KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode

If we take TSC-deadline mode timer out of the picture, the Intel SDM
does not say that the timer is disable when the timer mode is change,
either from one-shot to periodic or vice versa.

After this patch, the timer is no longer disarmed on change of mode, so
the counter (TMCCT) keeps counting down.

So what does a write to LVTT changes ? On baremetal, the change of mode
is probably taken into account only when the counter reach 0. When this
happen, LVTT is use to figure out if the counter should restard counting
down from TMICT (so periodic mode) or stop counting (if one-shot mode).

This patch is based on observation of the behavior of the APIC timer on
baremetal as well as check that they does not go against the description
written in the Intel SDM.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
[Fixed rate limiting of periodic timer.]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/lapic.c

@@ -1308,7 +1308,7 @@ static void limit_periodic_timer_frequency(struct kvm_lapic *apic)
 	 * interval, since the hrtimers are not throttled by the host
 	 * scheduler.
 	 */
-	if (apic_lvtt_period(apic)) {
+	if (apic_lvtt_period(apic) && apic->lapic_timer.period) {
 		s64 min_period = min_timer_period_us * 1000LL;
 
 		if (apic->lapic_timer.period < min_period) {
@@ -1329,10 +1329,12 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
 
 	if (apic->lapic_timer.timer_mode != timer_mode) {
 		if (apic_lvtt_tscdeadline(apic) != (timer_mode ==
-				APIC_LVT_TIMER_TSCDEADLINE))
+				APIC_LVT_TIMER_TSCDEADLINE)) {
 			kvm_lapic_set_reg(apic, APIC_TMICT, 0);
+			hrtimer_cancel(&apic->lapic_timer.timer);
+		}
 		apic->lapic_timer.timer_mode = timer_mode;
-		hrtimer_cancel(&apic->lapic_timer.timer);
+		limit_periodic_timer_frequency(apic);
 	}
 }