From: Simon Brown <smb@700c.org>
To: Rik van Riel <riel@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel
Date: Thu, 13 Jun 2013 23:32:06 +0100 [thread overview]
Message-ID: <3704356.7sSoN0NIpj@phaedrus> (raw)
In-Reply-To: <51B8D59B.1040801@redhat.com>
On Wednesday 12 Jun 2013 16:10:03 Rik van Riel wrote:
> 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.
The table looks like this:
[ 0.000000] BIOS-provided physical RAM map:
[ 0.000000] BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
[ 0.000000] BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
[ 0.000000] BIOS-e820: 00000000000e4000 - 0000000000100000 (reserved)
[ 0.000000] BIOS-e820: 0000000000100000 - 000000007ff80000 (usable)
[ 0.000000] BIOS-e820: 000000007ff80000 - 000000007ff8e000 (ACPI data)
[ 0.000000] BIOS-e820: 000000007ff8e000 - 000000007ffe0000 (ACPI NVS)
[ 0.000000] BIOS-e820: 000000007ffe0000 - 0000000080000000 (reserved)
[ 0.000000] BIOS-e820: 00000000fee00000 - 00000000fee01000 (reserved)
[ 0.000000] BIOS-e820: 00000000fff00000 - 0000000100000000 (reserved)
[ 0.000000] BIOS-e820: 0000000100000000 - 0000000180000000 (usable)
So the BIOS has reserved the entire upper half. Can I do anything about that?
Simon
next prev parent reply other threads:[~2013-06-13 22:32 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
2013-06-12 21:33 ` JA Magallón
2013-06-13 22:38 ` Simon Brown
2013-06-13 22:32 ` Simon Brown [this message]
2013-06-14 0:14 ` Rik van Riel
2013-06-14 0:31 ` Yinghai Lu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=3704356.7sSoN0NIpj@phaedrus \
--to=smb@700c.org \
--cc=linux-kernel@vger.kernel.org \
--cc=riel@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.