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Message-ID: <CAMj1kXH+FaddHV5--kqB_wVgw_M682MvchPB1BoCuDuA6vVyvg@mail.gmail.com>
Date: Fri, 15 Mar 2024 09:06:31 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Yuntao Liu <liuyuntao12@...wei.com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
arnd@...db.de, geert@...ux-m68k.org, linux@...linux.org.uk, afd@...com,
akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
geert+renesas@...der.be, corbet@....net, rppt@...nel.org, robh@...nel.org,
tglx@...utronix.de, linus.walleij@...aro.org, maskray@...gle.com
Subject: Re: [PATCH-next v3] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
On Fri, 15 Mar 2024 at 07:37, Yuntao Liu <liuyuntao12@...wei.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@...wei.com>
> Tested-by: Arnd Bergmann <arnd@...db.de>
> Reviewed-by: Arnd Bergmann <arnd@...db.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?
> /*
> @@ -125,7 +129,7 @@ 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..d60f6e83a9f7 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 \
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 6150a716828c..f01d23a220e6 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -1065,6 +1065,7 @@ vector_addrexcptn:
> .globl vector_fiq
>
> .section .vectors, "ax", %progbits
> + .reloc .text, R_ARM_NONE, .
> W(b) vector_rst
> W(b) vector_und
> ARM( .reloc ., R_ARM_LDR_PC_G0, .L__vector_swi )
> @@ -1078,6 +1079,7 @@ THUMB( .reloc ., R_ARM_THM_PC12, .L__vector_swi )
>
> #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
> ARM( .reloc ., R_ARM_LDR_PC_G0, .L__vector_bhb_loop8_swi )
> @@ -1090,6 +1092,7 @@ THUMB( .reloc ., R_ARM_THM_PC12, .L__vector_bhb_loop8_swi )
> W(b) vector_bhb_loop8_fiq
>
> .section .vectors.bhb.bpiall, "ax", %progbits
> + .reloc .text, R_ARM_NONE, .
> W(b) vector_rst
> W(b) vector_bhb_bpiall_und
> ARM( .reloc ., R_ARM_LDR_PC_G0, .L__vector_bhb_bpiall_swi )
> diff --git a/arch/arm/kernel/vmlinux-xip.lds.S b/arch/arm/kernel/vmlinux-xip.lds.S
> index c16d196b5aad..5eddb75a7174 100644
> --- a/arch/arm/kernel/vmlinux-xip.lds.S
> +++ b/arch/arm/kernel/vmlinux-xip.lds.S
> @@ -63,7 +63,7 @@ SECTIONS
> . = ALIGN(4);
> __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
> __start___ex_table = .;
> - ARM_MMU_KEEP(*(__ex_table))
> + ARM_MMU_KEEP(KEEP(*(__ex_table)))
> __stop___ex_table = .;
> }
>
> @@ -83,7 +83,7 @@ SECTIONS
> }
> .init.arch.info : {
> __arch_info_begin = .;
> - *(.arch.info.init)
> + KEEP(*(.arch.info.init))
> __arch_info_end = .;
> }
> .init.tagtable : {
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index bd9127c4b451..de373c6c2ae8 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -74,7 +74,7 @@ SECTIONS
> . = ALIGN(4);
> __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) {
> __start___ex_table = .;
> - ARM_MMU_KEEP(*(__ex_table))
> + ARM_MMU_KEEP(KEEP(*(__ex_table)))
> __stop___ex_table = .;
> }
>
> @@ -99,7 +99,7 @@ SECTIONS
> }
> .init.arch.info : {
> __arch_info_begin = .;
> - *(.arch.info.init)
> + KEEP(*(.arch.info.init))
> __arch_info_end = .;
> }
> .init.tagtable : {
> @@ -116,7 +116,7 @@ SECTIONS
> #endif
> .init.pv_table : {
> __pv_table_begin = .;
> - *(.pv_table)
> + KEEP(*(.pv_table))
> __pv_table_end = .;
> }
>
> --
> 2.34.1
>
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