[PATCH v2 3/4] KVM: LAPIC: Apply change to TDCR right away to the timer

[Wanpeng Li <kernellwp@gmail.com>]

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

The description in the Intel SDM of how the divide configuration
register is used: "The APIC timer frequency will be the processor's bus
clock or core crystal clock frequency divided by the value specified in
the divide configuration register."

Observation of baremetal shown that when the TDCR is change, the TMCCT
does not change or make a big jump in value, but the rate at which it
count down change.

The patch update the emulation to APIC timer to so that a change to the
divide configuration would be reflected in the value of the counter and
when the next interrupt is triggered.

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 | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 946c11b..6bafd06 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1432,7 +1432,7 @@ static void start_sw_period(struct kvm_lapic *apic)
 		HRTIMER_MODE_ABS_PINNED);
 }
 
-static bool set_target_expiration(struct kvm_lapic *apic, bool timer_update)
+static bool set_target_expiration(struct kvm_lapic *apic, bool timer_update, uint32_t old_divisor)
 {
 	ktime_t now, remaining;
 	u64 tscl = rdtsc(), delta;
@@ -1440,7 +1440,7 @@ static bool set_target_expiration(struct kvm_lapic *apic, bool timer_update)
 	/* 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;
+		* APIC_BUS_CYCLE_NS * old_divisor;
 
 	if (!apic->lapic_timer.period)
 		return false;
@@ -1485,6 +1485,12 @@ static bool set_target_expiration(struct kvm_lapic *apic, bool timer_update)
 	if (!delta)
 		return false;
 
+	if (apic->divide_count != old_divisor) {
+		apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
+			* APIC_BUS_CYCLE_NS * apic->divide_count;
+		delta = delta * apic->divide_count / old_divisor;
+	}
+
 	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
 		nsec_to_cycles(apic->vcpu, delta);
 	apic->lapic_timer.target_expiration = ktime_add_ns(now, delta);
@@ -1624,12 +1630,13 @@ void kvm_lapic_restart_hv_timer(struct kvm_vcpu *vcpu)
 	restart_apic_timer(apic);
 }
 
-static void start_apic_timer(struct kvm_lapic *apic, bool timer_update)
+static void start_apic_timer(struct kvm_lapic *apic, bool timer_update,
+				uint32_t old_divisor)
 {
 	atomic_set(&apic->lapic_timer.pending, 0);
 
 	if ((apic_lvtt_period(apic) || apic_lvtt_oneshot(apic))
-	    && !set_target_expiration(apic, timer_update))
+	    && !set_target_expiration(apic, timer_update, old_divisor))
 		return;
 
 	restart_apic_timer(apic);
@@ -1745,7 +1752,7 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 		val &= (apic_lvt_mask[0] | apic->lapic_timer.timer_mode_mask);
 		kvm_lapic_set_reg(apic, APIC_LVTT, val);
 		if (apic_update_lvtt(apic) && !apic_lvtt_tscdeadline(apic))
-			start_apic_timer(apic, true);
+			start_apic_timer(apic, true, apic->divide_count);
 		break;
 
 	case APIC_TMICT:
@@ -1754,16 +1761,20 @@ 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, false);
+		start_apic_timer(apic, false, apic->divide_count);
 		break;
 
-	case APIC_TDCR:
+	case APIC_TDCR: {
+		uint32_t current_divisor = apic->divide_count;
+
 		if (val & 4)
 			apic_debug("KVM_WRITE:TDCR %x\n", val);
 		kvm_lapic_set_reg(apic, APIC_TDCR, val);
 		update_divide_count(apic);
+		hrtimer_cancel(&apic->lapic_timer.timer);
+		start_apic_timer(apic, true, current_divisor);
 		break;
-
+	}
 	case APIC_ESR:
 		if (apic_x2apic_mode(apic) && val != 0) {
 			apic_debug("KVM_WRITE:ESR not zero %x\n", val);
@@ -1888,7 +1899,7 @@ 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, false);
+	start_apic_timer(apic, false, apic->divide_count);
 }
 
 void kvm_lapic_set_tpr(struct kvm_vcpu *vcpu, unsigned long cr8)
@@ -2254,7 +2265,7 @@ 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, false);
+	start_apic_timer(apic, false, apic->divide_count);
 	apic->irr_pending = true;
 	apic->isr_count = vcpu->arch.apicv_active ?
 				1 : count_vectors(apic->regs + APIC_ISR);
-- 
2.7.4