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From: Jason Wang <jasowang@redhat.com>
Date: Wed, 27 Mar 2024 10:34:34 +0800
Message-ID: <CACGkMEtyEo2QQSEh0ZnZDUJSXMMhzeO97Jp0wF4_rhzUBGk4Zg@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] vhost: Add smp_rmb() in vhost_vq_avail_empty()
To: Gavin Shan <gshan@redhat.com>
Cc: virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, 
	mst@redhat.com, davem@davemloft.net, stefanha@redhat.com, sgarzare@redhat.com, 
	keirf@google.com, yihyu@redhat.com, shan.gavin@gmail.com
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On Wed, Mar 27, 2024 at 7:39=E2=80=AFAM Gavin Shan <gshan@redhat.com> 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=3Dhost -cpu host          \
>   -smp maxcpus=3D1,cpus=3D1,sockets=3D1,clusters=3D1,cores=3D1,threads=3D=
1 \
>   -m 4096M,slots=3D16,maxmem=3D64G                                 \
>   -object memory-backend-ram,id=3Dmem0,size=3D4096M                \
>    :                                                           \
>   -netdev tap,id=3Dvnet0,vhost=3Dtrue                              \
>   -device virtio-net-pci,bus=3Dpcie.8,netdev=3Dvnet0,mac=3D52:54:00: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, s=
truct vhost_virtqueue *vq)
>         r =3D vhost_get_avail_idx(vq, &avail_idx);
>         if (unlikely(r))
>                 return false;
> +
>         vq->avail_idx =3D vhost16_to_cpu(vq, avail_idx);
> +       if (vq->avail_idx !=3D 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 =3D=3D vq->last_avail_idx;
> +       return true;
>  }
>  EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
>
> --
> 2.44.0
>