KVM: x86/mmu: Don't set dirty bits when disabling dirty logging w/ PML

Stop setting dirty bits for MMU pages when dirty logging is disabled for
a memslot, as PML is now completely disabled when there are no memslots
with dirty logging enabled.

This means that spurious PML entries will be created for memslots with
dirty logging disabled if at least one other memslot has dirty logging
enabled.  However, spurious PML entries are already possible since
dirty bits are set only when a dirty logging is turned off, i.e. memslots
that are never dirty logged will have dirty bits cleared.

In the end, it's faster overall to eat a few spurious PML entries in the
window where dirty logging is being disabled across all memslots.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 36 insertions, 51 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b9a8c8af9713..dca2c3333ef2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10789,7 +10789,18 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 
 	/*
 	 * Nothing more to do for RO slots (which can't be dirtied and can't be
-	 * made writable) or CREATE/MOVE/DELETE of a slot.  See comments below.
+	 * made writable) or CREATE/MOVE/DELETE of a slot.
+	 *
+	 * For a memslot with dirty logging disabled:
+	 * CREATE:      No dirty mappings will already exist.
+	 * MOVE/DELETE: The old mappings will already have been cleaned up by
+	 *		kvm_arch_flush_shadow_memslot()
+	 *
+	 * For a memslot with dirty logging enabled:
+	 * CREATE:      No shadow pages exist, thus nothing to write-protect
+	 *		and no dirty bits to clear.
+	 * MOVE/DELETE: The old mappings will already have been cleaned up by
+	 *		kvm_arch_flush_shadow_memslot().
 	 */
 	if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
 		return;
@@ -10802,55 +10813,31 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 	if (WARN_ON_ONCE(!((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)))
 		return;
 
-	/*
-	 * Dirty logging tracks sptes in 4k granularity, meaning that large
-	 * sptes have to be split.  If live migration is successful, the guest
-	 * in the source machine will be destroyed and large sptes will be
-	 * created in the destination. However, if the guest continues to run
-	 * in the source machine (for example if live migration fails), small
-	 * sptes will remain around and cause bad performance.
-	 *
-	 * Scan sptes if dirty logging has been stopped, dropping those
-	 * which can be collapsed into a single large-page spte.  Later
-	 * page faults will create the large-page sptes.
-	 *
-	 * There is no need to do this in any of the following cases:
-	 * CREATE:      No dirty mappings will already exist.
-	 * MOVE/DELETE: The old mappings will already have been cleaned up by
-	 *		kvm_arch_flush_shadow_memslot()
-	 */
-	if (!log_dirty_pages)
+	if (!log_dirty_pages) {
+		/*
+		 * Dirty logging tracks sptes in 4k granularity, meaning that
+		 * large sptes have to be split.  If live migration succeeds,
+		 * the guest in the source machine will be destroyed and large
+		 * sptes will be created in the destination.  However, if the
+		 * guest continues to run in the source machine (for example if
+		 * live migration fails), small sptes will remain around and
+		 * cause bad performance.
+		 *
+		 * Scan sptes if dirty logging has been stopped, dropping those
+		 * which can be collapsed into a single large-page spte.  Later
+		 * page faults will create the large-page sptes.
+		 */
 		kvm_mmu_zap_collapsible_sptes(kvm, new);
-
-	/*
-	 * Enable or disable dirty logging for the slot.
-	 *
-	 * For KVM_MR_DELETE and KVM_MR_MOVE, the shadow pages of the old
-	 * slot have been zapped so no dirty logging updates are needed for
-	 * the old slot.
-	 * For KVM_MR_CREATE and KVM_MR_MOVE, once the new slot is visible
-	 * any mappings that might be created in it will consume the
-	 * properties of the new slot and do not need to be updated here.
-	 *
-	 * When PML is enabled, the kvm_x86_ops dirty logging hooks are
-	 * called to enable/disable dirty logging.
-	 *
-	 * When disabling dirty logging with PML enabled, the D-bit is set
-	 * for sptes in the slot in order to prevent unnecessary GPA
-	 * logging in the PML buffer (and potential PML buffer full VMEXIT).
-	 * This guarantees leaving PML enabled for the guest's lifetime
-	 * won't have any additional overhead from PML when the guest is
-	 * running with dirty logging disabled.
-	 *
-	 * When enabling dirty logging, large sptes are write-protected
-	 * so they can be split on first write.  New large sptes cannot
-	 * be created for this slot until the end of the logging.
-	 * See the comments in fast_page_fault().
-	 * For small sptes, nothing is done if the dirty log is in the
-	 * initial-all-set state.  Otherwise, depending on whether pml
-	 * is enabled the D-bit or the W-bit will be cleared.
-	 */
-	if (log_dirty_pages) {
+	} else {
+		/*
+		 * Large sptes are write-protected so they can be split on first
+		 * write. New large sptes cannot be created for this slot until
+		 * the end of the logging. See the comments in fast_page_fault().
+		 *
+		 * For small sptes, nothing is done if the dirty log is in the
+		 * initial-all-set state.  Otherwise, depending on whether pml
+		 * is enabled the D-bit or the W-bit will be cleared.
+		 */
 		if (kvm_x86_ops.cpu_dirty_log_size) {
 			if (!kvm_dirty_log_manual_protect_and_init_set(kvm))
 				kvm_mmu_slot_leaf_clear_dirty(kvm, new);
@@ -10870,8 +10857,6 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 			 */
 			kvm_mmu_slot_remove_write_access(kvm, new, level);
 		}
-	} else if (kvm_x86_ops.cpu_dirty_log_size) {
-		kvm_mmu_slot_set_dirty(kvm, new);
 	}
 }