x86/Documentation: Add PTI description

x86/Documentation: Add PTI description

commit 01c9b17bf673b05bb401b76ec763e9730ccf1376 upstream.

Add some details about how PTI works, what some of the downsides
are, and how to debug it when things go wrong.

Also document the kernel parameter: 'pti/nopti'.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Moritz Lipp <moritz.lipp@iaik.tugraz.at>
Cc: Daniel Gruss <daniel.gruss@iaik.tugraz.at>
Cc: Michael Schwarz <michael.schwarz@iaik.tugraz.at>
Cc: Richard Fellner <richard.fellner@student.tugraz.at>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andi Lutomirsky <luto@kernel.org>
Link: https://lkml.kernel.org/r/20180105174436.1BC6FA2B@viggo.jf.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>