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    SubjectRe: [PATCH 04/17] Fix deep C-state TSC desynchronization
    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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