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Message-ID: <845c7729-a967-48b3-b93b-8b0d14007bc3@huawei.com>
Date: Tue, 23 Apr 2024 20:10:33 +0800
From: "liuyuntao (F)" <liuyuntao12@...wei.com>
To: Wei Yang <richard.weiyang@...il.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>,
<ruanjinjie@...wei.com>, <ndesaulniers@...gle.com>, <jgross@...e.com>
Subject: Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
On 2024/4/23 8:59, Wei Yang wrote:
> Hi, Yuntao
>
> I am not that familiar with this, so got some questions below.
>
> On Mon, Apr 22, 2024 at 06:05:56AM +0000, Yuntao Liu wrote:
>> 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.
>>
>> 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.
>> ---
>> arch/x86/Kconfig | 1 +
>> arch/x86/kernel/vmlinux.lds.S | 24 ++++++++++++------------
>> scripts/link-vmlinux.sh | 2 +-
>> 3 files changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index a902680b6537..92dfbc8ee4e7 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -247,6 +247,7 @@ config X86
>> select HAVE_FUNCTION_ERROR_INJECTION
>> select HAVE_KRETPROBES
>> select HAVE_RETHOOK
>> + select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !CONFIG_UNWINDER_ORC && !CONFIG_MITIGATION_RETPOLINE
>
> How to make sure only these two config affect the function?
>
I have tested all the parameters of objtool, and only when these two
config are removed can gc-sections take effect.
>> select HAVE_LIVEPATCH if X86_64
>> select HAVE_MIXED_BREAKPOINTS_REGS
>> select HAVE_MOD_ARCH_SPECIFIC
>> diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
>> index 3509afc6a672..aeee2b9b6a6a 100644
>> --- a/arch/x86/kernel/vmlinux.lds.S
>> +++ b/arch/x86/kernel/vmlinux.lds.S
>> @@ -253,7 +253,7 @@ SECTIONS
>>
>> .x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
>> __x86_cpu_dev_start = .;
>> - *(.x86_cpu_dev.init)
>> + KEEP(*(.x86_cpu_dev.init))
>
> How you choose the sections to keep?
>
> If my understanding is correct, keep the sections is for gc-section. Sounds we
> need to keep all possible section, otherwise it would mis-behave?
>
Nicholas Piggin has mentioned it before.
> FYI, easiest way to check if you forgot to KEEP a linker table is
> to look at `readelf -S vmlinux` differences, and to see what is
> being trimmed, look at nm differences or use --print-gc-sections
> LD option to see what symbols you're trimming. Linker tables,
> boot entry, and exception entry tends to require anchoring.
https://lore.kernel.org/all/20170709031333.29443-1-npiggin@gmail.com/
>> __x86_cpu_dev_end = .;
>> }
>>
>> @@ -261,7 +261,7 @@ SECTIONS
>> .x86_intel_mid_dev.init : AT(ADDR(.x86_intel_mid_dev.init) - \
>> LOAD_OFFSET) {
>> __x86_intel_mid_dev_start = .;
>> - *(.x86_intel_mid_dev.init)
>> + KEEP(*(.x86_intel_mid_dev.init))
>> __x86_intel_mid_dev_end = .;
>> }
>> #endif
>> @@ -275,21 +275,21 @@ SECTIONS
>> . = ALIGN(8);
>> .retpoline_sites : AT(ADDR(.retpoline_sites) - LOAD_OFFSET) {
>> __retpoline_sites = .;
>> - *(.retpoline_sites)
>> + KEEP(*(.retpoline_sites))
>> __retpoline_sites_end = .;
>> }
>>
>> . = ALIGN(8);
>> .return_sites : AT(ADDR(.return_sites) - LOAD_OFFSET) {
>> __return_sites = .;
>> - *(.return_sites)
>> + KEEP(*(.return_sites))
>> __return_sites_end = .;
>> }
>>
>> . = ALIGN(8);
>> .call_sites : AT(ADDR(.call_sites) - LOAD_OFFSET) {
>> __call_sites = .;
>> - *(.call_sites)
>> + KEEP(*(.call_sites))
>> __call_sites_end = .;
>> }
>> #endif
>> @@ -298,7 +298,7 @@ SECTIONS
>> . = ALIGN(8);
>> .ibt_endbr_seal : AT(ADDR(.ibt_endbr_seal) - LOAD_OFFSET) {
>> __ibt_endbr_seal = .;
>> - *(.ibt_endbr_seal)
>> + KEEP(*(.ibt_endbr_seal))
>> __ibt_endbr_seal_end = .;
>> }
>> #endif
>> @@ -307,7 +307,7 @@ SECTIONS
>> . = ALIGN(8);
>> .cfi_sites : AT(ADDR(.cfi_sites) - LOAD_OFFSET) {
>> __cfi_sites = .;
>> - *(.cfi_sites)
>> + KEEP(*(.cfi_sites))
>> __cfi_sites_end = .;
>> }
>> #endif
>> @@ -320,7 +320,7 @@ SECTIONS
>> . = ALIGN(8);
>> .altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) {
>> __alt_instructions = .;
>> - *(.altinstructions)
>> + KEEP(*(.altinstructions))
>> __alt_instructions_end = .;
>> }
>>
>> @@ -330,13 +330,13 @@ SECTIONS
>> * get the address and the length of them to patch the kernel safely.
>> */
>> .altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
>> - *(.altinstr_replacement)
>> + KEEP(*(.altinstr_replacement))
>> }
>>
>> . = ALIGN(8);
>> .apicdrivers : AT(ADDR(.apicdrivers) - LOAD_OFFSET) {
>> __apicdrivers = .;
>> - *(.apicdrivers);
>> + KEEP(*(.apicdrivers));
>> __apicdrivers_end = .;
>> }
>>
>> @@ -406,7 +406,7 @@ SECTIONS
>> .brk : AT(ADDR(.brk) - LOAD_OFFSET) {
>> __brk_base = .;
>> . += 64 * 1024; /* 64k alignment slop space */
>> - *(.bss..brk) /* areas brk users have reserved */
>> + KEEP(*(.bss..brk)) /* areas brk users have reserved */
>> __brk_limit = .;
>> }
>>
>> @@ -432,7 +432,7 @@ SECTIONS
>> . = ALIGN(HPAGE_SIZE);
>> .init.scratch : AT(ADDR(.init.scratch) - LOAD_OFFSET) {
>> __init_scratch_begin = .;
>> - *(.init.scratch)
>> + KEEP(*(.init.scratch))
>> . = ALIGN(HPAGE_SIZE);
>> __init_scratch_end = .;
>> }
>> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> index 7862a8101747..7287b5a9f17d 100755
>> --- a/scripts/link-vmlinux.sh
>> +++ b/scripts/link-vmlinux.sh
>> @@ -60,7 +60,7 @@ vmlinux_link()
>> # skip output file argument
>> shift
>>
>> - if is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT; then
>> + if [ is_enabled CONFIG_LTO_CLANG || is_enabled CONFIG_X86_KERNEL_IBT ] && [ ! is_enabled CONFIG_LD_DEAD_CODE_DATA_ELIMINATION ]; then
>> # Use vmlinux.o instead of performing the slow LTO link again.
>> objs=vmlinux.o
>> libs=
>> --
>> 2.34.1
>
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