linux-kernel - Re: [PATCH] trim memory not covered by WB MTRRs

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Message-Id: <200706260807.45362.jbarnes@virtuousgeek.org>
Date:	Tue, 26 Jun 2007 08:07:45 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jesse Barnes <jesse.barnes@...el.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Justin Piszcz <jpiszcz@...idpixels.com>,
	Yinghai Lu <yhlu.kernel@...il.com>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs

On Tuesday, June 26, 2007 8:03:48 am Andi Kleen wrote:
> On Mon, Jun 25, 2007 at 08:30:52PM -0700, Jesse Barnes wrote:
> > Oh, and FYI I've seen new systems with a default mapping type of WB, with
> > a few uncached holes for MMIO.  That means PAT will be absolutely
> > required on those systems if we want to use WC for the framebuffer or
> > other memory.
>
> Why? As long as there are still MTRRs free you could use them for
> WC even in this setup.

According to the manual, you can't use WC or WT MTRRs on top of a WB MTRR 
range.  Only UC MTRRs can override WB MTRRs, but PAT doesn't have this 
restriction.

Jesse

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