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

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Date:	Thu, 13 Jun 2013 20:14:42 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Simon Brown <smb@...c.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel

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.


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