x32: Warn and disable rather than error if binutils too old

If X32 is enabled in .config, but the binutils can't build it, issue a
warning and disable the feature rather than erroring out.

In order to support this, have CONFIG_X86_X32 be the option set in
Kconfig, and CONFIG_X86_X32_ABI be the option set by the Makefile when
it is enabled and binutils has been found to be functional.

Requested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: H. J. Lu <hjl.tools@gmail.com>
Link: http://lkml.kernel.org/r/1329696488-16970-1-git-send-email-hpa@zytor.com

arch/x86/Kconfig

@@ -2175,7 +2175,7 @@ config IA32_AOUT
 	---help---
 	  Support old a.out binaries in the 32bit emulation.
 
-config X86_X32_ABI
+config X86_X32
 	bool "x32 ABI for 64-bit mode (EXPERIMENTAL)"
 	depends on X86_64 && IA32_EMULATION && EXPERIMENTAL
 	---help---
@@ -2190,7 +2190,7 @@ config X86_X32_ABI
 
 config COMPAT
 	def_bool y
-	depends on IA32_EMULATION || X86_X32_ABI
+	depends on IA32_EMULATION || X86_X32
 
 config COMPAT_FOR_U64_ALIGNMENT
 	def_bool COMPAT

arch/x86/Makefile

@@ -82,6 +82,22 @@ ifdef CONFIG_CC_STACKPROTECTOR
         endif
 endif
 
+ifdef CONFIG_X86_X32
+	x32_ld_ok := $(call try-run,\
+			/bin/echo -e '1: .quad 1b' | \
+			$(CC) $(KBUILD_AFLAGS) -c -xassembler -o "$$TMP" - && \
+			$(OBJCOPY) -O elf32-x86-64 "$$TMP" "$$TMPO" && \
+			$(LD) -m elf32_x86_64 "$$TMPO" -o "$$TMP",y,n)
+	ifeq ($(x32_ld_ok),y)
+		CONFIG_X86_X32_ABI := y
+		KBUILD_AFLAGS += -DCONFIG_X86_X32_ABI
+		KBUILD_CFLAGS += -DCONFIG_X86_X32_ABI
+	else
+		$(warning CONFIG_X86_X32 enabled but no binutils support)
+	endif
+endif
+export CONFIG_X86_X32_ABI
+
 # Don't unroll struct assignments with kmemcheck enabled
 ifeq ($(CONFIG_KMEMCHECK),y)
 	KBUILD_CFLAGS += $(call cc-option,-fno-builtin-memcpy)