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x86/pat: Document the PAT initialization sequence

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Update PAT documentation to describe how PAT is initialized under
various configurations.

Signed-off-by: Toshi Kani <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Luis R. Rodriguez <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Toshi Kani <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Toshi Kani 2016-03-23 15:42:03 -06:00 • committed by Ingo Molnar
commit b6350c21cf
1 changed files with 32 additions and 0 deletions
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32
Documentation/x86/pat.txt
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@ -196,3 +196,35 @@ Another, more verbose way of getting PAT related debug messages is with
"debugpat" boot parameter. With this parameter, various debug messages are
printed to dmesg log.
PAT Initialization
------------------
The following table describes how PAT is initialized under various
configurations. The PAT MSR must be updated by Linux in order to support WC
and WT attributes. Otherwise, the PAT MSR has the value programmed in it
by the firmware. Note, Xen enables WC attribute in the PAT MSR for guests.
MTRR PAT Call Sequence PAT State PAT MSR
=========================================================
E E MTRR -> PAT init Enabled OS
E D MTRR -> PAT init Disabled -
D E MTRR -> PAT disable Disabled BIOS
D D MTRR -> PAT disable Disabled -
- np/E PAT -> PAT disable Disabled BIOS
- np/D PAT -> PAT disable Disabled -
E !P/E MTRR -> PAT init Disabled BIOS
D !P/E MTRR -> PAT disable Disabled BIOS
!M !P/E MTRR stub -> PAT disable Disabled BIOS
Legend
------------------------------------------------
E Feature enabled in CPU
D Feature disabled/unsupported in CPU
np "nopat" boot option specified
!P CONFIG_X86_PAT option unset
!M CONFIG_MTRR option unset
Enabled PAT state set to enabled
Disabled PAT state set to disabled
OS PAT initializes PAT MSR with OS setting
BIOS PAT keeps PAT MSR with BIOS setting