Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Masahiro Yamada]

On Mon, Apr 22, 2024 at 3:41 PM Yuntao Liu <liuyuntao12@xxxxxxxxxx> wrote:
>
> The current x86 architecture does not yet support the
> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. x86 is widely used in
> embedded scenarios, and enabling this feature would be beneficial for
> reducing the size of the kernel image.
>
> In order to make this work, we keep the necessary tables by annotating
> them with KEEP, also it requires further changes to linker script to KEEP
> some tables and wildcard compiler generated sections into the right place.
>
> Enabling CONFIG_UNWINDER_ORC or CONFIG_MITIGATION_RETPOLINE will enable
> the objtool's --orc and --retpoline parameters, which will alter the
> layout of the binary file, thereby preventing gc-sections from functioning
> properly. Therefore, HAVE_LD_DEAD_CODE_DATA_ELIMINATION should only be
> selected when they are not enabled.
>
> Enabling CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT will use vmlinux.o
> instead of performing the slow LTO link again. This can also prevent
> gc-sections from functioning properly. Therefore, using this optimization
> when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not enabled.
>
> The size comparison of zImage is as follows:
> x86_def_defconfig  i386_defconfig    tinyconfig
> 10892288           10826240          607232          no dce
> 10748928           10719744          529408          dce
> 1.3%               0.98%             12.8%           shrink
>
> When using smaller config file, there is a significant reduction in the
> size of the zImage.
> ---

> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 7862a8101747..7287b5a9f17d 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -60,7 +60,7 @@ vmlinux_link()
>         # skip output file argument
>         shift
>
> -       if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
> +       if [ is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT ] && [ ! is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION ]; then
>                 # Use vmlinux.o instead of performing the slow LTO link again.
>                 objs=vmlinux.o
>                 libs=
> --


This is wrong.
You should not put is_enabled inside [ ... ]

(is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT) && !
is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION

is still weird.


When CONFIG_LTO_CLANG=y and CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y,
the result of LTO will be discarded.


-- 
Best Regards
Masahiro Yamada