linux-kernel - Re: [PATCH -v3] x86, tsc: Remove CPU frequency calibration on AMD

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Message-ID: <20100826071948.GA980@aftab>
Date:	Thu, 26 Aug 2010 09:19:48 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Alok Kataria <akataria@...are.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Greg KH <gregkh@...e.de>, "greg@...ah.com" <greg@...ah.com>,
	"ksrinivasan@...ell.com" <ksrinivasan@...ell.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -v3] x86, tsc: Remove CPU frequency calibration on AMD

From: Alok Kataria <akataria@...are.com>
Date: Wed, Aug 25, 2010 at 06:33:08PM -0400

> 
> On Wed, 2010-08-25 at 09:28 -0700, Borislav Petkov wrote:
> > 6b37f5a20c0e5c334c010a587058354215433e92 introduced the CPU frequency
> > calibration code for AMD CPUs whose TSCs didn't increment with the
> > core's P0 frequency. From F10h, revB onward, however, the TSC increment
> > rate is denoted by MSRC001_0015[24] and when this bit is set (which
> > should be done by the BIOS) the TSC increments with the P0 frequency
> > so the calibration is not needed and booting can be a couple of mcecs
> > faster on those machines.
> > 
> > Besides, there should be virtually no machines out there which don't
> > have this bit set, therefore this calibration can be safely removed. It
> > is a shaky hack anyway since it assumes implicitly that the core is in
> > P0 when BIOS hands off to the OS, which might not always be the case.
> 
> Nice... this works for us too, we don't muck with that MSR bit either,
> its directly passed as is from the h/w to the guest. So no additional
> changes would be needed for us with this.

That's nice, KVM appears to not hit it either due to unsynchronized
TSCs.

> Hope that the 3rd time is a charm for you too :)

Yeah, I think it is :). Sorry for taking so long but removing code which
is actively executed from the kernel is not such a light decision. But
the hw guys made sure that this bit is always set so we don't need the
calibration. It wouldn't work in all cases anyway (hint: boosted cores).

Thanks.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach
General Managers: Alberto Bozzo, Andrew Bowd
Registration: Dornach, Gemeinde Aschheim, Landkreis Muenchen
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