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

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Date:   Sat, 20 Aug 2022 01:52:04 +0300
From:   Dmitrii Bundin <dmitrii.bundin.a@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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>
Subject: Re: [PATCH v3] kbuild: add debug level and macro defs options

On Fri, Aug 19, 2022 at 8:42 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> Is any of this really necessary?

Consider the case if CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y.
Prior to GCC11/Clang12 -gsplit-dwarf implied -g2. So on newer
compilers with -gsplit-dwarf in use there would be no debug symbols
produced. -gdwarf-4/5 still implies -g2, but in case
CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT=y neither of the options are
set. So it seems like a reasonable choice to provide a debug info
level config that would explicitly set the level without relying on
implicits. The default value of the config is set to -g2 to not change
the build behavior that was before introducing the option. And it
works for both older and newer versions of GCC/Clang in the same way.
The benefits of the -g1 option are indeed questionable except that it
produces an image with ~20% less in size.

> It seems like a great way to bloat
> vmlinux artifacts built with CONFIG_DEBUG_INFO even further.
The defaults were chosen to not change the build behavior that was
before introducing the options. Or did you mean something else?

> The
> above link mentions "when debugging with GDB."  In that case, please
> don't add new Kconfigs for these; just set -g3 when
> CONFIG_GDB_SCRIPTS=y.

CONFIG_GDB_SCRIPTS does not necessarily mean that -g3 is wanted, -g2
(default) is usually a reasonable choice. The -g3 option is very
useful when debugging macro-intensive code, but requires much more
disk space to build. I documented it explicitly in the help section of
DEBUG_INFO_LEVEL. GCC and Clang use different options to include macro
definitions so it was handled depending on the compiler used.

Regards
Dmitrii