Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring

[Jason Wang]

On 2018å05æ08æ 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,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â __u16 off_wrap, __u16 new,
ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â __u16 old)
{
ÂÂÂ bool wrap = off_wrap >> 15;
ÂÂÂ int off = off_wrap & ~(1 << 15);
ÂÂÂ __u16 d1, d2;

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

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

ÂÂÂ return d1 < d2;
}

Thanks