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

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Date:	Mon, 29 Oct 2007 23:48:52 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Zachary Amsden <zach@...are.com>
Cc:	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	rusty@...tcorp.com.au, jeremy@...p.org, --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


* 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.

	Ingo
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