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

[Jason Wang]

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@xxxxxxxxxx>
> ---
>  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)
>   

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