linux-kernel - Re: Opteron box and 4Gb memory

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Message-ID: <47211172.9070606@zytor.com>
Date:	Thu, 25 Oct 2007 14:58:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"J.A. Magallon" <magallon@...zar.es>
CC:	"Linux-Kernel, " <linux-kernel@...r.kernel.org>
Subject: Re: Opteron box and 4Gb memory

J.A. Magallon wrote:
> Hi...
> 
> I have some Quad-Opteron boxes with 4Gb memory and two of them are
> running two different Linux distros.
> 
> Box one sees 4Gb of memory, but box two just sees 3.
> Their mtrr setups are different:
> 
> Why ? Is it a bios setup problem ? A kernel problem ?
> grep HIGHMEN in configs for both kernels does not give anything, so
> I still understand less this thing...
> 

It would depend on how the BIOS programmed the memory controllers.  For 
32-bit (and lots of device) compatibility, a memory hole is required 
below 4 GB.  Not all memory controllers can remap memory in the 3-4 GB 
range above the 4 GB memory; I'm not sure if that varies with the 
different Opteron processors.

Also, if you run a 32-bit distribution, you need to have HIGHMEM_64G 
enabled in the kernel.

	-hpa
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