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

From: Masahiro Yamada <masahiroy@kernel.org>
To: Dmitrii Bundin <dmitrii.bundin.a@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Eric Dumazet <edumazet@google.com>,
	Isabella Basso <isabbasso@riseup.net>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Fangrui Song <maskray@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH v3] kbuild: add debug level and macro defs options
Date: Fri, 30 Sep 2022 06:06:46 +0900	[thread overview]
Message-ID: <CAK7LNATeW+c5+Kxnj9M4N+yNSv+7ot7bLTHzO3Z0Xb_XEW_6Nw@mail.gmail.com> (raw)
In-Reply-To: <CANXV_XzdTTYc2w7Ur8zY=ijOofg91yfF7RLhedbVH0rmi3c2yA@mail.gmail.com>

On Tue, Aug 23, 2022 at 7:42 AM Dmitrii Bundin
<dmitrii.bundin.a@gmail.com> wrote:
>
> On Tue, Aug 23, 2022 at 12:36 AM Nick Desaulniers
> <ndesaulniers@google.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.

This was fixed by commit 32ef9e5054ec0321b9336058c58ec749e9c6b0fe,
which is now in the mainline.

> > 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.

I think that would be much saner than this patch.

CONFIG_DEBUG_INFO_LEVEL is a direct way to specify the debug level.

CONFIG_DEBUG_MACRO_DEFINITIONS is feature-driven.

Do not mix two different ways.

CONFIG_DEBUG_INFO_LEVEL is here just because Andrew Morton suggested that.

The debug level is compiler-specific. There is no guarantee
that there is a common range.

The debug level range of GCC is 0-3.
Clang accepts 3, but -g3 has no effect.
The debug level range of Rustc is 0-2.

See how badly scripts/Makefile.debug looks in linux-next.

How should Rustc behave for CONFIG_DEBUG_INFO_LEVEL=3 ?

-Cdebuginfo=3 is a compile error.

  RUSTC L rust/core.o
error: debug info level needs to be between 0-2 (instead was `3`)

You cannot directly specify the debug level number given that
we support multiple compilers with different policy for
debug level options.

> > 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?
>
> Regards
> Dmitrii

-- 
Best Regards
Masahiro Yamada