vax_sgmap_reserve
int vax_sgmap_reserve(bus_addr_t, bus_size_t, struct vax_sgmap *);
vax_sgmap_reserve(start*VAX_NBPG,
vax_sgmap_reserve(start*VAX_NBPG, (pgnum - start)*VAX_NBPG,
vax_sgmap_reserve(0x3fe000, 0x400000 - 0x3fe000, &sc->uv_sgmap);