xs_ring_avail
if (xs_ring_avail(xs->xs_ring, 1) > 0)
uint32_t avail = xs_ring_avail(xsr, 1);
uint32_t avail = xs_ring_avail(xsr, 0);
avail = xs_ring_avail(xsr, 0);
if ((avail = xs_ring_avail(xsr, 0)) > 0)