1 files changed, 31 insertions, 13 deletions

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 0ae7f913d782..1597dd0b0cc6 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2126,23 +2126,41 @@ static bool set_target_expiration(struct kvm_lapic *apic, u32 count_reg)
 
 static void advance_periodic_target_expiration(struct kvm_lapic *apic)
 {
+	struct kvm_timer *ktimer = &apic->lapic_timer;
 	ktime_t now = ktime_get();
 	u64 tscl = rdtsc();
 	ktime_t delta;
 
 	/*
-	 * Synchronize both deadlines to the same time source or
-	 * differences in the periods (caused by differences in the
-	 * underlying clocks or numerical approximation errors) will
-	 * cause the two to drift apart over time as the errors
-	 * accumulate.
+	 * Use kernel time as the time source for both the hrtimer deadline and
+	 * TSC-based deadline so that they stay synchronized.  Computing each
+	 * deadline independently will cause the two deadlines to drift apart
+	 * over time as differences in the periods accumulate, e.g. due to
+	 * differences in the underlying clocks or numerical approximation errors.
 	 */
-	apic->lapic_timer.target_expiration =
-		ktime_add_ns(apic->lapic_timer.target_expiration,
-				apic->lapic_timer.period);
-	delta = ktime_sub(apic->lapic_timer.target_expiration, now);
-	apic->lapic_timer.tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
-		nsec_to_cycles(apic->vcpu, delta);
+	ktimer->target_expiration = ktime_add_ns(ktimer->target_expiration,
+						 ktimer->period);
+
+	/*
+	 * If the new expiration is in the past, e.g. because userspace stopped
+	 * running the VM for an extended duration, then force the expiration
+	 * to "now" and don't try to play catch-up with the missed events.  KVM
+	 * will only deliver a single interrupt regardless of how many events
+	 * are pending, i.e. restarting the timer with an expiration in the
+	 * past will do nothing more than waste host cycles, and can even lead
+	 * to a hard lockup in extreme cases.
+	 */
+	if (ktime_before(ktimer->target_expiration, now))
+		ktimer->target_expiration = now;
+
+	/*
+	 * Note, ensuring the expiration isn't in the past also prevents delta
+	 * from going negative, which could cause the TSC deadline to become
+	 * excessively large due to it an unsigned value.
+	 */
+	delta = ktime_sub(ktimer->target_expiration, now);
+	ktimer->tscdeadline = kvm_read_l1_tsc(apic->vcpu, tscl) +
+			      nsec_to_cycles(apic->vcpu, delta);
 }
 
 static void start_sw_period(struct kvm_lapic *apic)
@@ -2970,9 +2988,9 @@ static enum hrtimer_restart apic_timer_fn(struct hrtimer *data)
 
 	apic_timer_expired(apic, true);
 
-	if (lapic_is_periodic(apic)) {
+	if (lapic_is_periodic(apic) && !WARN_ON_ONCE(!apic->lapic_timer.period)) {
 		advance_periodic_target_expiration(apic);
-		hrtimer_add_expires_ns(&ktimer->timer, ktimer->period);
+		hrtimer_set_expires(&ktimer->timer, ktimer->target_expiration);
 		return HRTIMER_RESTART;
 	} else
 		return HRTIMER_NORESTART;