linux-kernel - Re: [PATCH v3] kbuild: add debug level and macro defs options

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Date:   Wed, 24 Aug 2022 16:25:19 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Dmitrii Bundin <dmitrii.bundin.a@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Isabella Basso <isabbasso@...eup.net>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Doug Anderson <dianders@...omium.org>
Subject: Re: [PATCH v3] kbuild: add debug level and macro defs options

On Mon, Aug 22, 2022 at 3:42 PM Dmitrii Bundin
<dmitrii.bundin.a@...il.com> wrote:
>
> On Tue, Aug 23, 2022 at 12:36 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > or perhaps that simply needs to be `-g -gsplit-dwarf`?  In which case,
> > that if/else could just be re-arranged.
>
> How about simply assigning DEBUG_CFLAGS   := -g at the very beginning
> without any conditions? This would provide the default with the
> possibility of overriding later and -gsplit-dwarf does not necessarily
> come with -g implicitly.

SGTM; I'd accept such a patch.

> > Honestly, I really don't think we need to be wrapping every compiler
> > command line flag under the sun in a kconfig option.
>
> This indeed sounds reasonable to me. So the key point here is to not
> bloat the kconfig with options related to every compiler flag. But I
> think it still might be useful to provide some option that would
> include sort of full debug information compilers may produce. With
> this approach there would be, in fact 3 different levels of debug
> information supported by Kconfig: reduced, default and full. The full
> level would increase everything like -g3, and -fdebug-macro for Clang,
> and probably others.

Has anyone asked for that though?  It seems like your intent with this
patch is to fix the surprising+user hostile behavior of compilers
requiring -g to be used when -gsplit-dwarf is used.

If someone using GDB_SCRIPTS or KGDB was like "man, I wish I could
debug macros" then sure I'd be more likely to accept this. Without the
need, this is just wrapping every compiler option in a kconfig, which
frustrates randconfig testing bots.  This will slow them down and
bloat their artifacts when randconfig selects -g3, so I'd like someone
to come forward saying they need this and why.

>
> > Or add -g1 to CONFIG_DEBUG_INFO_REDUCED.
>
> I ran some tests and there was indeed some decrease in size. That
> combination probably might be useful.
>
> Any thoughts?

I think there's also -gmlt; when that is preferable to -g1 IDK. Why
either of those weren't used in the first place, IDK.

The help text in DEBUG_INFO_REDUCED in lib/Kconfig.debug makes it
sound like -gmlt is what is wanted.  Maybe that should be updated.

But I think DEBUG_INFO_REDUCED is redundant if we were to accept
DEBUG_INFO_LEVEL. Both don't need to exist IMO.
-- 
Thanks,
~Nick Desaulniers