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

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Message-ID: <4C8F3FF3.9080803@redhat.com>
Date:	Tue, 14 Sep 2010 11:27:15 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Zachary Amsden <zamsden@...hat.com>, kvm@...r.kernel.org,
	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

  On 09/14/2010 11:10 AM, Jan Kiszka wrote:
> 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?
>
>

Most likely, time went backwards, and some 'future - past' calculation 
resulted in a negative sleep value which was then interpreted as 
unsigned and resulted in a 2342525634 year sleep.

Does your guest use kvmclock, tsc, or some other time source?

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

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