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

[Stefano Garzarella]

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@xxxxxxxxxx> # v4.11+
> Reported-by: Yihuang Yu <yihyu@xxxxxxxxxx>
> Suggested-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> ---
> drivers/vhost/vhost.c | 12 +++++++++++-
> 1 file changed, 11 insertions(+), 1 deletion(-)

Reviewed-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>

> 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