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Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring
From:   Jason Wang <jasowang@redhat.com>
To:     Tiwei Bie <tiwei.bie@intel.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
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Date:   Tue, 8 May 2018 13:40:40 +0800
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On 2018年05月08日 11:05, Jason Wang wrote:
>>
>> Because in virtqueue_enable_cb_delayed(), we may set an
>> event_off which is bigger than new and both of them have
>> wrapped. And in this case, although new is smaller than
>> event_off (i.e. the third param -- old), new shouldn't
>> add vq->num, and actually we are expecting a very big
>> idx diff.
>
> Yes, so to calculate distance correctly between event and new, we just 
> need to compare the warp counter and return false if it doesn't match 
> without the need to try to add vq.num here.
>
> Thanks 

Sorry, looks like the following should work, we need add vq.num if 
used_wrap_counter does not match:

static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq,
                       __u16 off_wrap, __u16 new,
                       __u16 old)
{
     bool wrap = off_wrap >> 15;
     int off = off_wrap & ~(1 << 15);
     __u16 d1, d2;

     if (wrap != vq->used_wrap_counter)
         d1 = new + vq->num - off - 1;
     else
         d1 = new - off - 1;

     if (new > old)
         d2 = new - old;
     else
         d2 = new + vq->num - old;

     return d1 < d2;
}

Thanks