linux-kernel - Re: [RFC] x86: mtrr: Constrain WB MTRR to max phys mem prior to cleanup

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Message-ID: <20120929201107.GB8329@khazad-dum.debian.net>
Date:	Sat, 29 Sep 2012 17:11:07 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	"zhenzhong.duan@...cle.com" <zhenzhong.duan@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] x86: mtrr: Constrain WB MTRR to max phys mem prior to
 cleanup

On Sat, 29 Sep 2012, H. Peter Anvin wrote:
> PAT support are lacking only in the Pentium Pro and Pentium II.  Sorry, if
> you're using crap that old, you don't get to screw up the kernel for
> everyone else.

PAT is blacklisted for x86_model < 15 on Intel, which covers a lot
more boxes than p-pro and pII.  There is a comment on kernel/cpu/intel.c:

        /*
         * There is a known erratum on Pentium III and Core Solo
         * and Core Duo CPUs.
         * " Page with PAT set to WC while associated MTRR is UC
         *   may consolidate to UC "
         * Because of this erratum, it is better to stick with
         * setting WC in MTRR rather than using PAT on these CPUs.
         *
         * Enable PAT WC only on P4, Core 2 or later CPUs.
         */
        if (c->x86 == 6 && c->x86_model < 15)
                clear_cpu_cap(c, X86_FEATURE_PAT);

Intel doesn't make it easy to get all processor specification updates at
once so that I could hunt down every processor which acknowledges the
existence of that errata before replying, so I will assume for the moment
that the comment is mostly correct.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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