[PATCH-next v3] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Ard Biesheuvel]

On Fri, 15 Mar 2024 at 11:23, liuyuntao (F) <liuyuntao12 at huawei.com> wrote:
>
>
>
> 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.
>

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