linux-kernel - Re: [RFC] CPUID usage for interaction between Hypervisors and Linux.

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Date:	Wed, 01 Oct 2008 18:21:56 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Zachary Amsden <zach@...are.com>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Alok Kataria <akataria@...are.com>,
	"avi@...hat.com" <avi@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Gerd Hoffmann <kraxel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Nakajima, Jun" <jun.nakajima@...el.com>,
	Daniel Hecht <dhecht@...are.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC] CPUID usage for interaction between	Hypervisors	and	Linux.

Zachary Amsden wrote:
> 
> I'm not suggesting using the nominal value.  I'm suggesting the
> measurement be done in the one and only place where there is perfect
> control of the system, the processor boot-strapping in the BIOS.
> 
> Only the platform designers themselves know the speed of the oscillator
> which is modulating the clock and so only they should be calibrating the
> speed of the TSC.
> 

No.  *Noone*, including the manufacturers, know the speed of the 
oscillator which is modulating the clock.  What you have to do is 
average over a timespan which is long enough that the SSM averages out 
(a relatively small fraction of a second.)

As for trusting the BIOS on this, that's a total joke.  Firmware vendors 
can't get the most basic details right.

> If this modulation really does alter the frequency by +/- 2% (seems high
> to me, but hey, I don't design motherboards), using an LFO, then
> basically all the calibration done in Linux is broken and has been for
> some time.  You can't calibrate only once, or risk being off by 2%, you
> can't calibrate repeatedly and take the fastest estimate, or you are off
> by 2%, and you can't calibrate repeatedly and take the average without
> risking SMI noise affecting the lowest clock speed measurement,
> contributing unknown error.

You have to calibrate over a sample interval long enough that the SSM 
averages out.

> Hmm.  Re-reading your e-mail, I see you are saying the nominal frequency
> may be off by 2% (and I easily believe that), not necessarily that the
> frequency modulation may be 2% (which I still think is high).  Does
> anyone know what the actual bounds on spread spectrum modulation are or
> how fast the clock is modulated?

No, I'm saying the frequency modulation may be up to 2%.  Typically it 
is something like [-2%,+0%].

	-hpa
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