linux-kernel - Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA

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Date: Thu, 25 Apr 2024 14:37:19 +0800
From: "liuyuntao (F)" <liuyuntao12@...wei.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
CC: <linux-kernel@...r.kernel.org>, <linux-kbuild@...r.kernel.org>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <hpa@...or.com>, <masahiroy@...nel.org>,
	<nathan@...nel.org>, <nicolas@...sle.eu>, <peterz@...radead.org>,
	<jpoimboe@...nel.org>, <leitao@...ian.org>, <petr.pavlu@...e.com>,
	<richard.weiyang@...il.com>, <ruanjinjie@...wei.com>,
	<ndesaulniers@...gle.com>, <jgross@...e.com>
Subject: Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION



On 2024/4/24 19:06, Alexander Lobakin wrote:
> From: Yuntao Liu <liuyuntao12@...wei.com>
> 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.