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

From: Rik van Riel
Date: Wed Jun 12 2013 - 16:10:40 EST

On 06/12/2013 12:54 PM, Simon Brown wrote:

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
Mem: 2012 491 1521 0 40

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.

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