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
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