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

["Michael S. Tsirkin" <mst@redhat.com>]

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@kernel.org>. 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@kernel.org> # v4.11+
> Reported-by: Yihuang Yu <yihyu@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  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.


> +		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.






> -- 
> 2.44.0