Re: [PATCH v3 1/3] vhost: Add smp_rmb() in vhost_vq_avail_empty()

[Stefano Garzarella <sgarzare@redhat.com>]

On Thu, Mar 28, 2024 at 10:21:47AM +1000, Gavin Shan wrote:
>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
>275bf960ac697 ("vhost: better detection of available buffers").
>
>Fixes: 275bf960ac69 ("vhost: better detection of available buffers")
>Cc: <stable@kernel.org> # v4.11+
>Reported-by: Yihuang Yu <yihyu@redhat.com>
>Suggested-by: Will Deacon <will@kernel.org>
>Signed-off-by: Gavin Shan <gshan@redhat.com>
>Acked-by: Jason Wang <jasowang@redhat.com>
>---
> drivers/vhost/vhost.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

>
>diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>index 045f666b4f12..29df65b2ebf2 100644
>--- a/drivers/vhost/vhost.c
>+++ b/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);
>
>-- 
>2.44.0
>