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

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Date:	Tue, 15 Jun 2010 11:09:35 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	mtosatti@...hat.com, glommer@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/17] Fix deep C-state TSC desynchronization

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@...hat.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).

-- 
error compiling committee.c: too many arguments to function

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