Messages in this thread | | | From | Simon Brown <> | Subject | Re: Accessing more than 2GB of memory with a 32 bit kernel | Date | Thu, 13 Jun 2013 23:32:06 +0100 |
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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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