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

[Jason Wang <jasowang@redhat.com>]

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