dat_ptep_xchg
dat_ptep_xchg(ptep, new_pte, gfn, walk->asce, false);
dat_ptep_xchg(ptep, _PTE_EMPTY, gfn, asce, has_skeys);
dat_ptep_xchg(ptep, _PTE_EMPTY, r_gfn, sg->asce, uses_skeys(sg));