[PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty()

[Gavin Shan]

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.
+		 */
+		smp_rmb();
+		return false;
+	}
 
-	return vq->avail_idx == vq->last_avail_idx;
+	return true;
 }
 EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
 
-- 
2.44.0