Re: [PATCH] Revert to KeMemoryBarrier() as poll loop barriers

[Owen Smith <owen.smith@xxxxxxxxxx>]

Reveiwed-by: Owen Smith <owen.smith@xxxxxxxxxx>

________________________________________
From: win-pv-devel <win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx> on behalf of Tu 
Dinh <ngoc-tu.dinh@xxxxxxxxxx>
Sent: 01 June 2026 9:15 AM
To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
Cc: Tu Dinh
Subject: [PATCH] Revert to KeMemoryBarrier() as poll loop barriers

312f9fa760bf ("ring: Weaken and delete unnecessary barriers") attempted
to replace barriers in BlkifRingPoll with their xen_mb/rmb/wmb
equivalents. Part of this involved replacing the full barrier at the end
of each poll loop with a "release barrier" consisting of xen_rmb +
xen_wmb.

This is fine on x86 since each store has release ordering. However, this
is incorrect in the Linux memory model, as rmb/wmb (and their smp_
equivalents) did not guarantee relative ordering of loads and subsequent
stores, unlike a release operation. Therefore, without specializing for
each architecture, a release barrier in this model would require a full
memory barrier (i.e. mb()).

Inconveniently, WDK doesn't provide a release barrier nor a release
store. I couldn't measure a performance difference from 312f9fa760bf in
my limited testing, so revert the change.

Fixes: 312f9fa760bf ("ring: Weaken and delete unnecessary barriers")
Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
 src/xenvbd/ring.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/xenvbd/ring.c b/src/xenvbd/ring.c
index 10ca6ee..d05dfc3 100644
--- a/src/xenvbd/ring.c
+++ b/src/xenvbd/ring.c
@@ -1228,7 +1228,7 @@ BlkifRingPoll(
         RING_IDX            rsp_prod;
         RING_IDX            rsp_cons;

-        xen_mb();
+        KeMemoryBarrier();

         rsp_prod = BlkifRing->Shared->rsp_prod;
         rsp_cons = BlkifRing->Front.rsp_cons;
@@ -1236,7 +1236,7 @@ BlkifRingPoll(
         if (rsp_cons == rsp_prod || Retry)
             break;

-        xen_rmb();
+        KeMemoryBarrier();

         while (rsp_cons != rsp_prod && !Retry) {
             blkif_response_t    *rsp;
@@ -1260,8 +1260,7 @@ BlkifRingPoll(
                 Retry = TRUE;
         }

-        xen_rmb();
-        xen_wmb();
+        KeMemoryBarrier();

         BlkifRing->Front.rsp_cons = rsp_cons;
         BlkifRing->Shared->rsp_event = rsp_cons + 1;
--
2.54.0.windows.1



--
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