Re: [PATCH v2 2/2] kbuild: link lib-y objects to vmlinux forcibly when CONFIG_MODULES=y

[Masahiro Yamada]

On Thu, Mar 12, 2020 at 7:38 AM Masahiro Yamada <masahiroy@xxxxxxxxxx> wrote:
>
> Kbuild supports not only obj-y but also lib-y to list objects linked to
> vmlinux.
>
> The difference between them is that all the objects from obj-y are
> forcibly linked to vmlinux by using --whole-archive, whereas the objects
> from lib-y are linked as needed; if there is no user of a lib-y object,
> it is not linked.
>
> lib-y is intended to list utility functions that may be called from all
> over the place (and may be unused at all), but it is a problem for
> EXPORT_SYMBOL(). Even if there is no call-site in the vmlinux, we need
> to keep exported symbols for the use from loadable modules.
>
> Commit 7f2084fa55e6 ("[kbuild] handle exports in lib-y objects reliably")
> worked around it by linking a dummy object, lib-ksyms.o, which contains
> references to all the symbols exported from lib.a in that directory.
> It uses the linker script command, EXTERN. Unfortunately, the meaning of
> EXTERN of ld.lld is different from that of ld.bfd. Therefore, this does
> not work with LD=ld.lld (CBL issue #515).
>
> Anyway, the build rule of lib-ksyms.o is somewhat tricky. So, I want to
> get rid of it.
>
> At first, I was thinking of accumulating lib-y objects into obj-y
> (or even replacing lib-y with obj-y entirely), but the lib-y syntax
> is used beyond the ordinary use in lib/ and arch/*/lib/.
>
> Examples:
>
>  - drivers/firmware/efi/libstub/Makefile builds lib.a, which is linked
>    into vmlinux in the own way (arm64), or linked to the decompressor
>    (arm, x86).
>
>  - arch/alpha/lib/Makefile builds lib.a which is linked not only to
>    vmlinux, but also to bootloaders in arch/alpha/boot/Makefile.
>
>  - arch/xtensa/boot/lib/Makefile builds lib.a for use from
>    arch/xtensa/boot/boot-redboot/Makefile.
>
> One more thing, adding everything to obj-y would increase the vmlinux
> size of allnoconfig (or tinyconfig).
>
> For less impact, I tweaked the destination of lib.a at the top Makefile;
> when CONFIG_MODULES=y, lib.a goes to KBUILD_VMLINUX_OBJS, which is
> forcibly linked to vmlinux, otherwise lib.a goes to KBUILD_VMLINUX_LIBS
> as before.
>
> The size impact for normal usecases is quite small since at lease one
> symbol in every lib-y object is eventually called by someone. In case
> you are intrested, here are the figures.
>
> x86_64_defconfig:
>
>    text    data     bss     dec     hex filename
> 19566602 5422072 1589328 26578002 1958c52 vmlinux.before
> 19566932 5422104 1589328 26578364 1958dbc vmlinux.after
>
> The case with the biggest impact is allnoconfig + CONFIG_MODULES=y.
>
> ARCH=x86 allnoconfig + CONFIG_MODULES=y:
>
>    text    data     bss     dec     hex filename
> 1175162  254740 1220608 2650510  28718e vmlinux.before
> 1177974  254836 1220608 2653418  287cea vmlinux.after
>
> Hopefully this is still not a big deal. The per-file trimming with the
> static library is not so effective after all.
>
> If fine-grained optimization is desired, some architectures support
> CONFIG_LD_DEAD_CODE_DATA_ELIMINATION, which trims dead code per-symbol
> basis. When LTO is supported in mainline, even better optimization will
> be possible.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/515
> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> ---

Applied to linux-kbuild.

I will rebase my branch during this MW,
so the commit ID will be unstable.
Please do not record it until it lands in Linus' tree.





-- 
Best Regards
Masahiro Yamada