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

[Gavin Shan]

On 3/27/24 12:41, Jason Wang wrote:
> On Wed, Mar 27, 2024 at 7:39 AM Gavin Shan <gshan@xxxxxxxxxx> wrote:
> > A smp_rmb() has been missed in vhost_enable_notify(), inspired 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_enable_notify(). Note that it
> > should be safe until vq->avail_idx is changed by commit d3bb267bbdcb
> > ("vhost: cache avail index in vhost_enable_notify()").
> >
> > Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
> > Cc: <stable@xxxxxxxxxx> # v5.18+
> > 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 00445ab172b3..58f9d6a435f0 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2847,9 +2847,18 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> >                         &vq->avail->idx, 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.
> > +                */
> > +               smp_rmb();
> > +               return true;
> > +       }
> >
> > -       return vq->avail_idx != vq->last_avail_idx;
> > +       return false;
>
> So we only care about the case when vhost_enable_notify() returns true.
>
> In that case, I think you want to order with vhost_get_vq_desc():
>
> last_avail_idx = vq->last_avail_idx;
>
> if (vq->avail_idx == vq->last_avail_idx) { /* false */
> }
>
> vhost_get_avail_head(vq, &ring_head, last_avail_idx)
>
> Assuming I understand the patch correctly.
>
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Jason, thanks for your review and comments. Your understanding is exactly
what I understood.

> >   }
> >   EXPORT_SYMBOL_GPL(vhost_enable_notify);

Thanks,
Gavin