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

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Date:	Tue, 14 Sep 2010 12:40:37 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Zachary Amsden <zamsden@...hat.com>,
	"kvm@...r.kernel.org" <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" <linux-kernel@...r.kernel.org>
Subject: Re: [KVM timekeeping 10/35] Fix deep C-state TSC desynchronization

Am 14.09.2010 11:27, Avi Kivity wrote:
>   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.

Looks like that's the case on first glance at the apic state.

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

A kernel that has kvmclock support even hangs in SMP mode. The others
pick hpet or acpi_pm. TSC is considered unstable.

Jan

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