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

[Jason Wang]

On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@xxxxxxxxxx> wrote:
>
> 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.
> +                */

We need to be more verbose here. For example, which load needs to be
ordered with which load.

The rmb in vhost_get_vq_desc() is used to order the load of avail idx
and the load of head. It is paired with e.g virtio_wmb() in
virtqueue_add_split().

vhost_vq_avail_empty() are mostly used as a hint in
vhost_net_busy_poll() which is under the protection of the vq mutex.

An exception is the tx_can_batch(), but in that case it doesn't even
want to read the head.

Thanks


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