x86/mm/pkeys: Fix compact mode by removing protection keys' XSAVE buffer manipulation

x86/mm/pkeys: Fix compact mode by removing protection keys' XSAVE buffer manipulation

The Memory Protection Keys "rights register" (PKRU) is
XSAVE-managed, and is saved/restored along with the FPU state.

When kernel code accesses FPU regsisters, it does a delicate
dance with preempt.  Otherwise, the context switching code can
get confused as to whether the most up-to-date state is in the
registers themselves or in the XSAVE buffer.

But, PKRU is not a normal FPU register.  Using it does not
generate the normal device-not-available (#NM) exceptions which
means we can not manage it lazily, and the kernel completley
disallows using lazy mode when it is enabled.

The dance with preempt *only* occurs when managing the FPU
lazily.  Since we never manage PKRU lazily, we do not have to do
the dance with preempt; we can access it directly.  Doing it
this way saves a ton of complicated code (and is faster too).

Further, the XSAVES reenabling failed to patch a bit of code
in fpu__xfeature_set_state() the checked for compacted buffers.
That check caused fpu__xfeature_set_state() to silently refuse to
work when the kernel is using compacted XSAVE buffers.  This
broke execute-only and future pkey_mprotect() support when using
compact XSAVE buffers.

But, removing fpu__xfeature_set_state() gets rid of this issue,
in addition to the nice cleanup and speedup.

This fixes the same thing as a fix that Sai posted:

  https://lkml.org/lkml/2016/7/25/637

The fix that he posted is a much more obviously correct, but I
think we should just do this instead.

Reported-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Yu-Cheng Yu <yu-cheng.yu@intel.com>
Link: http://lkml.kernel.org/r/20160727232040.7D060DAD@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>