x86/pkeys: Update documentation

There are a few items that have gotten stale in the protection
keys documentation.  The config option description only applied
to the execute-only support and is not accurate for the current
code.  There was also a typo with the number of system calls.  I
also wanted to call out that pkey_set() is not a kernel-provided
facility, and where to find an implementation.

Signed-off-by: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave@sr71.net>
Cc: linux-doc@vger.kernel.org
Cc: corbet@lwn.net
Link: http://lkml.kernel.org/r/20161004163857.71E0D6F6@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/x86/protection-keys.txt

@@ -20,7 +20,7 @@ instruction fetches.
 
 =========================== Syscalls ===========================
 
-There are 2 system calls which directly interact with pkeys:
+There are 3 system calls which directly interact with pkeys:
 
 	int pkey_alloc(unsigned long flags, unsigned long init_access_rights)
 	int pkey_free(int pkey);
@@ -52,6 +52,10 @@ is no longer in use:
 	munmap(ptr, PAGE_SIZE);
 	pkey_free(pkey);
 
+(Note: pkey_set() is a wrapper for the RDPKRU and WRPKRU instructions.
+ An example implementation can be found in
+ tools/testing/selftests/x86/protection_keys.c)
+
 =========================== Behavior ===========================
 
 The kernel attempts to make protection keys consistent with the
@@ -79,11 +83,3 @@ with a read():
 The kernel will send a SIGSEGV in both cases, but si_code will be set
 to SEGV_PKERR when violating protection keys versus SEGV_ACCERR when
 the plain mprotect() permissions are violated.
-
-=========================== Config Option ===========================
-
-This config option adds approximately 1.5kb of text. and 50 bytes of
-data to the executable.  A workload which does large O_DIRECT reads
-of holes in XFS files was run to exercise get_user_pages_fast().  No
-performance delta was observed with the config option
-enabled or disabled.