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Date: Sat, 9 Mar 2024 14:42:08 +0800
From: "liuyuntao (F)" <liuyuntao12@...wei.com>
To: Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>
CC: <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
Fangrui Song <maskray@...gle.com>, Russell King <linux@...linux.org.uk>,
Andrew Davis <afd@...com>, Andrew Morton <akpm@...ux-foundation.org>, "Kirill
A. Shutemov" <kirill.shutemov@...ux.intel.com>, Geert Uytterhoeven
<geert+renesas@...der.be>, Jonathan Corbet <corbet@....net>, Mike Rapoport
<rppt@...nel.org>, Rob Herring <robh@...nel.org>, Thomas Gleixner
<tglx@...utronix.de>, Linus Walleij <linus.walleij@...aro.org>,
<llvm@...ts.linux.dev>
Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
On 2024/3/8 22:27, Ard Biesheuvel wrote:
> On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd@...db.de> wrote:
>>
>> On Thu, Mar 7, 2024, at 16:12, Yuntao Liu 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.
>>>
>>> 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>
>>
>> I've retested with both gcc-13 and clang-18, and so no
>> more build issues. Your previous version already worked
>> fine for me.
>>
>> I did some tests combining this with CONFIG_TRIM_UNUSED_KSYMS,
>> which showed a significant improvement as expected. I also
>> tried combining it with an experimental CONFIG_LTO_CLANG
>> patch, but that did not show any further improvements.
>>
>> Tested-by: Arnd Bergmann <arnd@...db.de>
>> Reviewed-by: Arnd Bergmann <arnd@...db.de>
>>
>> Adding Ard Biesheuvel and Fangrui Song to Cc, so they can comment
>> on the ARM_VECTORS_TEXT workaround. I don't understand enough of
>> the details of what is going on here.
>>
>
> Thanks for the cc
>
>> Full quote of the patch below so they can see the whole thing.
>>
>> If they are also happy with the patch, I think you can send it
>> into Russell's patch tracker at
>> https://www.armlinux.org.uk/developer/patches/info.php
>>
>
> No, not happy at all :-)
>
> The resulting kernel does not boot (built with GCC or Clang). And the
> patch is buggy (see below)
>
>>> ---
>>> v2:
>>> - Support config XIP_KERNEL.
>>> - Support LLVM compilation.
>>>
>>> v1: https://lore.kernel.org/all/20240220081527.23408-1-liuyuntao12@huawei.com/
>>> ---
>>> arch/arm/Kconfig | 1 +
>>> arch/arm/boot/compressed/vmlinux.lds.S | 4 ++--
>>> arch/arm/include/asm/vmlinux.lds.h | 18 +++++++++++++++++-
>>> arch/arm/kernel/vmlinux-xip.lds.S | 8 ++++++--
>>> arch/arm/kernel/vmlinux.lds.S | 10 +++++++---
>>> 5 files changed, 33 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>>> index 0af6709570d1..de78ceb821df 100644
>>> --- a/arch/arm/Kconfig
>>> +++ b/arch/arm/Kconfig
>>> @@ -113,6 +113,7 @@ 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..da21244aa892 100644
>>> --- a/arch/arm/boot/compressed/vmlinux.lds.S
>>> +++ b/arch/arm/boot/compressed/vmlinux.lds.S
>>> @@ -89,7 +89,7 @@ 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
>>> */
>>> - *(.data.efistub .bss.efistub)
>>> + *(.data.* .bss.*)
>
> Why is this necessary? There is a reason we don't allow .data in the
> decompressor.
>
Arnd previously asked about this bit also. When CONFIG_EFI and
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION was enabled,I came across
a link failure using ld:
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
So I changed .data.efistub to .data.*., the same to .bss.efistub.
Perhaps it looks clearer like this.
--- 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 = .;
/*
Which approach do you prefer, or do you have a better way?
> On Tue, Feb 20, 2024, at 10:53, liuyuntao (F) wrote:
>> 在 2024/2/20 16:40, Arnd Bergmann 写道:
>>> On Tue, Feb 20, 2024, at 09:15, Yuntao Liu wrote:
>> #
>> # ARM discards the .data section because it disallows r/w data in the
>> # decompressor. So move our .data to .data.efistub and .bss to .bss.efistub,
>> # which are preserved explicitly by the decompressor linker script.
>> #
>> STUBCOPY_FLAGS-$(CONFIG_ARM) += --rename-section .data=.data.efistub \
>> --rename-section .bss=.bss.efistub,load,alloc
>>
>> ---
>>
>> I think that .data.efistub represents the entire .data section, the same
>> applies to .bss as well,
>>
>> so i move all .data and .bss into the stub here.
>>
>
> Ok, I see.
>>> __pecoff_data_end = .;
>>>
>>> /*
>>> @@ -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..dfe2b6ad6b51 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 \
>>> @@ -87,6 +87,22 @@
>>> *(.vfp11_veneer) \
>>> *(.v4_bx)
>>>
>>> +/*
>>> +When CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is enabled, it is important
>>> to
>>> +annotate .vectors sections with KEEP. While linking with ld, it is
>>> +acceptable to directly use KEEP with .vectors sections in ARM_VECTORS.
>>> +However, when using ld.lld for linking, KEEP is not recognized within
>>> the
>>> +OVERLAY command; it is treated as a regular string. Hence, it is
>>> advisable
>>> +to define a distinct section here that explicitly retains the .vectors
>>> +sections when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is turned on.
>>> +*/
>>> +#define ARM_VECTORS_TEXT \
>>> + .vectors.text : { \
>>> + KEEP(*(.vectors)) \
>>> + KEEP(*(.vectors.bhb.loop8)) \
>>> + KEEP(*(.vectors.bhb.bpiall)) \
>>> + }
>>> +
>
> This looks fishy to me. How is this supposed to work? You cannot emit
> these sections into some random other place in the binary.
>
> And also, ARM_VECTORS_TEXT is never used (by accident, see below)
Yes, this way to KEEP .vectors section is not good.
>
>>> #define ARM_TEXT \
>>> IDMAP_TEXT \
>>> __entry_text_start = .; \
>>> diff --git a/arch/arm/kernel/vmlinux-xip.lds.S
>>> b/arch/arm/kernel/vmlinux-xip.lds.S
>>> index c16d196b5aad..035fa18060b3 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 : {
>>> @@ -135,6 +135,10 @@ SECTIONS
>>> ARM_TCM
>>> #endif
>>>
>>> +#ifdef LD_DEAD_CODE_DATA_ELIMINATION
>
> This should be CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
My mistake, and thank you for pointing out it.
>
>>> + ARM_VECTORS_TEXT
>>> +#endif
>>> +
>>> /*
>>> * End of copied data. We need a dummy section to get its LMA.
>>> * Also located before final ALIGN() as trailing padding is not stored
>>> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
>>> index bd9127c4b451..2cfb890c93fb 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 = .;
>>> }
>>
>> I previously asked about this bit, since it appeared that this
>> might prevent a lot of code from being discarded when
>> CONFIG_ARM_PATCH_PHYS_VIRT is set. I tested this again now,
>> and found this makes very little practical difference, so
>> it's all good.
>>
>>> @@ -134,6 +134,10 @@ SECTIONS
>>> ARM_TCM
>>> #endif
>>>
>>> +#ifdef LD_DEAD_CODE_DATA_ELIMINATION
>
> Same here
>
>
>>> + ARM_VECTORS_TEXT
>>> +#endif
>>> +
>>> #ifdef CONFIG_STRICT_KERNEL_RWX
>>> . = ALIGN(1<<SECTION_SHIFT);
>>> #else
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