x86/kvm: fix LAPIC timer drift when guest uses periodic mode

This change “x86/kvm: fix LAPIC timer drift when guest uses periodic mode” (commit d8f2f49) in Linux kernel is authored by David Vrabel <david.vrabel [at] nutanix.com> on Fri May 18 16:55:46 2018 +0100.

Description of "x86/kvm: fix LAPIC timer drift when guest uses periodic mode"

The change “x86/kvm: fix LAPIC timer drift when guest uses periodic mode” introduces changes as follows.

x86/kvm: fix LAPIC timer drift when guest uses periodic mode

Since 4.10, commit 8003c9ae204e (KVM: LAPIC: add APIC Timer
periodic/oneshot mode VMX preemption timer support), guests using
periodic LAPIC timers (such as FreeBSD 8.4) would see their timers
drift significantly over time.

Differences in the underlying clocks and numerical errors means the
periods of the two timers (hv and sw) are not the same. This
difference will accumulate with every expiry resulting in a large
error between the hv and sw timer.

This means the sw timer may be running slow when compared to the hv
timer. When the timer is switched from hv to sw, the now active sw
timer will expire late. The guest VCPU is reentered and it switches to
using the hv timer. This timer catches up, injecting multiple IRQs
into the guest (of which the guest only sees one as it does not get to
run until the hv timer has caught up) and thus the guest's timer rate
is low (and becomes increasing slower over time as the sw timer lags
further and further behind).

I believe a similar problem would occur if the hv timer is the slower
one, but I have not observed this.

Fix this by synchronizing the deadlines for both timers to the same
time source on every tick. This prevents the errors from accumulating.

Fixes: 8003c9ae204e21204e49816c5ea629357e283b06
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: David Vrabel <david.vrabel@nutanix.com>
Cc: stable@vger.kernel.org
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

Linux kernel releases containing commit d8f2f49

The Linux kernel releases containing this commit are as follows.

Linux kernel code changes from "x86/kvm: fix LAPIC timer drift when guest uses periodic mode"

There are 16 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/kvm/lapic.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b74c9c1405b9..3773c4625114 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1522,11 +1522,23 @@ static bool set_target_expiration(struct kvm_lapic *apic)
 static void advance_periodic_target_expiration(struct kvm_lapic *apic)
-	apic->lapic_timer.tscdeadline +=
-		nsec_to_cycles(apic->vcpu, apic->lapic_timer.period);
+	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.
+	 */
 	apic->lapic_timer.target_expiration =
+	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);
 static void start_sw_period(struct kvm_lapic *apic)

The commit for this change in Linux stable tree is d8f2f49 (patch).

Last modified: 2020/02/09