x32: Allow x32 to be configured

At this point, one should be able to build an x32 kernel.

Note that for now we depend on CONFIG_IA32_EMULATION.  Long term, x32
and IA32 should be detangled.
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/Kconfig

@@ -2165,9 +2165,9 @@ config IA32_EMULATION
 	depends on X86_64
 	select COMPAT_BINFMT_ELF
 	---help---
-	  Include code to run 32-bit programs under a 64-bit kernel. You should
-	  likely turn this on, unless you're 100% sure that you don't have any
-	  32-bit programs left.
+	  Include code to run legacy 32-bit programs under a
+	  64-bit kernel. You should likely turn this on, unless you're
+	  100% sure that you don't have any 32-bit programs left.
 
 config IA32_AOUT
 	tristate "IA32 a.out support"
@@ -2175,9 +2175,22 @@ config IA32_AOUT
 	---help---
 	  Support old a.out binaries in the 32bit emulation.
 
+config X86_X32_ABI
+	bool "x32 ABI for 64-bit mode (EXPERIMENTAL)"
+	depends on X86_64 && IA32_EMULATION && EXPERIMENTAL
+	---help---
+	  Include code to run binaries for the x32 native 32-bit ABI
+	  for 64-bit processors.  An x32 process gets access to the
+	  full 64-bit register file and wide data path while leaving
+	  pointers at 32 bits for smaller memory footprint.
+
+	  You will need a recent binutils (2.22 or later) with
+	  elf32_x86_64 support enabled to compile a kernel with this
+	  option set.
+
 config COMPAT
 	def_bool y
-	depends on IA32_EMULATION
+	depends on IA32_EMULATION || X86_X32_ABI
 
 config COMPAT_FOR_U64_ALIGNMENT
 	def_bool COMPAT