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

Jason Wang
2010-06-16T08:10:52Z

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.
>
>
Is there any method to do the compensation on host TSC when recovering
from C-state? It should be simpler than do it on guest TSC.
> 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)
>