Messages in this thread | ![/](/images/icornerl.gif) | | Date | Tue, 23 Apr 2024 20:10:33 +0800 | Subject | Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION | From | "liuyuntao (F)" <> |
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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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