diff options
authorSean Christopherson <>2019-04-17 10:15:31 -0700
committerPaolo Bonzini <>2019-04-18 18:53:18 +0200
commit57bf67e73ce9bcce2258890f5abf2adf5f619f1a (patch)
parentda66761c2d93a46270d69001abb5692717495a68 (diff)
KVM: lapic: Disable timer advancement if adaptive tuning goes haywire
To minimize the latency of timer interrupts as observed by the guest, KVM adjusts the values it programs into the host timers to account for the host's overhead of programming and handling the timer event. Now that the timer advancement is automatically tuned during runtime, it's effectively unbounded by default, e.g. if KVM is running as L1 the advancement can measure in hundreds of milliseconds. Disable timer advancement if adaptive tuning yields an advancement of more than 5000ns, as large advancements can break reasonable assumptions of the guest, e.g. that a timer configured to fire after 1ms won't arrive on the next instruction. Although KVM busy waits to mitigate the case of a timer event arriving too early, complications can arise when shifting the interrupt too far, e.g. kvm-unit-test's vmx.interrupt test will fail when its "host" exits on interrupts as KVM may inject the INTR before the guest executes STI+HLT. Arguably the unit test is "broken" in the sense that delaying a timer interrupt by 1ms doesn't technically guarantee the interrupt will arrive after STI+HLT, but it's a reasonable assumption that KVM should support. Furthermore, an unbounded advancement also effectively unbounds the time spent busy waiting, e.g. if the guest programs a timer with a very large delay. 5000ns is a somewhat arbitrary threshold. When running on bare metal, which is the intended use case, timer advancement is expected to be in the general vicinity of 1000ns. 5000ns is high enough that false positives are unlikely, while not being so high as to negatively affect the host's performance/stability. Note, a future patch will enable userspace to disable KVM's adaptive tuning, which will allow priveleged userspace will to specifying an advancement value in excess of this arbitrary threshold in order to satisfy an abnormal use case. Cc: Liran Alon <> Cc: Wanpeng Li <> Cc: Fixes: 3b8a5df6c4dc6 ("KVM: LAPIC: Tune lapic_timer_advance_ns automatically") Signed-off-by: Sean Christopherson <> Signed-off-by: Paolo Bonzini <>
1 files changed, 4 insertions, 0 deletions
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 6cb990dbc15a..bbe1402309e3 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1522,6 +1522,10 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
lapic_timer_advance_adjust_done = true;
+ if (unlikely(lapic_timer_advance_ns > 5000)) {
+ lapic_timer_advance_ns = 0;
+ lapic_timer_advance_adjust_done = true;
+ }