x86/PV32: avoid TLB flushing after mod_l3_entry()

32-bit guests may not depend upon the side effect of using ordinary
4-level paging when running on a 64-bit hypervisor. For L3 entry updates
to take effect, they have to use a CR3 reload. Therefore there's no need
to issue a paging structure invalidating TLB flush in this case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/mm.c

@@ -4069,7 +4069,8 @@ long do_mmu_update(
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
                     if ( !rc &&
                          (page->u.inuse.type_info & PGT_count_mask) >
-                         1 + !!(page->u.inuse.type_info & PGT_pinned) )
+                         1 + !!(page->u.inuse.type_info & PGT_pinned) &&
+                         !is_pv_32bit_domain(pt_owner) )
                         flush_linear_pt = true;
                     break;