mtrr cleanup

having just done an upgrade, i went through the logs and found some mtrr #fail issues.

$ dmesg | grep -i mtrr
[    0.000000] MTRR default type: uncachable
[    0.000000] MTRR fixed ranges enabled:
[    0.000000] MTRR variable ranges enabled:
[    5.811624] mtrr: type mismatch for d0000000,10000000 old: write-back new: write-combining
[    5.811628] [drm] MTRR allocation failed.  Graphics performance may suffer.

$ cat /proc/mtrr
reg00: base=0x0c0000000 ( 3072MB), size= 1024MB, count=1: uncachable
reg01: base=0x000000000 (    0MB), size= 4096MB, count=1: write-back
reg02: base=0x100000000 ( 4096MB), size= 1024MB, count=1: write-back
reg03: base=0x0bf700000 ( 3063MB), size=    1MB, count=1: uncachable
reg04: base=0x0bf800000 ( 3064MB), size=    8MB, count=1: uncachable

the above doesn't seem to add up right. note that i have 4gb of memory on this machine.

debian testing 3.2.0-2-amd64 #1 SMP Fri Jun 1 17:49:08 UTC 2012 x86_64 GNU/Linux

i found many recommendations to enable_mtrr_cleanup, including this: http://forums.gentoo.org/viewtopic-t-857061-start-0-postdays-0-postorder-asc-highlight-.html. so i added the kernel bootoptions

enable_mtrr_cleanup mtrr_spare_reg_nr=1

to the grub commandline, and here is the result.

$ dmesg | grep -i mtrr
[    0.000000] MTRR default type: uncachable
[    0.000000] MTRR fixed ranges enabled:
[    0.000000] MTRR variable ranges enabled:
[    0.000000] original variable MTRRs
[    0.000000] Found optimal setting for mtrr clean up
[    0.000000] New variable MTRRs

$ cat /proc/mtrr
reg00: base=0x000000000 (    0MB), size= 2048MB, count=1: write-back
reg01: base=0x080000000 ( 2048MB), size= 1024MB, count=1: write-back
reg02: base=0x0bf700000 ( 3063MB), size=    1MB, count=1: uncachable
reg03: base=0x0bf800000 ( 3064MB), size=    8MB, count=1: uncachable
reg04: base=0x100000000 ( 4096MB), size= 1024MB, count=1: write-back
reg05: base=0x0d0000000 ( 3328MB), size=  256MB, count=1: write-combining

now this look much better! i'm keeping this kernel bootoption. have a go yourself, and post your (before/after) results below.

update: i should use mtrr_spare_reg_nr=0, as the first reg is 00.