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Commit b6350c21 authored by Toshi Kani's avatar Toshi Kani Committed by Ingo Molnar
Browse files

x86/pat: Document the PAT initialization sequence 



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

Signed-off-by: default avatarToshi Kani <toshi.kani@hpe.com>
Reviewed-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Toshi Kani <toshi.kani@hp.com>
Cc: elliott@hpe.com
Cc: konrad.wilk@oracle.com
Cc: paul.gortmaker@windriver.com
Cc: xen-devel@lists.xenproject.org
Link: http://lkml.kernel.org/r/1458769323-24491-8-git-send-email-toshi.kani@hpe.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 88ba2811

Documentation/x86/pat.txt

+32 −0
Original line number Diff line number Diff line
@@ -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