linux-kernel - [PATCH] Documentation: x86: Correct effective memory type for MTRR=WC and PAT=UC-

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Message-Id: <20231106045356.5856-1-yan.y.zhao@intel.com>
Date:   Mon,  6 Nov 2023 12:53:56 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        x86@...nel.org
Cc:     dave.hansen@...ux.intel.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, corbet@....net, mcgrof@...e.com,
        andriy.shevchenko@...el.com, kai.huang@...el.com,
        Yan Zhao <yan.y.zhao@...el.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: [PATCH] Documentation: x86: Correct effective memory type for MTRR=WC and PAT=UC-

Correct effective memory type for a WC MTRR on a region with a PAT entry
UC- from UC to WC.

As per Intel SDM section "Selecting Memory Types for Pentium III and More
Recent Processor Families", table "Effective Page-Level Memory Types for
Pentium III and More Recent Processor Families",

================  ===============  =====================
MTRR Memory Type  PAT Entry Value  Effective Memory Type
================  ===============  =====================
      WC               UC-                  WC

the effect of a WC MTRR on a region with a PAT entry UC- will be WC.

There's a previous attempt to fix this issue at [1], but not sure why the
patch got lost.

Fixes: 2f9e897353fc ("x86/mm/mtrr, pat: Document Write Combining MTRR type effects on PAT / non-PAT pages").
Link: https://lore.kernel.org/all/1457131501-14855-1-git-send-email-mcgrof@kernel.org [1]
Cc: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Kai Huang <kai.huang@...el.com>
Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
---
 Documentation/arch/x86/pat.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Documentation/arch/x86/pat.rst b/Documentation/arch/x86/pat.rst
index 5d901771016d..414129efa316 100644
--- a/Documentation/arch/x86/pat.rst
+++ b/Documentation/arch/x86/pat.rst
@@ -138,19 +138,19 @@ otherwise not be effective.
   ====  =======  ===  =========================  =====================
   MTRR  Non-PAT  PAT  Linux ioremap value        Effective memory type
   ====  =======  ===  =========================  =====================
         PAT                                        Non-PAT |  PAT
         |PCD                                               |
         ||PWT                                              |
         |||                                                |
   WC    000      WB   _PAGE_CACHE_MODE_WB             WC   |   WC
   WC    001      WC   _PAGE_CACHE_MODE_WC             WC*  |   WC
-  WC    010      UC-  _PAGE_CACHE_MODE_UC_MINUS       WC*  |   UC
+  WC    010      UC-  _PAGE_CACHE_MODE_UC_MINUS       WC*  |   WC
   WC    011      UC   _PAGE_CACHE_MODE_UC             UC   |   UC
   ====  =======  ===  =========================  =====================
 
   (*) denotes implementation defined and is discouraged
 
 .. note:: -- in the above table mean "Not suggested usage for the API". Some
   of the --'s are strictly enforced by the kernel. Some others are not really
   enforced today, but may be enforced in future.
 

base-commit: 1aabbc532413ced293952f8e149ad0a607d6e470
-- 
2.17.1