Messages in this thread | | | Date | Sat, 9 Mar 2024 14:46:59 +0800 | Subject | Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION | From | "liuyuntao (F)" <> |
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On 2024/3/9 8:01, Ard Biesheuvel wrote: > On Fri, 8 Mar 2024 at 16:37, Ard Biesheuvel <ardb@kernel.org> wrote: >> >> On Fri, 8 Mar 2024 at 15:27, Ard Biesheuvel <ardb@kernel.org> wrote: >>> >>> On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd@arndb.de> wrote: >>>> >>>> On Thu, Mar 7, 2024, at 16:12, Yuntao Liu wrote: >>>>> The current arm32 architecture does not yet support the >>>>> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. arm32 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. >>>>> >>>>> It boots normally with defconfig, vexpress_defconfig and tinyconfig. >>>>> >>>>> The size comparison of zImage is as follows: >>>>> defconfig vexpress_defconfig tinyconfig >>>>> 5137712 5138024 424192 no dce >>>>> 5032560 4997824 298384 dce >>>>> 2.0% 2.7% 29.7% shrink >>>>> >>>>> When using smaller config file, there is a significant reduction in the >>>>> size of the zImage. >>>>> >>>>> We also tested this patch on a commercially available single-board >>>>> computer, and the comparison is as follows: >>>>> a15eb_config >>>>> 2161384 no dce >>>>> 2092240 dce >>>>> 3.2% shrink >>>>> >>>>> The zImage size has been reduced by approximately 3.2%, which is 70KB on >>>>> 2.1M. >>>>> >>>>> Signed-off-by: Yuntao Liu <liuyuntao12@huawei.com> >>>> >>>> I've retested with both gcc-13 and clang-18, and so no >>>> more build issues. Your previous version already worked >>>> fine for me. >>>> >>>> I did some tests combining this with CONFIG_TRIM_UNUSED_KSYMS, >>>> which showed a significant improvement as expected. I also >>>> tried combining it with an experimental CONFIG_LTO_CLANG >>>> patch, but that did not show any further improvements. >>>> >>>> Tested-by: Arnd Bergmann <arnd@arndb.de> >>>> Reviewed-by: Arnd Bergmann <arnd@arndb.de> >>>> >>>> Adding Ard Biesheuvel and Fangrui Song to Cc, so they can comment >>>> on the ARM_VECTORS_TEXT workaround. I don't understand enough of >>>> the details of what is going on here. >>>> >>> >>> Thanks for the cc >>> >>>> Full quote of the patch below so they can see the whole thing. >>>> >>>> If they are also happy with the patch, I think you can send it >>>> into Russell's patch tracker at >>>> https://www.armlinux.org.uk/developer/patches/info.php >>>> >>> >>> No, not happy at all :-) >>> >>> The resulting kernel does not boot (built with GCC or Clang). And the >>> patch is buggy (see below) >>> >>>>> --- >>>>> v2: >>>>> - Support config XIP_KERNEL. >>>>> - Support LLVM compilation. >>>>> >>>>> v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@huawei.com/ >>>>> --- >>>>> arch/arm/Kconfig | 1 + >>>>> arch/arm/boot/compressed/vmlinux.lds.S | 4 ++-- >>>>> arch/arm/include/asm/vmlinux.lds.h | 18 +++++++++++++++++- >>>>> arch/arm/kernel/vmlinux-xip.lds.S | 8 ++++++-- >>>>> arch/arm/kernel/vmlinux.lds.S | 10 +++++++--- >>>>> 5 files changed, 33 insertions(+), 8 deletions(-) >>>>> >>>>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig >>>>> index 0af6709570d1..de78ceb821df 100644 >>>>> --- a/arch/arm/Kconfig >>>>> +++ b/arch/arm/Kconfig >>>>> @@ -113,6 +113,7 @@ config ARM >>>>> select HAVE_KERNEL_XZ >>>>> select HAVE_KPROBES if !XIP_KERNEL && !CPU_ENDIAN_BE32 && !CPU_V7M >>>>> select HAVE_KRETPROBES if HAVE_KPROBES >>>>> + select HAVE_LD_DEAD_CODE_DATA_ELIMINATION >>>>> select HAVE_MOD_ARCH_SPECIFIC >>>>> select HAVE_NMI >>>>> select HAVE_OPTPROBES if !THUMB2_KERNEL >>>>> diff --git a/arch/arm/boot/compressed/vmlinux.lds.S >>>>> b/arch/arm/boot/compressed/vmlinux.lds.S >>>>> index 3fcb3e62dc56..da21244aa892 100644 >>>>> --- a/arch/arm/boot/compressed/vmlinux.lds.S >>>>> +++ b/arch/arm/boot/compressed/vmlinux.lds.S >>>>> @@ -89,7 +89,7 @@ SECTIONS >>>>> * The EFI stub always executes from RAM, and runs strictly before >>>>> the >>>>> * decompressor, so we can make an exception for its r/w data, and >>>>> keep it >>>>> */ >>>>> - *(.data.efistub .bss.efistub) >>>>> + *(.data.* .bss.*) >>> >>> Why is this necessary? There is a reason we don't allow .data in the >>> decompressor. >>> >>>>> __pecoff_data_end = .; >>>>> >>>>> /* >>>>> @@ -125,7 +125,7 @@ SECTIONS >>>>> >>>>> . = BSS_START; >>>>> __bss_start = .; >>>>> - .bss : { *(.bss) } >>>>> + .bss : { *(.bss .bss.*) } >>>>> _end = .; >>>>> >>>>> . = ALIGN(8); /* the stack must be 64-bit aligned */ >>>>> diff --git a/arch/arm/include/asm/vmlinux.lds.h >>>>> b/arch/arm/include/asm/vmlinux.lds.h >>>>> index 4c8632d5c432..dfe2b6ad6b51 100644 >>>>> --- a/arch/arm/include/asm/vmlinux.lds.h >>>>> +++ b/arch/arm/include/asm/vmlinux.lds.h >>>>> @@ -42,7 +42,7 @@ >>>>> #define PROC_INFO \ >>>>> . = ALIGN(4); \ >>>>> __proc_info_begin = .; \ >>>>> - *(.proc.info.init) \ >>>>> + KEEP(*(.proc.info.init)) \ >>>>> __proc_info_end = .; >>>>> >>>>> #define IDMAP_TEXT \ >>>>> @@ -87,6 +87,22 @@ >>>>> *(.vfp11_veneer) \ >>>>> *(.v4_bx) >>>>> >>>>> +/* >>>>> +When CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled, it is important >>>>> to >>>>> +annotate .vectors sections with KEEP. While linking with ld, it is >>>>> +acceptable to directly use KEEP with .vectors sections in ARM_VECTORS. >>>>> +However, when using ld.lld for linking, KEEP is not recognized within >>>>> the >>>>> +OVERLAY command; it is treated as a regular string. Hence, it is >>>>> advisable >>>>> +to define a distinct section here that explicitly retains the .vectors >>>>> +sections when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is turned on. >>>>> +*/ >>>>> +#define ARM_VECTORS_TEXT \ >>>>> + .vectors.text : { \ >>>>> + KEEP(*(.vectors)) \ >>>>> + KEEP(*(.vectors.bhb.loop8)) \ >>>>> + KEEP(*(.vectors.bhb.bpiall)) \ >>>>> + } >>>>> + >>> >>> This looks fishy to me. How is this supposed to work? You cannot emit >>> these sections into some random other place in the binary. >>> >>> And also, ARM_VECTORS_TEXT is never used (by accident, see below) >>> >> >> The below appears to work for me: >> >> --- a/arch/arm/kernel/entry-armv.S >> +++ b/arch/arm/kernel/entry-armv.S >> @@ -1076,7 +1076,12 @@ >> W(b) vector_irq >> W(b) vector_fiq >> >> + .text >> + .reloc ., R_ARM_NONE, .vectors >> #ifdef CONFIG_HARDEN_BRANCH_HISTORY >> + .reloc ., R_ARM_NONE, .vectors.bhb.loop8 >> + .reloc ., R_ARM_NONE, .vectors.bhb.bpiall >> + >> .section .vectors.bhb.loop8, "ax", %progbits >> W(b) vector_rst >> W(b) vector_bhb_loop8_und > > ... or even better: > > --- a/arch/arm/kernel/entry-armv.S > +++ b/arch/arm/kernel/entry-armv.S > @@ -1066,4 +1066,5 @@ > > .section .vectors, "ax", %progbits > + .reloc .text, R_ARM_NONE, . > W(b) vector_rst > W(b) vector_und > @@ -1079,4 +1080,5 @@ > #ifdef CONFIG_HARDEN_BRANCH_HISTORY > .section .vectors.bhb.loop8, "ax", %progbits > + .reloc .text, R_ARM_NONE, . > W(b) vector_rst > W(b) vector_bhb_loop8_und > @@ -1091,4 +1093,5 @@ > > .section .vectors.bhb.bpiall, "ax", %progbits > + .reloc .text, R_ARM_NONE, . > W(b) vector_rst > W(b) vector_bhb_bpiall_und
I used `.reloc ., R_ARM_NONE, .vectors` to KEEP .vectors section, but it failed. It seems now that I did not use the reloc directive correctly. Thanks Ard, and your approach is concise and effctive. and, could I submit a v3 patch to apply these new changes?
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