From: Jesse Barnes <jbarnes@virtuousgeek.org>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: [PATCH] trim memory not covered by WB MTRRs
Date: Tue, 26 Jun 2007 08:07:45 -0700
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Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
       Jesse Barnes <jesse.barnes@intel.com>, linux-kernel@vger.kernel.org,
       akpm@linux-foundation.org, Justin Piszcz <jpiszcz@lucidpixels.com>,
       Yinghai Lu <yhlu.kernel@gmail.com>
References: <200706251434.43863.jesse.barnes@intel.com> <200706252030.52611.jbarnes@virtuousgeek.org> <20070626150348.GB7877@one.firstfloor.org>
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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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