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

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Date:	Thu, 13 Jun 2013 23:38:22 +0100
From:	Simon Brown <smb@...c.org>
To:	JA Magallón <jamagallon@....com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel

On Wednesday 12 Jun 2013 23:33:26 JA Magallón wrote:
> On 06/12/2013 10:10 PM, 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.
> 
> Acording to manual, that mobo has an option to "Memory remap feature"
> in BIOS that looks like that...

I don't understand that option in the BIOS. If I disable the option the e820 
table is the same as before except missing the last line and the BIOS boot 
screen only reports 2GB. The choice seems to be map it high or lose it.

  [    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)

Apologies for the slow follow up,

Simon
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