On 2024/4/23 0:01, Masahiro Yamada wrote:
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 [ ... ]
My mistake.
(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.
On arm and risc-v, these two configs can both be enabled without any
issues, i think, it should be the same for x86 as well.
