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Date: Fri, 21 Aug 2009 23:43:34 -0600
From: Robert Hancock <hancockrwd@gmail.com>
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To: Volker Armin Hemmann <volkerarmin@googlemail.com>
CC: linux-kernel@vger.kernel.org
Subject: Re: amd64 + mtrr: only 3.1gb of 8gb are covered, kernel 2.6.30 and
 earlier.
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On 08/21/2009 10:14 AM, Volker Armin Hemmann wrote:
> Hi,
>
> I have an amd64 system with 8GB ram. Of these only 3.1GB are currently covered
> with mtrr:
> cat /proc/mtrr
> reg00: base=0x000000000 (    0MB), size= 2048MB, count=1: write-back
> reg01: base=0x080000000 ( 2048MB), size= 1024MB, count=1: write-back
> reg02: base=0x0c0000000 ( 3072MB), size=  128MB, count=1: write-back
>
> This is with a Phenom II X4. Mobo is an Asrock A770Crossfire with AMD 770
> chipset. Ram is 1066 ddr2 running at 800.
>
> Before that I had a X2 6000 where I had one more mtrr, covering additional
> 256mb with 6gb ram total.
>
> When I use the opensource driver, I get a 'mtrr disabled' message when X
> starts, nothing like that with fglrx. The results are the same.
>
> Is there anything that I can do about it? I already played with mtrr cleanup
> options - without any results.

I seem to recall hearing that on some newer AMD CPUs the memory above 
4GB is either hardcoded to cacheable or set that way via some other 
mechanism such that MTRRs aren't needed for it. Maybe someone else can 
provide more details.