linux-kernel - Re: [PATCH] x86: mostly disable '-maccumulate-outgoing-args'

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Date:   Thu, 16 Mar 2017 15:06:48 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, Andy Lutomirski <luto@...capital.net>,
        Pavel Machek <pavel@....cz>,
        kernel list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86: mostly disable '-maccumulate-outgoing-args'

On Thu, 16 Mar 2017 13:36:35 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Thu, Mar 16, 2017 at 01:32:01PM -0400, Steven Rostedt wrote:
> > On Thu, 16 Mar 2017 10:42:08 -0500
> > Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >   
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > > ---
> > >  arch/x86/Makefile        | 29 +++++++++++++++++++++++++----
> > >  arch/x86/Makefile_32.cpu | 18 ------------------
> > >  arch/x86/kernel/ftrace.c |  6 ++++++
> > >  scripts/Kbuild.include   |  4 ++++
> > >  4 files changed, 35 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > > index 2d44933..fa45989b 100644
> > > --- a/arch/x86/Makefile
> > > +++ b/arch/x86/Makefile
> > > @@ -120,10 +120,6 @@ else
> > >          # -funit-at-a-time shrinks the kernel .text considerably
> > >          # unfortunately it makes reading oopses harder.
> > >          KBUILD_CFLAGS += $(call cc-option,-funit-at-a-time)
> > > -
> > > -        # this works around some issues with generating unwind tables in older gccs
> > > -        # newer gccs do it by default
> > > -        KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args)
> > >  endif
> > >  
> > >  ifdef CONFIG_X86_X32
> > > @@ -147,6 +143,31 @@ ifeq ($(CONFIG_KMEMCHECK),y)
> > >  	KBUILD_CFLAGS += $(call cc-option,-fno-builtin-memcpy)
> > >  endif
> > >  
> > > +# If the function graph tracer is used with mcount instead of fentry,
> > > +# '-maccumulate-outgoing-args' is needed to prevent gcc bug  
> > 
> > 			"to prevent a gcc bug"  
> 
> It was
> 
>   "to prevent gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42109"
> 
> where "gcc bug" was an adjective and the URL was a noun.  But yeah,
> that's kind of confusing, and the line wrap made it more so.  Maybe I'll
> change it to
> 
>   "to prevent a gcc bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42109)"

Hmm, "the" would have made it work too.

> 
> and a similar change for the jump label bug comment.
> 
> > > +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42109
> > > +ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > > +  ifndef CONFIG_HAVE_FENTRY
> > > +	ACCUMULATE_OUTGOING_ARGS := 1
> > > +  else
> > > +    ifeq ($(call cc-option, -mfentry),)  
> > 
> > Hmm, the blank entry makes me nervous. I wonder if it would be better
> > if we had ifneq ($(call cc-option-yn, -mfentry),y)
> > 
> > Unfortunately, there's one of each in the existing kernel, so there is
> > really no precedence.  
> 
> Either way seems fine.  I'll go with your suggested change.
> 
> > > +	ACCUMULATE_OUTGOING_ARGS := 1
> > > +    endif
> > > +  endif
> > > +endif
> > > +
> > > +# Jump labels need '-maccumulate-outgoing-args' for gcc < 4.5.2 to prevent  
> > 
> > Can we make a test instead? I hate testing versions, and things get
> > backported all the time. We usually like to have a test case instead of
> > relying on versions. Not to mention, a newer gcc may one day break.  
> 
> Tests are generally better, but I'm not sure how to test for this
> cleanly.  The test is rather big for embedding in a makefile:
> 
>   https://gcc.gnu.org/bugzilla/attachment.cgi?id=22199
> 
> Any ideas?
> 

I'd reply but I see you figured it out yourself.

-- Steve