linux-kernel - Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux

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Message-ID: <202005211506.2700F86@keescook>
Date:   Thu, 21 May 2020 15:08:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: reuse vmlinux.o in vmlinux_link

On Thu, May 21, 2020 at 01:27:16PM -0700, Sami Tolvanen wrote:
> Instead of linking all compilation units again each time vmlinux_link is
> called, reuse vmlinux.o from modpost_link.
> 
> With x86_64 allyesconfig, vmlinux_link is called three times and reusing
> vmlinux.o reduces the build time ~38 seconds on my system (59% reduction
> in the time spent in vmlinux_link).

Nice! Any time savings at final link is a big cumulative win.

> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>  scripts/link-vmlinux.sh | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index d09ab4afbda4..c6cc4305950c 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -77,11 +77,8 @@ vmlinux_link()
>  
>  	if [ "${SRCARCH}" != "um" ]; then
>  		objects="--whole-archive			\
> -			${KBUILD_VMLINUX_OBJS}			\
> +			vmlinux.o				\
>  			--no-whole-archive			\
> -			--start-group				\
> -			${KBUILD_VMLINUX_LIBS}			\
> -			--end-group				\
>  			${@}"
>  
>  		${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux}	\

I think the "um" case can be updated as well too, yes?

Also, I think the comment above modpost_link() needs to be updated now
to reflect the nature of how vmlinux.o gets used after this patch.

-- 
Kees Cook