linux-kernel - [PATCH 4.6 29/56] x86/pat: Document the PAT initialization sequence

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Message-Id: <20160814202506.132428006@linuxfoundation.org>
Date:	Sun, 14 Aug 2016 22:37:33 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Toshi Kani <toshi.kani@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Borislav Petkov <bp@...en8.de>, Borislav Petkov <bp@...e.de>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Toshi Kani <toshi.kani@...com>, elliott@....com,
	konrad.wilk@...cle.com, paul.gortmaker@...driver.com,
	xen-devel@...ts.xenproject.org, Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 4.6 29/56] x86/pat: Document the PAT initialization sequence

4.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Toshi Kani <toshi.kani@....com>

commit b6350c21cfe8aa9d65e189509a23c0ea4b8362c2 upstream.

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

Signed-off-by: Toshi Kani <toshi.kani@....com>
Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Borislav Petkov <bp@...e.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Juergen Gross <jgross@...e.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@...e.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Toshi Kani <toshi.kani@...com>
Cc: elliott@....com
Cc: konrad.wilk@...cle.com
Cc: paul.gortmaker@...driver.com
Cc: xen-devel@...ts.xenproject.org
Link: http://lkml.kernel.org/r/1458769323-24491-8-git-send-email-toshi.kani@hpe.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 Documentation/x86/pat.txt |   32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

--- a/Documentation/x86/pat.txt
+++ b/Documentation/x86/pat.txt
@@ -196,3 +196,35 @@ Another, more verbose way of getting PAT
 "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
+