From: Rik van Riel <riel@redhat.com>
To: Simon Brown <lists@700c.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel
Date: Wed, 12 Jun 2013 16:10:03 -0400 [thread overview]
Message-ID: <51B8D59B.1040801@redhat.com> (raw)
In-Reply-To: <1371056055.24587.140661243104889.29DDEEB4@webmail.messagingengine.com>
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.
--
All rights reversed
next prev parent reply other threads:[~2013-06-12 20:10 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-12 16:54 Accessing more than 2GB of memory with a 32 bit kernel Simon Brown
2013-06-12 17:35 ` JA Magallón
2013-06-12 20:10 ` Rik van Riel [this message]
2013-06-12 21:33 ` JA Magallón
2013-06-13 22:38 ` Simon Brown
2013-06-13 22:32 ` Simon Brown
2013-06-14 0:14 ` Rik van Riel
2013-06-14 0:31 ` Yinghai Lu
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