linux-kernel - Re: Accessing more than 2GB of memory with a 32 bit kernel

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Message-ID: <51B8D59B.1040801@redhat.com>
Date:	Wed, 12 Jun 2013 16:10:03 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Simon Brown <lists@...c.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel

On 06/12/2013 12:54 PM, Simon Brown wrote:
> Hello,
>
> For the sake of an old prototype peripheral I'm using a non PAE 32 bit
> x86 kernel and I'm having trouble accessing memory above 2 GB. The
> system has 4GB installed and all is well with a PAE kernel.
>
> I'm obviously expecting to lose some memory due to memory mapped devices
> but I wasn't expecting to lose 2GB. Instead I'm suspecting a BIOS bug.
> The system reports:
> free -m
>               total       used       free     shared    buffers
>               cached
> Mem:          2012        491       1521          0         40
> 277
>
> The mtrr table looked odd so I enabled sanitisation:
> [    0.000000] original variable MTRRs
> [    0.000000] reg 0, base: 2GB, range: 2GB, type UC
> [    0.000000] reg 1, base: 0GB, range: 4GB, type WB
> [    0.000000] reg 2, base: 4GB, range: 2GB, type WB
> [    0.000000] total RAM covered: 4096M
> [    0.000000] Found optimal setting for mtrr clean up
> [    0.000000]  gran_size: 64K  chunk_size: 64K         num_reg: 2
> lose cover RAM: 0G
> [    0.000000] New variable MTRRs
> [    0.000000] reg 0, base: 0GB, range: 2GB, type WB
> [    0.000000] reg 1, base: 4GB, range: 2GB, type WB
>
> I don't understand the gap in the new table.

Check the e820 table. Chances are the BIOS is reserving 2GB to
map various devices (especially video cards) below the 4GB limit.


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