KVM: x86: detect LVTT changes under APICv

APIC-write VM exits are "trap-like": they save CS:RIP values for the
instruction after the write, and more importantly, the handler will
already see the new value in the virtual-APIC page.  This means that
apic_reg_write cannot use kvm_apic_get_reg to omit timer cancelation
when mode changes.

timer_mode_mask shouldn't be changing as it depends on cpuid.
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -1242,17 +1242,20 @@ static int apic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 
 		break;
 
-	case APIC_LVTT:
-		if ((kvm_apic_get_reg(apic, APIC_LVTT) &
-		    apic->lapic_timer.timer_mode_mask) !=
-		   (val & apic->lapic_timer.timer_mode_mask))
+	case APIC_LVTT: {
+		u32 timer_mode = val & apic->lapic_timer.timer_mode_mask;
+
+		if (apic->lapic_timer.timer_mode != timer_mode) {
+			apic->lapic_timer.timer_mode = timer_mode;
 			hrtimer_cancel(&apic->lapic_timer.timer);
+		}
 
 		if (!kvm_apic_sw_enabled(apic))
 			val |= APIC_LVT_MASKED;
 		val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
 		apic_set_reg(apic, APIC_LVTT, val);
 		break;
+	}
 
 	case APIC_TMICT:
 		if (apic_lvtt_tscdeadline(apic))
@@ -1483,6 +1486,7 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu)
 
 	for (i = 0; i < APIC_LVT_NUM; i++)
 		apic_set_reg(apic, APIC_LVTT + 0x10 * i, APIC_LVT_MASKED);
+	apic->lapic_timer.timer_mode = 0;
 	apic_set_reg(apic, APIC_LVT0,
 		     SET_APIC_DELIVERY_MODE(0, APIC_MODE_EXTINT));
 

arch/x86/kvm/lapic.h

@@ -11,6 +11,7 @@
 struct kvm_timer {
 	struct hrtimer timer;
 	s64 period; 				/* unit: ns */
+	u32 timer_mode;
 	u32 timer_mode_mask;
 	u64 tscdeadline;
 	atomic_t pending;			/* accumulated triggered timers */