x86/mm/pat: Add support of non-default PAT MSR setting

In preparation for fixing a regression caused by:

  9cd25aac ("x86/mm/pat: Emulate PAT when it is disabled")'

... PAT needs to support a case that PAT MSR is initialized with a
non-default value.

When pat_init() is called and PAT is disabled, it initializes the
PAT table with the BIOS default value. Xen, however, sets PAT MSR
with a non-default value to enable WC. This causes inconsistency
between the PAT table and PAT MSR when PAT is set to disable on Xen.

Change pat_init() to handle the PAT disable cases properly.  Add
init_cache_modes() to handle two cases when PAT is set to disable.

 1. CPU supports PAT: Set PAT table to be consistent with PAT MSR.
 2. CPU does not support PAT: Set PAT table to be consistent with
    PWT and PCD bits in a PTE.

Note, __init_cache_modes(), renamed from pat_init_cache_modes(),
will be changed to a static function in a later patch.

Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Reviewed-by: Thomas 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/1458769323-24491-2-git-send-email-toshi.kani@hpe.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>