Re: [Qemu-devel] [PATCH 3/8] virtio: add empty check for packed ring

[Jason Wang]

On 2018年06月04日 01:44, Wei Xu wrote:
> > > +static int virtio_queue_empty_packed_rcu(VirtQueue *vq)
> > > +{
> > > +    struct VRingDescPacked desc;
> > > +    VRingMemoryRegionCaches *cache;
> > > +
> > > +    if (unlikely(!vq->packed.desc)) {
> > > +        return 1;
> > > +    }
> > > +
> > > +    cache = vring_get_region_caches(vq);
> > > +    vring_desc_read_packed(vq->vdev, &desc, &cache->desc_packed, 
> > > vq->last_avail_idx);
> > > +
> > > +    /* Make sure we see the updated flag */
> > > +    smp_mb();
> > What we need here is to make sure flag is read before all other fields,
> > looks like this barrier can't.
> Isn't flag updated yet if it has been read?

Consider the case of event index, you need make sure flag is read before 
off_wrap.

Thanks