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

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Date:	Thu, 13 Jun 2013 23:32:06 +0100
From:	Simon Brown <smb@...c.org>
To:	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: Accessing more than 2GB of memory with a 32 bit kernel

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

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