linux-kernel - Re: [PATCH 1/5] Add a global synchronization point for pvclock

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Message-ID: <4BCD7557.9090502@redhat.com>
Date:	Tue, 20 Apr 2010 12:35:19 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Zachary Amsden <zamsden@...hat.com>
Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock

On 04/20/2010 04:57 AM, Marcelo Tosatti wrote:
>
>> Marcelo can probably confirm it, but he has a nehalem with an appearently
>> very good tsc source. Even this machine warps.
>>
>> It stops warping if we only write pvclock data structure once and forget it,
>> (which only updated tsc_timestamp once), according to him.
>>      
> Yes. So its not as if the guest visible TSCs go out of sync (they don't
> on this machine Glauber mentioned, or even on a multi-core Core 2 Duo),
> but the delta calculation is very hard (if not impossible) to get right.
>
> The timewarps i've seen were in the 0-200ns range, and very rare (once
> every 10 minutes or so).
>    

Might be due to NMIs or SMIs interrupting the rdtsc(); ktime_get() 
operation which establishes the timeline.  We could limit it by having a 
loop doing rdtsc(); ktime_get(); rdtsc(); and checking for some bound, 
but it isn't worthwhile (and will break nested virtualization for 
sure).  Better to have the option to calibrate kvmclock just once on 
machines with X86_FEATURE_NONSTOP_TRULY_RELIABLE _TSC_HONESTLY.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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