x86/build: convert function graph '-Os' error to warning

For pre-4.6.0 versions of GCC, which don't have '-mfentry', the
'-maccumulate-outgoing-args' option is required for function graph
tracing in order to avoid GCC bug 42109.

However, GCC ignores '-maccumulate-outgoing-args' when '-Os' is
also set.

Currently we force a build error to prevent that scenario, but that
breaks randconfigs. So change the error to a warning which also
disables CONFIG_CC_OPTIMIZE_FOR_SIZE.

Reported-by: Andi Kleen <[e-mail]>
Signed-off-by: Josh Poimboeuf <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: kbuild test robot <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/20170418214429.o7fbwbmf4nqosezy@treble
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/Makefile

···
   else
     ifeq ($(call cc-option-yn, -mfentry), n)
 	ACCUMULATE_OUTGOING_ARGS := 1
+
+	# GCC ignores '-maccumulate-outgoing-args' when used with '-Os'.
+	# If '-Os' is enabled, disable it and print a warning.
+        ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
+          undefine CONFIG_CC_OPTIMIZE_FOR_SIZE
+	  $(warning Disabling CONFIG_CC_OPTIMIZE_FOR_SIZE.  Your compiler does not have -mfentry so you cannot optimize for size with CONFIG_FUNCTION_GRAPH_TRACER.)
+        endif
+
     endif
   endif
 endif

arch/x86/kernel/ftrace.c

···
 #include <asm/ftrace.h>
 #include <asm/nops.h>
 
-#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && \
-	!defined(CC_USING_FENTRY) && \
-	!defined(CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE)
-# error The following combination is not supported: ((compiler missing -mfentry) || (CONFIG_X86_32 and !CONFIG_DYNAMIC_FTRACE)) && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE
-#endif
-
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 int ftrace_arch_code_modify_prepare(void)