linux-kernel - Re: [PATCH] raise tsc clocksource rating

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Message-ID: <47266431.1010500@goop.org>
Date:	Mon, 29 Oct 2007 15:52:33 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Zachary Amsden <zach@...are.com>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	rusty@...tcorp.com.au, --cc@...hat.com, avi@...amnet.com,
	kvm-devel@...ts.sourceforge.net,
	Glauber de Oliveira Costa <glauber@....localdomain>,
	Dan Hecht <dhecht@...are.com>,
	Garrett Smith <garrett@...are.com>
Subject: Re: [PATCH] raise tsc clocksource rating

Ingo Molnar wrote:
> * Zachary Amsden <zach@...are.com> wrote:
>
>   
>> On Mon, 2007-10-29 at 20:10 -0300, Glauber de Oliveira Costa wrote:
>>     
>>> From: Glauber de Oliveira Costa <glauber@....localdomain>
>>>
>>> tsc is very good time source (when it does not have drifts, does not
>>> change it's frequency, i.e. when it works), so it should have its rating
>>> raised to a value greater than, or equal 400.
>>>
>>> Since it's being a tendency among paravirt clocksources to use values
>>> around 400, we should declare tsc as even better: So we use 500.
>>>       
>> Why is the TSC better than a paravirt clocksource?  In our case this 
>> is definitely inaccurate.  Paravirt clocksources should be preferred 
>> to TSC, and both must be made available in hardware for platforms 
>> which do not support paravirt.
>>     
>
> if it's inaccurate why are you exposing it to the guest then? Native 
> only uses the TSC if it's safe and accurate to do so.
>   

It is used as part of the Xen clocksource as a short term extrapolator,
with correction parameters supplied by the hypervisor.  It should never
be used directly.

    J
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