[PATCH v3 0/7] Enhance PAT init to fix Xorg crashes

[Toshi Kani]

A Xorg failure on qemu32 was reported as a regression [1] caused by
'commit 9cd25aac1f44 ("x86/mm/pat: Emulate PAT when it is disabled")'.
This patch-set fixes the regression.

Negative effects of this regression were two failures [2] in Xorg on
QEMU with QEMU CPU model "qemu32" (-cpu qemu32), which were triggered
by the fact that its virtual CPU does not support MTRR.
 #1. copy_process() failed in the check in reserve_pfn_range()
 #2. error path in copy_process() then hit WARN_ON_ONCE in
     untrack_pfn().

These negative effects are caused by two separate bugs, but they can be
addressed in separate patches.  This patch-set addresses the root cause,
a long-standing PAT initialization issue.

Please see the changelog in patch 4/7 for the details of the issue.

- Patch 1-2 make necessary enhancement to PAT for the fix without
  breaking Xen.
- Patch 3 is cleanup.
- Patch 4 fixes the regression.
- Patch 5 fixes an MTRR issue related with PAT init.
- Patch 6 removes PAT init code from Xen.
- Patch 7 adds PAT init to documentation.

[1]: https://lkml.org/lkml/2016/3/3/828
[2]: https://lkml.org/lkml/2016/3/4/775

I'd appreciate if someone can test this patch-set on Xen to verify that
there is no change in "x86/PAT: Configuration [0-7] .." message in dmesg.

---
v3:
 - Change a new func name to init_cache_modes(). (Borislav Petkov)
 - Add check with __pat_enabled, and use WARN_ONCE() for a bug check
   in pat_disable(). (Borislav Petkov)
 - Update changelog, comments, and doc per review. (Borislav Petkov)

v2:
 - Divide patch-set into a single change. (Borislav Petkov)
 - Xen's case must be handled properly. (Luis R. Rodriguez)
 - Change changelog and title to describe the issue. (Ingo Molnar)
 - Update an error message. (Robert Elliott, Borislav Petkov)

---
Toshi Kani (7):
 1/7 x86/mm/pat: Add support of non-default PAT MSR setting
 2/7 x86/mm/pat: Add pat_disable() interface
 3/7 x86/mm/pat: Replace cpu_has_pat with boot_cpu_has
 4/7 x86/mtrr: Fix Xorg crashes in Qemu sessions
 5/7 x86/mtrr: Fix PAT init handling when MTRR is disabled
 6/7 x86/xen,pat: Remove PAT table init code from Xen
 7/7 x86/pat: Document PAT initialization

---
 Documentation/x86/pat.txt          | 32 ++++++++++++++
 arch/x86/include/asm/mtrr.h        |  6 ++-
 arch/x86/include/asm/pat.h         |  2 +-
 arch/x86/kernel/cpu/mtrr/generic.c | 24 +++++-----
 arch/x86/kernel/cpu/mtrr/main.c    | 13 +++++-
 arch/x86/kernel/cpu/mtrr/mtrr.h    |  1 +
 arch/x86/mm/pat.c                  | 90 ++++++++++++++++++++++++++++----------
 arch/x86/xen/enlighten.c           |  9 ----
 8 files changed, 132 insertions(+), 45 deletions(-)