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

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Message-ID: <20071029225528.GA31293@elte.hu>
Date:	Mon, 29 Oct 2007 23:55:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> > 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.

that's totally broken then. You cannot create an SMP-safe monotonic 
clocksource via interpolation - native does not do it either. Good thing 
this problem got exposed, it needs to be fixed.

	Ingo
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