Re: [PATCH-next v3] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
From: liuyuntao (F)
Date: Fri Mar 15 2024 - 07:49:02 EST
On 2024/3/15 18:46, Ard Biesheuvel wrote:
On Fri, 15 Mar 2024 at 11:23, liuyuntao (F) <liuyuntao12@xxxxxxxxxx> wrote:
On 2024/3/15 16:06, Ard Biesheuvel wrote:
On Fri, 15 Mar 2024 at 07:37, Yuntao Liu <liuyuntao12@xxxxxxxxxx> 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@xxxxxxxxxx>
Tested-by: Arnd Bergmann <arnd@xxxxxxxx>
Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
---
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@xxxxxxxxxx/
v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@xxxxxxxxxx/
---
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 ...