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From: Jason Wang <jasowang@redhat.com>
Message-ID: <182fcc5e-5996-24e3-d190-be66b0c9f1bd@redhat.com>
Date: Mon, 4 Jun 2018 16:32:49 +0800
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Subject: Re: [Qemu-devel] [PATCH 3/8] virtio: add empty check for packed ring
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To: Wei Xu <wexu@redhat.com>
Cc: jfreimann@redhat.com, qemu-devel@nongnu.org, tiwei.bie@intel.com, mst@redhat.com



On 2018=E5=B9=B406=E6=9C=8804=E6=97=A5 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 =3D 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=20
off_wrap.

Thanks