linux-kernel - Re: [PATCH] ensure guest's kvmclock never goes backwards when TSC jumps backward

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Date:	Wed, 16 Jul 2014 16:55:37 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH] ensure guest's kvmclock never goes backwards when TSC
 jumps backward

Il 16/07/2014 16:51, Igor Mammedov ha scritto:
>>>> I'm not sure that a per-CPU value is enough; your patch can make
>>>> the problem much less frequent of course, but I'm not sure neither
>>>> detection nor correction are 100% reliable.  Your addition is
>>>> basically a faster but less reliable version of the last_value
>>>> logic.
>>>
>>> How is it less reliable than last_value logic?
>>
>> Suppose CPU 1 is behind by 3 nanoseconds
>>
>>     CPU 0                      CPU 1
>>     time = 100                              (at time 100)
>>                                time = 99    (at time 102)
>>     time = 104                              (at time 104)
>>                                time = 105   (at time 108)
>>
>> Your patch will not detect this.
> Is it possible for each cpu to have it's own time?

Yes, that's one of the reasons for TSC not to be stable (it could also 
happen simply because the value of TSC_ADJUST MSR is bogus).

> tsc_wrap_test starts to fail almost imediately,
>
> I'll check how much tries it takes to fail for the first time, if it is
> not too much I guess we could add check to check_system_tsc_reliable().

Thanks!

Paolo
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