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

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Message-ID: <47271D4A.8030707@redhat.com>
Date:	Tue, 30 Oct 2007 10:02:18 -0200
From:	Glauber de Oliveira Costa <gcosta@...hat.com>
To:	Zachary Amsden <zach@...are.com>
CC:	Ingo Molnar <mingo@...e.hu>, 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

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Zachary Amsden escreveu:
> On Tue, 2007-10-30 at 00:02 +0100, Ingo Molnar wrote:
>> * Zachary Amsden <zach@...are.com> wrote:
> 
>>> Not every guest support paravirt, but for correctness, all guests 
>>> require TSC, which must be exposed all the way up to userspace, no 
>>> matter what the efficiency or accuracy may be.
>> but if there's a perfect TSC available (there is such hardware) then the 
>> TSC _is_ the best clocksource. Paravirt now turns it off unconditionally 
>> in essence.
> 
> No, if no paravirt clocksource is detected, nothing can override the
> perfect TSC hardware clocksource rating of 400.  And if a paravirt
> clocksource is detected, it is always better than TSC.

Why always? tsc is the best possible alternative the _platform_ can
provide. So the paravirt clocksource will be at best, as good as tsc.
And if it is the case: why bother with communication mechanisms of all
kinds, calculations, etc, if we can just read the tsc?

Noting again: If the tsc does not arrive accurate to the guest, it will
fail the tsc clocksource tests. So it will be rated zero anyway



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