vhost: Add smp_rmb() in vhost_vq_avail_empty()

A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by
Will. 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(). When tx_can_batch()
returns true, it means there's still pending tx buffers. Since it might
read indices, so it still can bypass the smp_rmb() in vhost_get_vq_desc().
Note that it should be safe until vq->avail_idx is changed by commit
275bf96 ("vhost: better detection of available buffers").

Fixes: 275bf96 ("vhost: better detection of available buffers")
Cc: <[email protected]> # v4.11+
Reported-by: Yihuang Yu <[email protected]>
Suggested-by: Will Deacon <[email protected]>
Signed-off-by: Gavin Shan <[email protected]>
Acked-by: Jason Wang <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Stefano Garzarella <[email protected]>

Author: Gavin Shan

drivers/vhost/vhost.c

@@ -2799,9 +2799,19 @@ 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) {
+		/* 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.
+		 */
+		smp_rmb();
+		return false;
+	}
 
-	return vq->avail_idx == vq->last_avail_idx;
+	return true;
 }
 EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);