This option has been added in v2.6.26 as a default-disabled
feature and went through several revisions since then.

The feature fixes a wide range of MTRR setup problems that BIOSes
leave us with: slow system, slow Xorg, slow system when adding lots
of RAM, etc., so we want to enable it by default for v2.6.28.

See:

  [Bug 10508] Upgrade to 4GB of RAM messes up MTRRs
  http://bugzilla.kernel.org/show_bug.cgi?id=10508

and the test results in:

   http://lkml.org/lkml/2008/9/29/273

1. hpa
reg00: base=0xc0000000 (3072MB), size=1024MB: uncachable, count=1
reg01: base=0x13c000000 (5056MB), size=  64MB: uncachable, count=1
reg02: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
reg03: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
reg04: base=0xbf700000 (3063MB), size=   1MB: uncachable, count=1
reg05: base=0xbf800000 (3064MB), size=   8MB: uncachable, count=1

will get
Found optimal setting for mtrr clean up
gran_size: 1M   chunk_size: 128M        num_reg: 6      lose RAM: 0M
range0: 0000000000000000 - 00000000c0000000
Setting variable MTRR 0, base: 0MB, range: 2048MB, type WB
Setting variable MTRR 1, base: 2048MB, range: 1024MB, type WB
hole: 00000000bf700000 - 00000000c0000000
Setting variable MTRR 2, base: 3063MB, range: 1MB, type UC
Setting variable MTRR 3, base: 3064MB, range: 8MB, type UC
range0: 0000000100000000 - 0000000140000000
Setting variable MTRR 4, base: 4096MB, range: 1024MB, type WB
hole: 000000013c000000 - 0000000140000000
Setting variable MTRR 5, base: 5056MB, range: 64MB, type UC

2. Dylan Taft
reg00: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
reg01: base=0x100000000 (4096MB), size= 512MB: write-back, count=1
reg02: base=0x120000000 (4608MB), size= 256MB: write-back, count=1
reg03: base=0xd0000000 (3328MB), size= 256MB: uncachable, count=1
reg04: base=0xe0000000 (3584MB), size= 512MB: uncachable, count=1
reg05: base=0xc7e00000 (3198MB), size=   2MB: uncachable, count=1
reg06: base=0xc8000000 (3200MB), size= 128MB: uncachable, count=1

will get
Found optimal setting for mtrr clean up
gran_size: 1M   chunk_size: 4M  num_reg: 6      lose RAM: 0M
range0: 0000000000000000 - 00000000c8000000
Setting variable MTRR 0, base: 0MB, range: 2048MB, type WB
Setting variable MTRR 1, base: 2048MB, range: 1024MB, type WB
Setting variable MTRR 2, base: 3072MB, range: 128MB, type WB
hole: 00000000c7e00000 - 00000000c8000000
Setting variable MTRR 3, base: 3198MB, range: 2MB, type UC
rangeX: 0000000100000000 - 0000000130000000
Setting variable MTRR 4, base: 4096MB, range: 512MB, type WB
Setting variable MTRR 5, base: 4608MB, range: 256MB, type WB

3. Gabriel
reg00: base=0xd0000000 (3328MB), size= 256MB: uncachable, count=1
reg01: base=0xe0000000 (3584MB), size= 512MB: uncachable, count=1
reg02: base=0x00000000 (   0MB), size=4096MB: write-back, count=1
reg03: base=0x100000000 (4096MB), size= 512MB: write-back, count=1
reg04: base=0x120000000 (4608MB), size= 128MB: write-back, count=1
reg05: base=0x128000000 (4736MB), size=  64MB: write-back, count=1
reg06: base=0xcf600000 (3318MB), size=   2MB: uncachable, count=1

will get
Found optimal setting for mtrr clean up
gran_size: 1M   chunk_size: 16M         num_reg: 7      lose RAM: 0M
range0: 0000000000000000 - 00000000d0000000
Setting variable MTRR 0, base: 0MB, range: 2048MB, type WB
Setting variable MTRR 1, base: 2048MB, range: 1024MB, type WB
Setting variable MTRR 2, base: 3072MB, range: 256MB, type WB
hole: 00000000cf600000 - 00000000cf800000
Setting variable MTRR 3, base: 3318MB, range: 2MB, type UC
rangeX: 0000000100000000 - 000000012c000000
Setting variable MTRR 4, base: 4096MB, range: 512MB, type WB
Setting variable MTRR 5, base: 4608MB, range: 128MB, type WB
Setting variable MTRR 6, base: 4736MB, range: 64MB, type WB

4. Mika Fischer
reg00: base=0xc0000000 (3072MB), size=1024MB: uncachable, count=1
reg01: base=0x00000000 ( 0MB), size=4096MB: write-back, count=1
reg02: base=0x100000000 (4096MB), size=1024MB: write-back, count=1
reg03: base=0xbf700000 (3063MB), size= 1MB: uncachable, count=1
reg04: base=0xbf800000 (3064MB), size= 8MB: uncachable, count=1

will get
Found optimal setting for mtrr clean up
gran_size: 1M   chunk_size: 16M         num_reg: 5      lose RAM: 0M
range0: 0000000000000000 - 00000000c0000000
Setting variable MTRR 0, base: 0MB, range: 2048MB, type WB
Setting variable MTRR 1, base: 2048MB, range: 1024MB, type WB
hole: 00000000bf700000 - 00000000c0000000
Setting variable MTRR 2, base: 3063MB, range: 1MB, type UC
Setting variable MTRR 3, base: 3064MB, range: 8MB, type UC
rangeX: 0000000100000000 - 0000000140000000
Setting variable MTRR 4, base: 4096MB, range: 1024MB, type WB
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>