x86/pti: Leave kernel text global for !PCID

Global pages are bad for hardening because they potentially let an
exploit read the kernel image via a Meltdown-style attack which
makes it easier to find gadgets.

But, global pages are good for performance because they reduce TLB
misses when making user/kernel transitions, especially when PCIDs
are not available, such as on older hardware, or where a hypervisor
has disabled them for some reason.

This patch implements a basic, sane policy: If you have PCIDs, you
only map a minimal amount of kernel text global.  If you do not have
PCIDs, you map all kernel text global.

This policy effectively makes PCIDs something that not only adds
performance but a little bit of hardening as well.

I ran a simple "lseek" microbenchmark[1] to test the benefit on
a modern Atom microserver.  Most of the benefit comes from applying
the series before this patch ("entry only"), but there is still a
signifiant benefit from this patch.

  No Global Lines (baseline  ): 6077741 lseeks/sec
  88 Global Lines (entry only): 7528609 lseeks/sec (+23.9%)
  94 Global Lines (this patch): 8433111 lseeks/sec (+38.8%)

[1.] https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180406205518.E3D989EB@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

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