From: Rik van Riel <riel@redhat.com>
To: Simon Brown <smb@700c.org>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel
Date: Thu, 13 Jun 2013 20:14:42 -0400 [thread overview]
Message-ID: <51BA6072.1020506@redhat.com> (raw)
In-Reply-To: <3704356.7sSoN0NIpj@phaedrus>
On 06/13/2013 06:32 PM, Simon Brown wrote:
> 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?
Besides use a 64 bit kernel? No.
--
All rights reversed
next prev parent reply other threads:[~2013-06-14 0:14 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
2013-06-14 0:14 ` Rik van Riel [this message]
2013-06-14 0:31 ` Yinghai Lu
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