vring_avail_event
if (putu16(vrh, &vring_avail_event(&vrh->vring),
&vring_avail_event(&vrh->vring));
READ_ONCE(vring_avail_event(&vq->split.vring)));
need = vring_need_event(vring_avail_event(&ring),
vring_avail_event(&ring) = host.used_idx;