== SRU Justification ==
This patch fixes a regression introduced by mainline commit
8003c9ae204e in v4.10. Without this new patch, OpenBSD guests encounter
timing issues when running under bionic kernel.
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.
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.
+ 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);
On Fri, Jun 29, 2018 at 09:22:52AM -0400, Joseph Salisbury wrote:
> BugLink: http://bugs.launchpad.net/bugs/1778486 >
> == SRU Justification ==
> This patch fixes a regression introduced by mainline commit
> 8003c9ae204e in v4.10. Without this new patch, OpenBSD guests encounter
> timing issues when running under bionic kernel.
> See also:
> https://www.spinics.net/lists/kvm/msg161311.html >
> == Fix ==
> d8f2f498d9ed ("x86/kvm: fix LAPIC timer drift when guest uses periodic mode")
> == Regression Potential ==
> Low. This fixes a current regression.
> == Test Case ==
> A test kernel was built with this patch and tested by the original bug reporter.
> The bug reporter states the test kernel resolved the bug.