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Message-ID: <20170316150648.2375e1e5@gandalf.local.home>
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
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