x86, mpx: Give MPX a real config option prompt

Give MPX a real config option. The CPUs that support it (referenced
here):

  https://software.intel.com/en-us/forums/topic/402393

are not available publicly yet. Right now only the software emulator
provides MPX for the general public.

[ tglx: Make it default off. There is no point in having it on right
        now as no hardware and no proper tooling support are available ]

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dave Hansen <dave@sr71.net>
Link: http://lkml.kernel.org/r/20141212183836.2569D58D@viggo.jf.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Documentation/x86/intel_mpx.txt

@@ -7,11 +7,15 @@ that can be used in conjunction with compiler changes to check memory
 references, for those references whose compile-time normal intentions are
 usurped at runtime due to buffer overflow or underflow.
 
+You can tell if your CPU supports MPX by looking in /proc/cpuinfo:
+
+	cat /proc/cpuinfo  | grep ' mpx '
+
 For more information, please refer to Intel(R) Architecture Instruction
 Set Extensions Programming Reference, Chapter 9: Intel(R) Memory Protection
 Extensions.
 
-Note: Currently no hardware with MPX ISA is available but it is always
+Note: As of December 2014, no hardware with MPX is available but it is
 possible to use SDE (Intel(R) Software Development Emulator) instead, which
 can be downloaded from
 http://software.intel.com/en-us/articles/intel-software-development-emulator

arch/x86/Kconfig

@@ -248,10 +248,6 @@ config HAVE_INTEL_TXT
 	def_bool y
 	depends on INTEL_IOMMU && ACPI
 
-config X86_INTEL_MPX
-	def_bool y
-	depends on CPU_SUP_INTEL
-
 config X86_32_SMP
 	def_bool y
 	depends on X86_32 && SMP
@@ -1575,6 +1571,32 @@ config X86_SMAP
 
 	  If unsure, say Y.
 
+config X86_INTEL_MPX
+	prompt "Intel MPX (Memory Protection Extensions)"
+	def_bool n
+	depends on CPU_SUP_INTEL
+	---help---
+	  MPX provides hardware features that can be used in
+	  conjunction with compiler-instrumented code to check
+	  memory references.  It is designed to detect buffer
+	  overflow or underflow bugs.
+
+	  This option enables running applications which are
+	  instrumented or otherwise use MPX.  It does not use MPX
+	  itself inside the kernel or to protect the kernel
+	  against bad memory references.
+
+	  Enabling this option will make the kernel larger:
+	  ~8k of kernel text and 36 bytes of data on a 64-bit
+	  defconfig.  It adds a long to the 'mm_struct' which
+	  will increase the kernel memory overhead of each
+	  process and adds some branches to paths used during
+	  exec() and munmap().
+
+	  For details, see Documentation/x86/intel_mpx.txt
+
+	  If unsure, say N.
+
 config EFI
 	bool "EFI runtime service support"
 	depends on ACPI