KVM: LAPIC: Apply change to TDCR right away to the timer

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>
[Fixed some whitespace and added a check for negative delta and running
 timer. - Radim]
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/lapic.c

@@ -1456,6 +1456,30 @@ static void start_sw_period(struct kvm_lapic *apic)
 		HRTIMER_MODE_ABS_PINNED);
 }
 
+static void update_target_expiration(struct kvm_lapic *apic, uint32_t old_divisor)
+{
+	ktime_t now, remaining;
+	u64 ns_remaining_old, ns_remaining_new;
+
+	apic->lapic_timer.period = (u64)kvm_lapic_get_reg(apic, APIC_TMICT)
+		* APIC_BUS_CYCLE_NS * apic->divide_count;
+	limit_periodic_timer_frequency(apic);
+
+	now = ktime_get();
+	remaining = ktime_sub(apic->lapic_timer.target_expiration, now);
+	if (ktime_to_ns(remaining) < 0)
+		remaining = 0;
+
+	ns_remaining_old = ktime_to_ns(remaining);
+	ns_remaining_new = mul_u64_u32_div(ns_remaining_old,
+	                                   apic->divide_count, old_divisor);
+
+	apic->lapic_timer.tscdeadline +=
+		nsec_to_cycles(apic->vcpu, ns_remaining_new) -
+		nsec_to_cycles(apic->vcpu, ns_remaining_old);
+	apic->lapic_timer.target_expiration = ktime_add_ns(now, ns_remaining_new);
+}
+
 static bool set_target_expiration(struct kvm_lapic *apic)
 {
 	ktime_t now;
@@ -1748,13 +1772,21 @@ int kvm_lapic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)
 		start_apic_timer(apic);
 		break;
 
-	case APIC_TDCR:
+	case APIC_TDCR: {
+		uint32_t old_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);
+		if (apic->divide_count != old_divisor &&
+				apic->lapic_timer.period) {
+			hrtimer_cancel(&apic->lapic_timer.timer);
+			update_target_expiration(apic, old_divisor);
+			restart_apic_timer(apic);
+		}
 		break;
-
+	}
 	case APIC_ESR:
 		if (apic_x2apic_mode(apic) && val != 0) {
 			apic_debug("KVM_WRITE:ESR not zero %x\n", val);