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

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Message-ID: <5066680E.3090907@zytor.com>
Date:	Fri, 28 Sep 2012 20:16:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Peter Hurley <peter@...leysoftware.com>
CC:	"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 09/28/2012 10:37 AM, Peter Hurley wrote:
> 
> An interesting side note: more recent revisions of this BIOS (rev. A11)
> report one less variable MTRR (so, IA32_MTRRCAP is writable?)
> 
>>> However, the right way to fix that is to use the PAT interfaces, which 
>>> doesn't have this drawback -- then MTRR cleanup becomes entirely 
>>> superfluous and the problem goes away.
>> Do you mean disable MTRR totally here?
> 
> Well, since PAT entries marked WC override all MTRR settings, whatever
> the BIOS set the variable MTRRs to becomes irrelevant, so not disabled
> but rather ignored.
> 

The whole point is that the display stuff should not use MTRR, but
rather use PAT to provide WC.  Then we don't need to "clean up" the
BIOS-set MTRRs.

	-hpa

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