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>

3 files changed, 34 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 486c7ac6d813..bcca0b80e0d0 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -4490,6 +4490,11 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
 		vmx_set_virtual_apic_mode(vcpu);
 	}
 
+	if (vmx->nested.update_vmcs01_cpu_dirty_logging) {
+		vmx->nested.update_vmcs01_cpu_dirty_logging = false;
+		vmx_update_cpu_dirty_logging(vcpu);
+	}
+
 	/* Unpin physical memory we referred to in vmcs02 */
 	if (vmx->nested.apic_access_page) {
 		kvm_release_page_clean(vmx->nested.apic_access_page);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index eb207ffb7873..50810d471462 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4277,7 +4277,12 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
 	*/
 	exec_control &= ~SECONDARY_EXEC_SHADOW_VMCS;
 
-	if (!enable_pml)
+	/*
+	 * PML is enabled/disabled when dirty logging of memsmlots changes, but
+	 * it needs to be set here when dirty logging is already active, e.g.
+	 * if this vCPU was created after dirty logging was enabled.
+	 */
+	if (!vcpu->kvm->arch.cpu_dirty_logging_count)
 		exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
 
 	if (cpu_has_vmx_xsaves()) {
@@ -7491,6 +7496,26 @@ static void vmx_sched_in(struct kvm_vcpu *vcpu, int cpu)
 		shrink_ple_window(vcpu);
 }
 
+void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_vmx *vmx = to_vmx(vcpu);
+
+	if (is_guest_mode(vcpu)) {
+		vmx->nested.update_vmcs01_cpu_dirty_logging = true;
+		return;
+	}
+
+	/*
+	 * Note, cpu_dirty_logging_count can be changed concurrent with this
+	 * code, but in that case another update request will be made and so
+	 * the guest will never run with a stale PML value.
+	 */
+	if (vcpu->kvm->arch.cpu_dirty_logging_count)
+		secondary_exec_controls_setbit(vmx, SECONDARY_EXEC_ENABLE_PML);
+	else
+		secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_ENABLE_PML);
+}
+
 static int vmx_pre_block(struct kvm_vcpu *vcpu)
 {
 	if (pi_pre_block(vcpu))
@@ -7698,6 +7723,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
 	.sched_in = vmx_sched_in,
 
 	.cpu_dirty_log_size = PML_ENTITY_NUM,
+	.update_cpu_dirty_logging = vmx_update_cpu_dirty_logging,
 
 	.pre_block = vmx_pre_block,
 	.post_block = vmx_post_block,
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index 12c53d05a902..89da5e1251f1 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -165,6 +165,7 @@ struct nested_vmx {
 
 	bool change_vmcs01_virtual_apic_mode;
 	bool reload_vmcs01_apic_access_page;
+	bool update_vmcs01_cpu_dirty_logging;
 
 	/*
 	 * Enlightened VMCS has been enabled. It does not mean that L1 has to
@@ -393,6 +394,7 @@ int vmx_find_loadstore_msr_slot(struct vmx_msrs *m, u32 msr);
 void vmx_ept_load_pdptrs(struct kvm_vcpu *vcpu);
 void vmx_set_intercept_for_msr(struct kvm_vcpu *vcpu,
 	u32 msr, int type, bool value);
+void vmx_update_cpu_dirty_logging(struct kvm_vcpu *vcpu);
 
 static inline u8 vmx_get_rvi(void)
 {