Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

From: Alexander Lobakin <aleksander.lobakin@intel.com>
To: Yuntao Liu <liuyuntao12@huawei.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: Wed, 24 Apr 2024 13:06:26 +0200	[thread overview]
Message-ID: <8cb7f43e-0ee9-423d-ad27-93b3c69809f5@intel.com> (raw)
In-Reply-To: <20240422060556.1226848-1-liuyuntao12@huawei.com>

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