linux-kernel - Re: [PATCH v7 1/2] Kbuild: make DWARF version a choice

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Date:   Wed, 3 Feb 2021 15:16:24 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Jakub Jelinek <jakub@...hat.com>,
        Fangrui Song <maskray@...gle.com>,
        Caroline Tice <cmtice@...gle.com>,
        Nick Clifton <nickc@...hat.com>, Yonghong Song <yhs@...com>,
        Jiri Olsa <jolsa@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH v7 1/2] Kbuild: make DWARF version a choice

On Wed, Feb 3, 2021 at 2:24 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Sat, Jan 30, 2021 at 10:52 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >
> > On Fri, Jan 29, 2021 at 04:44:00PM -0800, Nick Desaulniers wrote:
> > > Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice which is
> > > the default. Does so in a way that's forward compatible with existing
> > > configs, and makes adding future versions more straightforward.
> > >
> > > GCC since ~4.8 has defaulted to this DWARF version implicitly.
> > >
> > > Suggested-by: Arvind Sankar <nivedita@...m.mit.edu>
> > > Suggested-by: Fangrui Song <maskray@...gle.com>
> > > Suggested-by: Nathan Chancellor <nathan@...nel.org>
> > > Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> >
> > One comment below:
> >
> > Reviewed-by: Nathan Chancellor <nathan@...nel.org>
> >
> > > ---
> > >  Makefile          |  5 ++---
> > >  lib/Kconfig.debug | 16 +++++++++++-----
> > >  2 files changed, 13 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 95ab9856f357..d2b4980807e0 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -830,9 +830,8 @@ ifneq ($(LLVM_IAS),1)
> > >  KBUILD_AFLAGS        += -Wa,-gdwarf-2
> >
> > It is probably worth a comment somewhere that assembly files will still
> > have DWARF v2.
>
> I agree.
> Please noting the reason will be helpful.

Via a comment in the source, or in the commit message?

>
> Could you summarize Jakub's comment in short?
> https://patchwork.kernel.org/project/linux-kbuild/patch/20201022012106.1875129-1-ndesaulniers@google.com/#23727667

Via a comment in the source, or in the commit message?

> One more question.
>
>
> Can we remove -g option like follows?
>
>
>  ifdef CONFIG_DEBUG_INFO_SPLIT
>  DEBUG_CFLAGS   += -gsplit-dwarf
> -else
> -DEBUG_CFLAGS   += -g
>  endif
>
>
>
>
>
> In the current mainline code,
> -g is the only debug option
> if CONFIG_DEBUG_INFO_DWARF4 is disabled.
>
>
> The GCC manual says:
> https://gcc.gnu.org/onlinedocs/gcc-10.2.0/gcc/Debugging-Options.html#Debugging-Options
>
>
> -g
>
>     Produce debugging information in the operating system’s
>     native format (stabs, COFF, XCOFF, or DWARF).
>     GDB can work with this debugging information.
>
>
> Of course, we expect the -g option will produce
> the debug info in the DWARF format.
>
>
>
>
>
> With this patch set applied, it is very explicit.
>
> Only the format type, but also the version.
>
> The compiler will be given either
> -gdwarf-4 or -gdwarf-5,
> making the -g option redundant, I think.

Can I provide that as a separate patch?  I don't want any breakage
from that to delay DWARF v5 support further.  If so, should it be the
first patch or last?
-- 
Thanks,
~Nick Desaulniers