Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: "liuyuntao (F)" <liuyuntao12@huawei.com>
To: Alexander Lobakin <aleksander.lobakin@intel.com>
Cc: <linux-kernel@vger.kernel.org>, <linux-kbuild@vger.kernel.org>,
	<tglx@linutronix.de>, <mingo@redhat.com>, <bp@alien8.de>,
	<dave.hansen@linux.intel.com>, <hpa@zytor.com>,
	<masahiroy@kernel.org>, <nathan@kernel.org>, <nicolas@fjasle.eu>,
	<peterz@infradead.org>, <jpoimboe@kernel.org>,
	<leitao@debian.org>, <petr.pavlu@suse.com>,
	<richard.weiyang@gmail.com>, <ruanjinjie@huawei.com>,
	<ndesaulniers@google.com>, <jgross@suse.com>
Subject: Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Thu, 25 Apr 2024 14:37:19 +0800	[thread overview]
Message-ID: <d129733e-6c53-41b9-ae23-4a97c4eb5a30@huawei.com> (raw)
In-Reply-To: <8cb7f43e-0ee9-423d-ad27-93b3c69809f5@intel.com>


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