On 3/27/24 22:07, Michael S. Tsirkin wrote:
On Wed, Mar 27, 2024 at 09:38:45AM +1000, Gavin Shan wrote:
A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by
Will Deacon <will@xxxxxxxxxx>. Otherwise, it's not ensured the
available ring entries pushed by guest can be observed by vhost
in time, leading to stale available ring entries fetched by vhost
in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's
grace-hopper (ARM64) platform.
/home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \
-accel kvm -machine virt,gic-version=host -cpu host \
-smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \
-m 4096M,slots=16,maxmem=64G \
-object memory-backend-ram,id=mem0,size=4096M \
: \
-netdev tap,id=vnet0,vhost=true \
-device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0
:
guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM
virtio_net virtio0: output.0:id 100 is not a head!
Add the missed smp_rmb() in vhost_vq_avail_empty(). Note that it
should be safe until vq->avail_idx is changed by commit 275bf960ac697
("vhost: better detection of available buffers").
Fixes: 275bf960ac697 ("vhost: better detection of available buffers")
Cc: <stable@xxxxxxxxxx> # v4.11+
Reported-by: Yihuang Yu <yihyu@xxxxxxxxxx>
Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
---
drivers/vhost/vhost.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 045f666b4f12..00445ab172b3 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -2799,9 +2799,18 @@ bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
r = vhost_get_avail_idx(vq, &avail_idx);
if (unlikely(r))
return false;
+
vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+ if (vq->avail_idx != vq->last_avail_idx) {
+ /* Similar to what's done in vhost_get_vq_desc(), we need
+ * to ensure the available ring entries have been exposed
+ * by guest.
+ */
A slightly clearer comment:
/* Since we have updated avail_idx, the following call to
* vhost_get_vq_desc will read available ring entries.
* Make sure that read happens after the avail_idx read.
*/
Pls repost with that, and I will apply.
Also add suggested-by for will.
Sure, the suggested comments have been included to v3.
+ smp_rmb();
+ return false;
+ }
- return vq->avail_idx == vq->last_avail_idx;
+ return true;
}
EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
As a follow-up patch, we should clean out code duplication that
accumulated with 3 places reading avail idx in essentially
the same way - this duplication is what causes the mess in
the 1st place.
Yes, nice idea. I've added PATCH[v3 3/3] to improve vhost_get_avail_idx()
to handle the memory barrier since all the callers have the concern.
v3: https://lore.kernel.org/virtualization/20240328002149.1141302-1-gshan@xxxxxxxxxx/
Thanks,
Gavin
