vring_avail
size += sizeof(struct vring_avail) + (num * sizeof(uint16_t)) +
used_offset = (avail_offset + sizeof(struct vring_avail) +
vr->avail = (struct vring_avail *) (p + avail_offset);
struct vring_avail *avail;
vq->vq_avail = (struct vring_avail *)base;
struct vring_avail *vq_avail; /* the "avail" ring */