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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Tue, Apr 18, 2017 at 01:25:19PM -0700, Andi Kleen wrote:
> On Tue, Apr 18, 2017 at 03:19:42PM -0500, Josh Poimboeuf wrote:
> > On Tue, Apr 18, 2017 at 11:52:41AM -0700, Andi Kleen wrote:
> > > Josh Poimboeuf <jpoimboe@redhat.com> writes:
> > > >
> > > > The error is working as designed.  gcc < 4.6.0 doesn't have -mfentry, so
> > > > it fails the above check on x86.  Can you add a skip rule?  It should
> > > > skip building the following case:
> > > >
> > > >   x86 && ((gcc < 4.6.0) || (CONFIG_X86_32 and !CONFIG_DYNAMIC_FTRACE))
> > > > && CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_CC_OPTIMIZE_FOR_SIZE
> > > 
> > > This will also break everyone's randconfig builds. The better way
> > > would be to check this in the Makefile and disable one of the options
> > 
> > I don't know how to disable a config option from the Makefile like that.
> 
> You just don't do something in the Makefile, e.g. don't set the -Os 
> 
> For an example see the LTO Makefile here
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git/tree/scripts/Makefile.lto?h=lto-411-1

This seems to work:

----

From: Josh Poimboeuf <jpoimboe@redhat.com>
Subject: [PATCH] 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 <andi@firstfloor.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/Makefile        | 8 ++++++++
 arch/x86/kernel/ftrace.c | 6 ------
 2 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 73604e1..4430dd4 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -154,6 +154,14 @@ ifdef CONFIG_FUNCTION_GRAPH_TRACER
   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
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 0e5ceac..5b71535 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -29,12 +29,6 @@
 #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)
-- 
2.7.4