linux-kernel - Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

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Message-ID: <4C8F3C03.50306@siemens.com>
Date:	Tue, 14 Sep 2010 11:10:27 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Glauber Costa <glommer@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org
Subject: Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

Am 20.08.2010 10:07, 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 |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7fc4a55..52b6c21 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1866,7 +1866,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	}
>  
>  	kvm_x86_ops->vcpu_load(vcpu, cpu);
> -	if (unlikely(vcpu->cpu != cpu)) {
> +	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
>  		/* Make sure TSC doesn't go backwards */
>  		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
>  				native_read_tsc() - vcpu->arch.last_host_tsc;

For yet unknown reason, this commit breaks Linux guests here if they are
started with only a single VCPU. They hang during boot, obviously no
longer receiving interrupts.

I'm using kvm-kmod against a 2.6.34 host kernel, so this may be a side
effect of the wrapping, though I cannot imagine how.

Anyone any ideas?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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