[tip:x86/mm] x86/mm/mtrr: Remove kernel internal MTRR interfaces: unexport mtrr_add() and mtrr_del()

From: tip-bot for Luis R. Rodriguez
Date: Fri Aug 28 2015 - 04:14:19 EST

Commit-ID: 2baa891e42d84159b693eadd44f6fe1486285bdc
Gitweb: http://git.kernel.org/tip/2baa891e42d84159b693eadd44f6fe1486285bdc
Author: Luis R. Rodriguez <mcgrof@xxxxxxxx>
AuthorDate: Mon, 24 Aug 2015 12:13:33 -0700
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 28 Aug 2015 10:09:28 +0200

x86/mm/mtrr: Remove kernel internal MTRR interfaces: unexport mtrr_add() and mtrr_del()

The effort to replace mtrr_add() with architecture agnostic
arch_phys_wc_add() is complete, this will ensure write-combining
implementations (PAT on x86) is taken advantage instead of using
MTRR. With the effort done now, hide direct MTRR access for

The legacy user-space /proc/mtrr ABI is not affected.

Update x86 documentation on MTRR to reflect the completion of
the phasing out of direct access to MTRR, also add a note on
platform firmware code use of MTRRs based on the obituary
discussion of MTRRs on Linux [0].

[0] http://lkml.kernel.org/r/1438991330.3109.196.camel@xxxxxx

Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
Cc: <syrjala@xxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Andy Walls <awalls@xxxxxxxxxxxxxxxx>
Cc: Antonino Daplas <adaplas@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Dave Airlie <airlied@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dbueso@xxxxxxx>
Cc: Doug Ledford <dledford@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@xxxxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Suresh Siddha <sbsiddha@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tomi Valkeinen <tomi.valkeinen@xxxxxx>
Cc: Toshi Kani <toshi.kani@xxxxxx>
Cc: Ville SyrjÃlà <syrjala@xxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: airlied@xxxxxxxx
Cc: benh@xxxxxxxxxxxxxxxxxxx
Cc: bhelgaas@xxxxxxxxxx
Cc: dan.j.williams@xxxxxxxxx
Cc: konrad.wilk@xxxxxxxxxx
Cc: linux-fbdev@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-media@xxxxxxxxxxxxxxx
Cc: mst@xxxxxxxxxx
Cc: netdev@xxxxxxxxxxxxxxx
Cc: vinod.koul@xxxxxxxxx
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1440443613-13696-12-git-send-email-mcgrof@xxxxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Documentation/x86/mtrr.txt | 20 ++++++++++++++++----
arch/x86/kernel/cpu/mtrr/main.c | 2 --
2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/Documentation/x86/mtrr.txt b/Documentation/x86/mtrr.txt
index 860bc3a..dc3e703 100644
--- a/Documentation/x86/mtrr.txt
+++ b/Documentation/x86/mtrr.txt
@@ -6,10 +6,22 @@ Luis R. Rodriguez <mcgrof@xxxxxxxxxxxxxxxx> - April 9, 2015
Phasing out MTRR use

-MTRR use is replaced on modern x86 hardware with PAT. Over time the only type
-of effective MTRR that is expected to be supported will be for write-combining.
-As MTRR use is phased out device drivers should use arch_phys_wc_add() to make
-MTRR effective on non-PAT systems while a no-op on PAT enabled systems.
+MTRR use is replaced on modern x86 hardware with PAT. Direct MTRR use by
+drivers on Linux is now completely phased out, device drivers should use
+arch_phys_wc_add() in combination with ioremap_wc() to make MTRR effective on
+non-PAT systems while a no-op but equally effective on PAT enabled systems.
+Even if Linux does not use MTRRs directly, some x86 platform firmware may still
+set up MTRRs early before booting the OS. They do this as some platform
+firmware may still have implemented access to MTRRs which would be controlled
+and handled by the platform firmware directly. An example of platform use of
+MTRRs is through the use of SMI handlers, one case could be for fan control,
+the platform code would need uncachable access to some of its fan control
+registers. Such platform access does not need any Operating System MTRR code in
+place other than mtrr_type_lookup() to ensure any OS specific mapping requests
+are aligned with platform MTRR setup. If MTRRs are only set up by the platform
+firmware code though and the OS does not make any specific MTRR mapping
+requests mtrr_type_lookup() should always return MTRR_TYPE_INVALID.

For details refer to Documentation/x86/pat.txt.

diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
index e7ed0d8..f891b47 100644
--- a/arch/x86/kernel/cpu/mtrr/main.c
+++ b/arch/x86/kernel/cpu/mtrr/main.c
@@ -448,7 +448,6 @@ int mtrr_add(unsigned long base, unsigned long size, unsigned int type,
return mtrr_add_page(base >> PAGE_SHIFT, size >> PAGE_SHIFT, type,

* mtrr_del_page - delete a memory type region
@@ -537,7 +536,6 @@ int mtrr_del(int reg, unsigned long base, unsigned long size)
return -EINVAL;
return mtrr_del_page(reg, base >> PAGE_SHIFT, size >> PAGE_SHIFT);

* arch_phys_wc_add - add a WC MTRR and handle errors if PAT is unavailable
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