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

[Gavin Shan]

On 3/27/24 22:07, Michael S. Tsirkin wrote:
> 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@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.
> > +		 */
>
> 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.

Sure, the suggested comments have been included to v3.

> > +		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.

Yes, nice idea. I've added PATCH[v3 3/3] to improve vhost_get_avail_idx()
to handle the memory barrier since all the callers have the concern.

v3: https://lore.kernel.org/virtualization/20240328002149.1141302-1-gshan@xxxxxxxxxx/

Thanks,
Gavin