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

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Message-Id: <200710260045.44090.rjw@sisk.pl>
Date:	Fri, 26 Oct 2007 00:45:43 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"J.A. Magallon" <magallon@...zar.es>,
	"Linux-Kernel, " <linux-kernel@...r.kernel.org>
Subject: Re: Opteron box and 4Gb memory

On Thursday, 25 October 2007 23:58, H. Peter Anvin wrote:
> 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.

It shouldn't, AFAICS.

Greetings,
Rafael
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