[PATCH v2 6/6] x86/pat: Document PAT initializations

[Toshi Kani]

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

Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 Documentation/x86/pat.txt |   32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/Documentation/x86/pat.txt b/Documentation/x86/pat.txt
index 54944c7..f619e1d 100644
--- a/Documentation/x86/pat.txt
+++ b/Documentation/x86/pat.txt
@@ -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. PAT must be set to enable to initialize PAT MSR in order
+to support WC and WT attributes. Otherwise, PAT keeps PAT MSR value set
+by BIOS. Note, Xen enables WC attribute in BIOS setup for guests.
+
+ MTRR PAT   Call Sequence               PAT State  PAT MSR
+ =========================================================
+ E    E     MTRR -> pat_init()          Enable     OS
+ E    D     MTRR -> pat_init()          Disable    -
+ D    E     MTRR -> pat_disable()       Disable    BIOS
+ D    D     MTRR -> pat_disable()       Disable    -
+ -    np/E  nopat() -> pat_disable()    Disable    BIOS
+ -    np/D  nopat() -> pat_disable()    Disable    -
+ E    !P/E  MTRR -> pat_init()          Disable    BIOS
+ D    !P/E  MTRR -> pat_disable()       Disable    BIOS
+ !M   !P/E  MTRR stub -> pat_disable()  Disable    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
+ Enable   PAT state set to enable
+ Disable  PAT state set to disable
+ OS       PAT initializes PAT MSR with OS setup
+ BIOS     PAT keeps PAT MSR with BIOS setup
+