[PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
liuyuntao (F)
liuyuntao12 at huawei.com
Fri Mar 8 22:46:59 PST 2024
On 2024/3/9 8:01, Ard Biesheuvel wrote:
> On Fri, 8 Mar 2024 at 16:37, Ard Biesheuvel <ardb at kernel.org> wrote:
>>
>> On Fri, 8 Mar 2024 at 15:27, Ard Biesheuvel <ardb at kernel.org> wrote:
>>>
>>> On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd at 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 at 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 at arndb.de>
>>>> Reviewed-by: Arnd Bergmann <arnd at 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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