[PATCH-next v3] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[liuyuntao (F)]

On 2024/3/15 16:06, Ard Biesheuvel wrote:
> On Fri, 15 Mar 2024 at 07:37, Yuntao Liu <liuyuntao12 at 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 at huawei.com>
>> Tested-by: Arnd Bergmann <arnd at arndb.de>
>> Reviewed-by: Arnd Bergmann <arnd at 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.