XENEVT_RING_MASK
((d->ring_write + 1) & XENEVT_RING_MASK)) {
d->ring_write = (d->ring_write + 1) & XENEVT_RING_MASK;
ring_read = (ring_read + len) & XENEVT_RING_MASK;
ring_read = (ring_read + len) & XENEVT_RING_MASK;