linux-kernel - Re: Clock jumps

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Message-ID: <4BFEF0A5.8040301@redhat.com>
Date:	Thu, 27 May 2010 12:22:29 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Bernhard Schmidt <berni@...kenwald.de>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Clock jumps

On 05/27/2010 12:12 PM, Bernhard Schmidt wrote:
> On 27.05.2010 23:53, Zachary Amsden wrote:
>
> Hello Zachary,
>
>> I have server side fixes for this kvm-clock which seem to give me a
>> stable clock on this machine, but for true SMP stability, you will need
>> Glauber's guest side changes to kvmclock as well. It is impossible to
>> guarantee strictly monotonic clocksource across multiple CPUs when
>> frequency is dynamically changing (and also because of the C1E idle
>> problems).
>
> Is all this relevant only when the host is on TSC? Because I have seen 
> these jumps when the host was on HPET and the guests were using 
> kvm-clock.

It doesn't matter what the host uses (although the host on TSC with 
unstable TSC can make things worse), tsc and kvmclock sources in the 
guest will be unstable regardless.
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