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

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Date: Thu, 25 Apr 2024 17:41:42 +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 2024/4/25 17:21, Alexander Lobakin wrote:
> From: Liuyuntao (F) <liuyuntao12@...wei.com>
> Date: Thu, 25 Apr 2024 14:37:19 +0800
> 
>>
>>
>> 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?
> 
> Nope, it was a year ago and I haven't touched it since then. Did the
> low-level code change a lot?
> 
No, I'm not yet certain what changes have had an impact on it compared 
to a year ago.