xenbus_grant_ring
int xenbus_grant_ring(struct xenbus_device *, paddr_t, grant_ref_t *);
error = xenbus_grant_ring(sc->sc_xbusd, ma, &sc->sc_tx_ring_gntref);
error = xenbus_grant_ring(sc->sc_xbusd, ma, &sc->sc_rx_ring_gntref);
error = xenbus_grant_ring(sc->sc_xbusd, ma, &sc->sc_ring_gntref);
error = xenbus_grant_ring(sc->sc_xbusd, ma, &sc->sc_shared_gntref);