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    Subject[PATCH 04/17] Fix deep C-state TSC desynchronization
    Date
    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;
    }

    static int is_efer_nx(void)
    --
    1.7.1


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