[PATCH] x86: Detect CFI support in the assembler at runtime

... instead of using a CONFIG option. The config option still controls
if the resulting executable actually has unwind information.

This is useful to prevent compilation errors when users select
CONFIG_STACK_UNWIND on old binutils and also allows to use
CFI in the future for non kernel debugging applications.

Cc: jbeulich@novell.com
Cc: sam@ravnborg.org
Signed-off-by: NAndi Kleen <ak@suse.de>

Documentation/kbuild/makefiles.txt

@@ -421,6 +421,11 @@ more details, with real examples.
 	The second argument is optional, and if supplied will be used
 	if first argument is not supported.
 
+    as-instr
+	as-instr checks if the assembler reports a specific instruction
+	and then outputs either option1 or option2
+	C escapes are supported in the test instruction
+
     cc-option
 	cc-option is used to check if $(CC) supports a given option, and not
 	supported to use an optional second option.

arch/i386/Makefile

@@ -46,6 +46,10 @@ cflags-y += -ffreestanding
 # a lot more stack due to the lack of sharing of stacklots:
 CFLAGS				+= $(shell if [ $(call cc-version) -lt 0400 ] ; then echo $(call cc-option,-fno-unit-at-a-time); fi ;)
 
+# do binutils support CFI?
+cflags-y += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
+AFLAGS += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
+
 CFLAGS += $(cflags-y)
 
 # Default subarch .c files

arch/x86_64/Makefile

@@ -54,6 +54,9 @@ endif
 cflags-y += $(call cc-option,-funit-at-a-time)
 # prevent gcc from generating any FP code by mistake
 cflags-y += $(call cc-option,-mno-sse -mno-mmx -mno-sse2 -mno-3dnow,)
+# do binutils support CFI?
+cflags-y += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
+AFLAGS += $(call as-instr,.cfi_startproc\n.cfi_endproc,-DCONFIG_AS_CFI=1,)
 
 CFLAGS += $(cflags-y)
 CFLAGS_KERNEL += $(cflags-kernel-y)

include/asm-x86_64/dwarf2.h

@@ -13,7 +13,7 @@
    away for older version. 
  */
 
-#ifdef CONFIG_UNWIND_INFO
+#ifdef CONFIG_AS_CFI
 
 #define CFI_STARTPROC .cfi_startproc
 #define CFI_ENDPROC .cfi_endproc

scripts/Kbuild.include

@@ -63,6 +63,13 @@ as-option = $(shell if $(CC) $(CFLAGS) $(1) -Wa,-Z -c -o /dev/null \
 	     -xassembler /dev/null > /dev/null 2>&1; then echo "$(1)"; \
 	     else echo "$(2)"; fi ;)
 
+# as-instr
+# Usage: cflags-y += $(call as-instr, instr, option1, option2)
+
+as-instr = $(shell if echo -e "$(1)" | $(AS) -Z -o astest$$$$.out \
+		   2>&1 >/dev/null ; then echo "$(2)"; else echo "$(3)"; fi; \
+	           rm -f astest$$$$.out)
+
 # cc-option
 # Usage: cflags-y += $(call cc-option, -march=winchip-c6, -march=i586)