Re: [PATCH-next v3] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

["liuyuntao (F)" <liuyuntao12@huawei.com>]

On 2024/3/15 18:46, Ard Biesheuvel wrote:
> On Fri, 15 Mar 2024 at 11:23, liuyuntao (F) <liuyuntao12@huawei.com> wrote:
>>
>>
>>
>> On 2024/3/15 16:06, Ard Biesheuvel wrote:
>>> On Fri, 15 Mar 2024 at 07:37, Yuntao Liu <liuyuntao12@huawei.com> 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.
>>>> When using ld.lld for linking, the KEEP keyword is not recognized within
>>>> the OVERLAY command, Ard proposed a concise method to solve this problem.
>>>>
>>>> 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>
>>>> Tested-by: Arnd Bergmann <arnd@arndb.de>
>>>> Reviewed-by: Arnd Bergmann <arnd@arndb.de>
>>>> ---
>>>> v3:
>>>>      - A better way to KEEP .vectors section for ld.lld linking.
>>>>
>>>> v2:
>>>>      - Support config XIP_KERNEL.
>>>>      - Support LLVM compilation.
>>>>      - https://lore.kernel.org/all/20240307151231.654025-1-liuyuntao12@huawei.com/
>>>>
>>>> v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@huawei.com/
>>>> ---
>>>>    arch/arm/Kconfig                       | 1 +
>>>>    arch/arm/boot/compressed/vmlinux.lds.S | 6 +++++-
>>>>    arch/arm/include/asm/vmlinux.lds.h     | 2 +-
>>>>    arch/arm/kernel/entry-armv.S           | 3 +++
>>>>    arch/arm/kernel/vmlinux-xip.lds.S      | 4 ++--
>>>>    arch/arm/kernel/vmlinux.lds.S          | 6 +++---
>>>>    6 files changed, 15 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>>>> index b14aed3a17ab..45f25f6e7a62 100644
>>>> --- a/arch/arm/Kconfig
>>>> +++ b/arch/arm/Kconfig
>>>> @@ -114,6 +114,7 @@ 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..affd30714f01 100644
>>>> --- a/arch/arm/boot/compressed/vmlinux.lds.S
>>>> +++ b/arch/arm/boot/compressed/vmlinux.lds.S
>>>> @@ -89,7 +89,11 @@ 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
>>>>         */
>>>> +#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
>>>> +    *(.data.* .bss.*)
>>>> +#else
>>>>        *(.data.efistub .bss.efistub)
>>>> +#endif
>>>>        __pecoff_data_end = .;
>>>>
>>>
>>> This is still not right.
>>>
>>> Can you just add -fno-data-sections to cflags-$(CONFIG_ARM) in
>>> drivers/firmware/efi/libstub/Makefile?
>>>
>>
>> I rebuild kernel in such way:
>> --- a/drivers/firmware/efi/libstub/Makefile
>> +++ b/drivers/firmware/efi/libstub/Makefile
>> @@ -28,6 +28,7 @@ cflags-$(CONFIG_ARM)          += -DEFI_HAVE_STRLEN
>> -DEFI_HAVE_STRNLEN \
>>                                      -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
>>                                      -DEFI_HAVE_STRCMP -fno-builtin -fpic \
>>                                      $(call cc-option,-mno-single-pic-base)
>> +cflags-$(CONFIG_ARM)           += -fno-data-sections
>>    cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE -mno-relax
>>    cflags-$(CONFIG_LOONGARCH)     += -fpi
>> but I am still encountering this error:
>> arm-linux-gnueabi-ld: warning: orphan section `.data.efi_loglevel' from
>> `drivers/firmware/efi/libstub/printk.stub.o' being placed in section
>> `.data.efi_loglevel'
>> arm-linux-gnueabi-ld: warning: orphan section `.data.screen_info_guid'
>> from `drivers/firmware/efi/libstub/screen_info.stub.o' being placed in
>> section `.data.screen_info_guid'
>> arm-linux-gnueabi-ld: warning: orphan section `.data.cpu_state_guid'
>> from `drivers/firmware/efi/libstub/arm32-stub.stub.o' being placed in
>> section `.data.cpu_state_guid'
>> arm-linux-gnueabi-ld: warning: orphan section `.data.efi_nokaslr' from
>> `drivers/firmware/efi/libstub/efi-stub-helper.stub.o' being placed in
>> section `.data.efi_nokaslr'
>> arm-linux-gnueabi-ld: error: zImage file size is incorrect
>>
>> I am puzzled because I could not find any option named
>> -fno-data-sections for GCC.
>>
> 
> How about
> 
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -28,6 +28,7 @@
>                                     -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
>                                     -DEFI_HAVE_STRCMP -fno-builtin -fpic \
>                                     $(call cc-option,-mno-single-pic-base)
> +cflags-$(CONFIG_ARM)           := $(filter-out -fdata-sections, $(CFLAGS-y))
>   cflags-$(CONFIG_RISCV)         += -fpic -DNO_ALTERNATIVE -mno-relax
>   cflags-$(CONFIG_LOONGARCH)     += -fpie

Another error:
cannot initialize array of ‘short unsigned int’ from a string literal 
with type array of ‘unsigned int’
  17 | static const efi_char16_t shim_MokSBState_name[] = L"MokSBStateRT";

I checked the cflags-y variable, and the -fshort-wchar option does 
exist.cflags-y = -std=gnu11 -fshort-wchar -funsigned-char -fno-common 
   -fno-PIE -fno-strict-aliasing ...