linux-kernel - Re: [PATCH 1/2] x86: fix keeping track of AMD C1E

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Date:	Fri, 16 Jul 2010 09:22:44 +0200
From:	Borislav Petkov <borislav.petkov@....com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Michal Schmidt <mschmidt@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Herrmann3, Andreas" <Andreas.Herrmann3@....com>,
	Shaohua Li <shaohua.li@...el.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/2] x86: fix keeping track of AMD C1E

From: "H. Peter Anvin" <hpa@...or.com>
Date: Thu, Jul 15, 2010 at 05:56:40PM -0400

> >> This is a change of semantics from an AND to an OR across CPUs...
> >
> > You mean the c1e_detected variable and the CPUID flag, right? Well,
> > frankly and if I'm not missing anything, we actually only need to track
> > when either bits [27,28] get set in that MSR - MSR_K8_INT_PENDING_MSG -
> > in order to do timer broadcast.
> >
> > And strictly speaking, we don't need a variable for that at all (nor a
> > synthetic CPUID flag, for that matter) - we can simply read the MSR as
> > much as we'd like after we've detected that this CPU supports C1E.
> >
> > But having the value cached is faster and doesn't enlarge checking
> > code in acpi_processor_cstate_check().
> >
> > I think the reason for adding the syntetic cpuid flag is only to
> > communicate to the ACPI processor module that we don't support deeper
> > C-states on a C1E machine, see a8d6829044901a67732904be5f1eacdf8539604f.
> > So we don't strictly need it and we can only export c1e_detected to the
> > rest for simplicity.
> >
> 
> No, the difference between using a separate variable and the CPU feature 
> bit is that CPU feature bit is ANDed across all CPUs, whereas this 
> variable is set if it is set on *any* CPU.

... and that's ok because the MSR bits get set on all cores after BIOS
turns on C1E. Let me verify this though.

-- 
Regards/Gruss,
Boris.

Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach
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Registration: Dornach, Gemeinde Aschheim, Landkreis Muenchen
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