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

[Jason Wang]

On Wed, Mar 27, 2024 at 10:34 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
> 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.

Ok, if it is needed only in that path, maybe we can move the barriers there.

Thanks

>
> 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
> >