[PATCH v3 2/3] KVM: LAPIC: Keep timer running when switching between one-shot and periodic mode

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpeng.li@hotmail.com>

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>
---
 arch/x86/kvm/lapic.c | 45 +++++++++++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 6723e2c..df31048 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1301,18 +1301,21 @@ static void update_divide_count(struct kvm_lapic *apic)
 				   apic->divide_count);
 }
 
-static void apic_update_lvtt(struct kvm_lapic *apic)
+static bool apic_update_lvtt(struct kvm_lapic *apic)
 {
 	u32 timer_mode = kvm_lapic_get_reg(apic, APIC_LVTT) &
 			apic->lapic_timer.timer_mode_mask;
 
 	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);
+		return true;
 	}
+	return false;
 }
 
 static void apic_timer_expired(struct kvm_lapic *apic)
@@ -1433,11 +1436,12 @@ static void start_sw_period(struct kvm_lapic *apic)
 		HRTIMER_MODE_ABS_PINNED);
 }
 
-static bool set_target_expiration(struct kvm_lapic *apic)
+static bool set_target_expiration(struct kvm_lapic *apic, bool timer_update)
 {
-	ktime_t now;
-	u64 tscl = rdtsc();
+	ktime_t now, remaining;
+	u64 tscl = rdtsc(), delta;
 
+	/* Calculate the next time the timer should trigger an interrupt */
 	now = ktime_get();
 	apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
 		* APIC_BUS_CYCLE_NS * apic->divide_count;
@@ -1473,9 +1477,21 @@ static bool set_target_expiration(struct kvm_lapic *apic)
 		   ktime_to_ns(ktime_add_ns(now,
 				apic->lapic_timer.period)));
 
+	if (!timer_update)
+		delta = apic->lapic_timer.period;
+	else {
+		remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
+		if (ktime_to_ns(remaining) < 0)
+			remaining = 0;
+		delta = mod_64(ktime_to_ns(remaining), apic->lapic_timer.period);
+	}
+
+	if (!delta)
+		return false;
+
 	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
-		nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
-	apic->lapic_timer.target_expiration = ktime_add_ns(now, apic->lapic_timer.period);
+		nsec_to_cycles(apic->vcpu, delta);
+	apic->lapic_timer.target_expiration = ktime_add_ns(now, delta);
 
 	return true;
 }
@@ -1612,12 +1628,12 @@ void kvm_lapic_restart_hv_timer(struct kvm_vcpu *vcpu)
 	restart_apic_timer(apic);
 }
 
-static void start_apic_timer(struct kvm_lapic *apic)
+static void start_apic_timer(struct kvm_lapic *apic, bool timer_update)
 {
 	atomic_set(&apic->lapic_timer.pending, 0);
 
 	if ((apic_lvtt_period(apic) || apic_lvtt_oneshot(apic))
-	    && !set_target_expiration(apic))
+	    && !set_target_expiration(apic, timer_update))
 		return;
 
 	restart_apic_timer(apic);
@@ -1729,7 +1745,8 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 			val |= APIC_LVT_MASKED;
 		val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
 		kvm_lapic_set_reg(apic, APIC_LVTT, val);
-		apic_update_lvtt(apic);
+		if (apic_update_lvtt(apic) && !apic_lvtt_tscdeadline(apic))
+			start_apic_timer(apic, true);
 		break;
 
 	case APIC_TMICT:
@@ -1738,7 +1755,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 
 		hrtimer_cancel(&apic->lapic_timer.timer);
 		kvm_lapic_set_reg(apic, APIC_TMICT, val);
-		start_apic_timer(apic);
+		start_apic_timer(apic, false);
 		break;
 
 	case APIC_TDCR:
@@ -1872,7 +1889,7 @@ void kvm_set_lapic_tscdeadline_msr(struct kvm_vcpu *vcpu, u64 data)
 
 	hrtimer_cancel(&apic->lapic_timer.timer);
 	apic->lapic_timer.tscdeadline = data;
-	start_apic_timer(apic);
+	start_apic_timer(apic, false);
 }
 
 void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8)
@@ -2238,7 +2255,7 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
 	apic_update_lvtt(apic);
 	apic_manage_nmi_watchdog(apic, kvm_lapic_get_reg(apic, APIC_LVT0));
 	update_divide_count(apic);
-	start_apic_timer(apic);
+	start_apic_timer(apic, false);
 	apic->irr_pending = true;
 	apic->isr_count = vcpu->arch.apicv_active ?
 				1 : count_vectors(apic->regs + APIC_ISR);
-- 
2.7.4