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

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