On 2024/4/24 19:06, Alexander Lobakin wrote:
From: Yuntao Liu <liuyuntao12@xxxxxxxxxx>
Date: Mon, 22 Apr 2024 06:05:56 +0000
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.
Dunno, I have DCE enabled for years on my home kernel, see commit [0]
with both ORC and retpolines enabled, and I didn't have any issues.
vmlinux still shrinks well, even with Clang LTO.
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.
[0] https://github.com/solbjorn/linux/commit/25c4953ea73d
Thanks,
Olek
I apply your patch, and use LLVM toolchain to compile the kernel, it not
boot on QEMU.
I use the following command.
qemu-system-x86_64 -smp 2 -m 1024M -nographic -kernel mainline_linux/arch/x86/boot/bzImage -hda rootfs.img -append "root=/dev/sda console=ttyS0 rootfstype=ext4 init=/linuxrc rw"
Have you tested your patch on the latest mainline version?
Thanks.
Yuntao.
