[PATCH-next v4] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
From: Yuntao Liu
Date: Fri Mar 15 2024 - 22:44:53 EST
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, KEEP is not recognized within the OVERLAY
command, and 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@xxxxxxxxxx>
Tested-by: Arnd Bergmann <arnd@xxxxxxxx>
Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
---
v4:
- remove -fdata-sections flag from KBUILD_CFLAGS_KERNEL in drivers/firmware/efi/libstub/Makefile
v3:
- A better way to KEEP .vectors section for ld.lld linking.
- https://lore.kernel.org/all/20240315063154.696633-1-liuyuntao12@xxxxxxxxxx/
v2:
- Support config XIP_KERNEL.
- Support LLVM compilation.
- https://lore.kernel.org/all/20240307151231.654025-1-liuyuntao12@xxxxxxxxxx/
v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@xxxxxxxxxx/
---
arch/arm/Kconfig | 1 +
arch/arm/boot/compressed/vmlinux.lds.S | 2 +-
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 +++---
drivers/firmware/efi/libstub/Makefile | 4 ++++
7 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..d411abd4310e 100644
--- a/arch/arm/boot/compressed/vmlinux.lds.S
+++ b/arch/arm/boot/compressed/vmlinux.lds.S
@@ -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..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 = .;
}
diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 31eb1e287ce1..619f1f9d9ba9 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -56,6 +56,10 @@ KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_CFI), $(KBUILD_CFLAGS))
# disable LTO
KBUILD_CFLAGS := $(filter-out $(CC_FLAGS_LTO), $(KBUILD_CFLAGS))
+# The .data section would be renamed to .data.efistub, therefore, remove
+# `-fdata-sections` flag from KBUILD_CFLAGS_KERNEL
+KBUILD_CFLAGS_KERNEL := $(filter-out -fdata-sections, $(KBUILD_CFLAGS_KERNEL))
+
GCOV_PROFILE := n
# Sanitizer runtimes are unavailable and cannot be linked here.
KASAN_SANITIZE := n
--
2.34.1