Messages in this thread | | | Date | Mon, 14 Jun 2010 22:14:02 -1000 | From | Zachary Amsden <> | Subject | Re: [PATCH 04/17] Fix deep C-state TSC desynchronization |
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On 06/14/2010 10:09 PM, Avi Kivity wrote: > 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<zamsden@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). >
Blasted SVM / VMX differences! There are other, less elegant ways to accomplish this task however.
Good catch, BTW.
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