_gmap_ptep_xchg
pgste = _gmap_ptep_xchg(sg->parent, ptep_h, newpte, pgste, f->gfn, false);
pgste = _gmap_ptep_xchg(sg->parent, ptep, pte, pgste, p_gfn, false);
return _gmap_ptep_xchg(gmap, ptep, newpte, pgste, gfn, true);