On 06/15/2010 10:34 AM, Zachary Amsden wrote:
When CPUs with unstable TSCs enter deep C-state, TSC may stop running. This causes us to require resynchronization. Since we can't tell when this may potentially happen, we assume the worst by forcing re-compensation for it at every point the VCPU task is descheduled.Signed-off-by: Zachary Amsden<zams...@redhat.com> --- arch/x86/kvm/x86.c | 11 +++++++++++ 1 files changed, 11 insertions(+), 0 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index c8289d0..618c435 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) { kvm_x86_ops->vcpu_put(vcpu); kvm_put_guest_fpu(vcpu); + vcpu->arch.last_host_tsc = native_read_tsc(); + + /* + * When potentially leaving a CPU with unstable TSCs, we risk + * that the CPU enters deep C-state. If it does, the TSC may + * go out of sync but we will not recalibrate because the test + * vcpu->cpu != cpu can not detect this condition. So set + * vcpu->cpu = -1 to force the recalibration above. + */ + if (check_tsc_unstable()) + vcpu->cpu = -1; }
That will cause us to miss a vmclear later on. Also it invalidates an invariant that the per-cpu list vcpus_on_cpu has all the vcpus with vcpu->cpu == cpu on this cpu (try saything that fast).
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