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Date: Wed, 3 Jun 2026 18:38:58 -0700
From: Nathan Chancellor <nathan@kernel.org>
To: Wentao Guan <guanwentao@uniontech.com>
Cc: nsc@kernel.org, tamird@kernel.org, linux-kbuild@vger.kernel.org,
	linux-kernel@vger.kernel.org, petr.pavlu@suse.com
Subject: Re: [PATCH] kbuild: try readelf first in gen_symversions
Message-ID: <20260604013858.GB1329739@ax162>
References: <20260603161732.52477-1-guanwentao@uniontech.com>
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In-Reply-To: <20260603161732.52477-1-guanwentao@uniontech.com>

On Thu, Jun 04, 2026 at 12:17:32AM +0800, Wentao Guan wrote:
> Use readelf to dig out if <file>.o contain a __export_symbol_*.
> 
> Instead of nm, readelf is more faster, and significantly improve speed
> when enable CONFIG_MODVERSIONS.
> 
> Build x86_64_defconfigs in 2C4T cloud server with CONFIG_MODVERSIONS=y:
> With patch:
> real    17m21.019s
> user    61m48.388s
> sys     4m27.709s
> Without patch:
> real    17m39.435s
> user    62m24.686s
> sys     5m3.200s
> 
> Link: https://lore.kernel.org/all/tencent_2FA16E0A18D6D0C0703F5D49@qq.com/
> Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
> ---
>  scripts/Makefile.build | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 3498d25b15e85..54a91bc144cce 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -233,7 +233,7 @@ ifdef CONFIG_MODVERSIONS
>  #   be compiled and linked to the kernel and/or modules.
>  
>  gen_symversions =								\
> -	if $(NM) $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\
> +	if $(READELF) -sW $@ 2>/dev/null | grep -q ' __export_symbol_'; then		\

This breaks modversioning for Clang LTO builds, as llvm-nm can read LLVM
bitcode but llvm-readelf cannot, it expects strictly ELF.

Is there any performance gain with adding '-m1' to the grep command so
that it stops looking for a match after the first export symbol is
found?

>  		$(cmd_gensymtypes_$1) >> $(dot-target).cmd;			\
>  	fi
>  
> -- 
> 2.30.2
> 

-- 
Cheers,
Nathan