KVM: x86: Optimize NMI watchdog delivery

As suggested by Avi, this patch introduces a counter of VCPUs that have
LVT0 set to NMI mode. Only if the counter > 0, we push the PIT ticks via
all LAPIC LVT0 lines to enable NMI watchdog support.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -361,6 +361,7 @@ struct kvm_arch{
 	struct kvm_ioapic *vioapic;
 	struct kvm_pit *vpit;
 	struct hlist_head irq_ack_notifier_list;
+	int vapics_in_nmi_mode;
 
 	int round_robin_prev_vcpu;
 	unsigned int tss_addr;

arch/x86/kvm/i8254.c

@@ -620,11 +620,12 @@ static void __inject_pit_timer_intr(struct kvm *kvm)
 	 * LVT0 to NMI delivery. Other PIC interrupts are just sent to
 	 * VCPU0, and only if its LVT0 is in EXTINT mode.
 	 */
-	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
-		vcpu = kvm->vcpus[i];
-		if (vcpu)
-			kvm_apic_nmi_wd_deliver(vcpu);
-	}
+	if (kvm->arch.vapics_in_nmi_mode > 0)
+		for (i = 0; i < KVM_MAX_VCPUS; ++i) {
+			vcpu = kvm->vcpus[i];
+			if (vcpu)
+				kvm_apic_nmi_wd_deliver(vcpu);
+		}
 }
 
 void kvm_inject_pit_timer_irqs(struct kvm_vcpu *vcpu)

arch/x86/kvm/lapic.c

@@ -130,6 +130,11 @@ static inline int apic_lvtt_period(struct kvm_lapic *apic)
 	return apic_get_reg(apic, APIC_LVTT) & APIC_LVT_TIMER_PERIODIC;
 }
 
+static inline int apic_lvt_nmi_mode(u32 lvt_val)
+{
+	return (lvt_val & (APIC_MODE_MASK | APIC_LVT_MASKED)) == APIC_DM_NMI;
+}
+
 static unsigned int apic_lvt_mask[APIC_LVT_NUM] = {
 	LVT_MASK | APIC_LVT_TIMER_PERIODIC,	/* LVTT */
 	LVT_MASK | APIC_MODE_MASK,	/* LVTTHMR */
@@ -672,6 +677,20 @@ static void start_apic_timer(struct kvm_lapic *apic)
 					apic->timer.period)));
 }
 
+static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
+{
+	int nmi_wd_enabled = apic_lvt_nmi_mode(apic_get_reg(apic, APIC_LVT0));
+
+	if (apic_lvt_nmi_mode(lvt0_val)) {
+		if (!nmi_wd_enabled) {
+			apic_debug("Receive NMI setting on APIC_LVT0 "
+				   "for cpu %d\n", apic->vcpu->vcpu_id);
+			apic->vcpu->kvm->arch.vapics_in_nmi_mode++;
+		}
+	} else if (nmi_wd_enabled)
+		apic->vcpu->kvm->arch.vapics_in_nmi_mode--;
+}
+
 static void apic_mmio_write(struct kvm_io_device *this,
 			    gpa_t address, int len, const void *data)
 {
@@ -753,9 +772,7 @@ static void apic_mmio_write(struct kvm_io_device *this,
 		break;
 
 	case APIC_LVT0:
-		if (val == APIC_DM_NMI)
-			apic_debug("Receive NMI setting on APIC_LVT0 "
-				"for cpu %d\n", apic->vcpu->vcpu_id);
+		apic_manage_nmi_watchdog(apic, val);
 	case APIC_LVTT:
 	case APIC_LVTTHMR:
 	case APIC_LVTPC: