linux-kernel - Re: [PATCH v5 2/3] Kbuild: make DWARF version a choice

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Message-ID: <20210120204025.GA548985@ubuntu-m3-large-x86>
Date:   Wed, 20 Jan 2021 13:40:25 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-kbuild@...r.kernel.org, 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 v5 2/3] Kbuild: make DWARF version a choice

On Fri, Jan 15, 2021 at 01:06:15PM -0800, Nick Desaulniers wrote:
> Modifies CONFIG_DEBUG_INFO_DWARF4 to be a member of a choice. Adds an
> explicit CONFIG_DEBUG_INFO_DWARF2, which is the default. Does so in a
> way that's forward compatible with existing configs, and makes adding
> future versions more straightforward.
> 
> Suggested-by: Arvind Sankar <nivedita@...m.mit.edu>
> Suggested-by: Fangrui Song <maskray@...gle.com>
> Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
>  Makefile          | 13 ++++++-------
>  lib/Kconfig.debug | 21 ++++++++++++++++-----
>  2 files changed, 22 insertions(+), 12 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index d49c3f39ceb4..4eb3bf7ee974 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -826,13 +826,12 @@ else
>  DEBUG_CFLAGS	+= -g
>  endif
>  
> -ifneq ($(LLVM_IAS),1)
> -KBUILD_AFLAGS	+= -Wa,-gdwarf-2
> -endif

Aren't you regressing this with this patch? Why is the hunk from 3/3
that adds

ifdef CONFIG_CC_IS_CLANG
ifneq ($(LLVM_IAS),1)

not in this patch?

> -ifdef CONFIG_DEBUG_INFO_DWARF4
> -DEBUG_CFLAGS	+= -gdwarf-4
> -endif
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF2) := 2
> +dwarf-version-$(CONFIG_DEBUG_INFO_DWARF4) := 4
> +DEBUG_CFLAGS	+= -gdwarf-$(dwarf-version-y)
> +# Binutils 2.35+ required for -gdwarf-4+ support.
> +dwarf-aflag	:= $(call as-option,-Wa$(comma)-gdwarf-$(dwarf-version-y))
> +KBUILD_AFLAGS	+= $(dwarf-aflag)
>  
>  ifdef CONFIG_DEBUG_INFO_REDUCED
>  DEBUG_CFLAGS	+= $(call cc-option, -femit-struct-debug-baseonly) \
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index dd7d8d35b2a5..e80770fac4f0 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -256,13 +256,24 @@ config DEBUG_INFO_SPLIT
>  	  to know about the .dwo files and include them.
>  	  Incompatible with older versions of ccache.
>  
> +choice
> +	prompt "DWARF version"
> +	help
> +	  Which version of DWARF debug info to emit.
> +
> +config DEBUG_INFO_DWARF2
> +	bool "Generate DWARF Version 2 debuginfo"
> +	help
> +	  Generate DWARF v2 debug info.
> +
>  config DEBUG_INFO_DWARF4
> -	bool "Generate dwarf4 debuginfo"
> +	bool "Generate DWARF Version 4 debuginfo"
>  	help
> -	  Generate dwarf4 debug info. This requires recent versions
> -	  of gcc and gdb. It makes the debug information larger.
> -	  But it significantly improves the success of resolving
> -	  variables in gdb on optimized code.
> +	  Generate DWARF v4 debug info. This requires gcc 4.5+ and gdb 7.0+.
> +	  It makes the debug information larger, but it significantly
> +	  improves the success of resolving variables in gdb on optimized code.
> +
> +endchoice # "DWARF version"
>  
>  config DEBUG_INFO_BTF
>  	bool "Generate BTF typeinfo"
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>