_wu) {
-    tbe.actions.push(Event:SendDBIDResp_WU);
-    tbe.actions.pushNB(Event:WriteFEPipe);
-    tbe.actions.pushNB(Event:SendWriteUnique);
-    tbe.actions.pushNB(Event:SendComp_WU);
-  } else {
-    tbe.actions.push(Event:SendCompDBIDResp_WU);
-    tbe.actions.pushNB(Event:WriteFEPipe);
-    tbe.actions.pushNB(Event:SendWriteUnique);
-  }
+  tbe.actions.push(Event:WriteFEPipe);
+  tbe.actions.push(Event:SendWriteUnique);
+  tbe.actions.push(Event:SendCompDBIDResp_WU);
   tbe.actions.push(Event:WriteBEPipe);
   tbe.actions.push(Event:SendWUData);
   tbe.dataToBeInvalid := true;