x86: reduce Meltdown band-aid IPI overhead

In case we can detect single-threaded guest processes (by checking whether we can account for all root page table uses locally on the vCPU that's running), there's no point in issuing a sync IPI upon an L4 entry update, as no other vCPU of the guest will have that page table loaded. Signed-off-by: Jan Beulich <[email protected]> Acked-by: George Dunlap <[email protected]> Acked-by: Andrew Cooper <[email protected]>
MultipleCrashes · Feb 7, 2018 · a22320e · a22320e
1 parent c5d981c
commit a22320e
Showing 1 changed file with 12 additions and 2 deletions.
diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
@@ -3706,8 +3706,18 @@ long do_mmu_update(
                         break;
                     rc = mod_l4_entry(va, l4e_from_intpte(req.val), mfn,
                                       cmd == MMU_PT_UPDATE_PRESERVE_AD, v);
-                    if ( !rc )
-                        sync_guest = this_cpu(root_pgt);
+                    /*
+                     * No need to sync if all uses of the page can be accounted
+                     * to the page lock we hold, its pinned status, and uses on
+                     * this (v)CPU.
+                     */
+                    if ( !rc && this_cpu(root_pgt) &&
+                         ((page->u.inuse.type_info & PGT_count_mask) >
+                          (1 + !!(page->u.inuse.type_info & PGT_pinned) +
+                           (pagetable_get_pfn(curr->arch.guest_table) == mfn) +
+                           (pagetable_get_pfn(curr->arch.guest_table_user) ==
+                            mfn))) )
+                        sync_guest = true;
                     break;
 
                 case PGT_writable_page: