x86/mm/pat: Wrap pat_enabled into a function API

We use pat_enabled in x86-specific code to see if PAT is enabled
or not but we're granting full access to it even though readers
do not need to set it. If, for instance, we granted access to it
to modules later they then could override the variable
setting... no bueno.

This renames pat_enabled to a new static variable __pat_enabled.
Folks are redirected to use pat_enabled() now.

Code that sets this can only be internal to pat.c. Apart from
the early kernel parameter "nopat" to disable PAT, we also have
a few cases that disable it later and make use of a helper
pat_disable(). It is wrapped under an ifdef but since that code
cannot run unless PAT was enabled its not required to wrap it
with ifdefs, unwrap that. Likewise, since "nopat" doesn't really
change non-PAT systems just remove that ifdef as well.

Although we could add and use an early_param_off(), these
helpers don't use __read_mostly but we want to keep
__read_mostly for __pat_enabled as this is a hot path -- upon
boot, for instance, a simple guest may see ~4k accesses to
pat_enabled(). Since __read_mostly early boot params are not
that common we don't add a helper for them just yet.

Signed-off-by: Luis R. Rodriguez <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Walls <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Doug Ledford <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kyle McMartin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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