KVM: VMX: Dynamically enable/disable PML based on memslot dirty logging

Currently, if enable_pml=1 PML remains enabled for the entire lifetime
of the VM irrespective of whether dirty logging is enable or disabled.
When dirty logging is disabled, all the pages of the VM are manually
marked dirty, so that PML is effectively non-operational.  Setting
the dirty bits is an expensive operation which can cause severe MMU
lock contention in a performance sensitive path when dirty logging is
disabled after a failed or canceled live migration.

Manually setting dirty bits also fails to prevent PML activity if some
code path clears dirty bits, which can incur unnecessary VM-Exits.

In order to avoid this extra overhead, dynamically enable/disable PML
when dirty logging gets turned on/off for the first/last memslot.

Signed-off-by: Makarand Sonare <makarandsonare@google.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210213005015.1651772-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 31 insertions, 4 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c0d22f19aed0..b9a8c8af9713 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -8987,6 +8987,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			kvm_check_async_pf_completion(vcpu);
 		if (kvm_check_request(KVM_REQ_MSR_FILTER_CHANGED, vcpu))
 			static_call(kvm_x86_msr_filter_changed)(vcpu);
+
+		if (kvm_check_request(KVM_REQ_UPDATE_CPU_DIRTY_LOGGING, vcpu))
+			static_call(kvm_x86_update_cpu_dirty_logging)(vcpu);
 	}
 
 	if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win ||
@@ -10755,14 +10758,38 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	return 0;
 }
 
+
+static void kvm_mmu_update_cpu_dirty_logging(struct kvm *kvm, bool enable)
+{
+	struct kvm_arch *ka = &kvm->arch;
+
+	if (!kvm_x86_ops.cpu_dirty_log_size)
+		return;
+
+	if ((enable && ++ka->cpu_dirty_logging_count == 1) ||
+	    (!enable && --ka->cpu_dirty_logging_count == 0))
+		kvm_make_all_cpus_request(kvm, KVM_REQ_UPDATE_CPU_DIRTY_LOGGING);
+
+	WARN_ON_ONCE(ka->cpu_dirty_logging_count < 0);
+}
+
 static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 				     struct kvm_memory_slot *old,
 				     struct kvm_memory_slot *new,
 				     enum kvm_mr_change change)
 {
+	bool log_dirty_pages = new->flags & KVM_MEM_LOG_DIRTY_PAGES;
+
+	/*
+	 * Update CPU dirty logging if dirty logging is being toggled.  This
+	 * applies to all operations.
+	 */
+	if ((old->flags ^ new->flags) & KVM_MEM_LOG_DIRTY_PAGES)
+		kvm_mmu_update_cpu_dirty_logging(kvm, log_dirty_pages);
+
 	/*
-	 * Nothing 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.
+	 * 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.
 	 */
 	if ((change != KVM_MR_FLAGS_ONLY) || (new->flags & KVM_MEM_READONLY))
 		return;
@@ -10792,7 +10819,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 	 * MOVE/DELETE: The old mappings will already have been cleaned up by
 	 *		kvm_arch_flush_shadow_memslot()
 	 */
-	if (!(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
+	if (!log_dirty_pages)
 		kvm_mmu_zap_collapsible_sptes(kvm, new);
 
 	/*
@@ -10823,7 +10850,7 @@ static void kvm_mmu_slot_apply_flags(struct kvm *kvm,
 	 * initial-all-set state.  Otherwise, depending on whether pml
 	 * is enabled the D-bit or the W-bit will be cleared.
 	 */
-	if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) {
+	if (log_dirty_pages) {
 		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);