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Message-ID: <9b97e30d-62a5-4f35-943d-aa76014f0f9b@huawei.com>
Date: Sat, 9 Mar 2024 14:46:59 +0800
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Subject: Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
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To: Ard Biesheuvel <ardb@kernel.org>, Arnd Bergmann <arnd@arndb.de>
CC: <linux-arm-kernel@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
	Fangrui Song <maskray@google.com>, Russell King <linux@armlinux.org.uk>,
	Andrew Davis <afd@ti.com>, Andrew Morton <akpm@linux-foundation.org>, "Kirill
 A. Shutemov" <kirill.shutemov@linux.intel.com>, Geert Uytterhoeven
	<geert+renesas@glider.be>, Jonathan Corbet <corbet@lwn.net>, Mike Rapoport
	<rppt@kernel.org>, Rob Herring <robh@kernel.org>, Thomas Gleixner
	<tglx@linutronix.de>, Linus Walleij <linus.walleij@linaro.org>,
	<llvm@lists.linux.dev>
References: <20240307151231.654025-1-liuyuntao12@huawei.com>
 <58cc1053-7208-4b22-99cb-210fdf700569@app.fastmail.com>
 <CAMj1kXFEdRZsy8FovGrfWotoaws1KoKbFv5q+7yKL=pRV8zkZw@mail.gmail.com>
 <CAMj1kXF7fOO6gyvTJNOKp1_PVtAUi08ezz6k1g7jHNfUXAzXUw@mail.gmail.com>
 <CAMj1kXE+XeMTw6VdtiTDFw_zgWEngN_mFbUy+WECNckb5m0NAg@mail.gmail.com>
From: "liuyuntao (F)" <liuyuntao12@huawei.com>
In-Reply-To: <CAMj1kXE+XeMTw6VdtiTDFw_zgWEngN_mFbUy+WECNckb5m0NAg@mail.gmail.com>
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On 2024/3/9 8:01, Ard Biesheuvel wrote:
> On Fri, 8 Mar 2024 at 16:37, Ard Biesheuvel <ardb@kernel.org> wrote:
>>
>> On Fri, 8 Mar 2024 at 15:27, Ard Biesheuvel <ardb@kernel.org> wrote:
>>>
>>> On Fri, 8 Mar 2024 at 14:16, Arnd Bergmann <arnd@arndb.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@huawei.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@arndb.de>
>>>> Reviewed-by: Arnd Bergmann <arnd@arndb.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.
>>>
>>>>>       __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)
>>>
>>
>> The below appears to work for me:
>>
>> --- a/arch/arm/kernel/entry-armv.S
>> +++ b/arch/arm/kernel/entry-armv.S
>> @@ -1076,7 +1076,12 @@
>>          W(b)    vector_irq
>>          W(b)    vector_fiq
>>
>> +       .text
>> +       .reloc  ., R_ARM_NONE, .vectors
>>   #ifdef CONFIG_HARDEN_BRANCH_HISTORY
>> +       .reloc  ., R_ARM_NONE, .vectors.bhb.loop8
>> +       .reloc  ., R_ARM_NONE, .vectors.bhb.bpiall
>> +
>>          .section .vectors.bhb.loop8, "ax", %progbits
>>          W(b)    vector_rst
>>          W(b)    vector_bhb_loop8_und
> 
> ... or even better:
> 
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -1066,4 +1066,5 @@
> 
>          .section .vectors, "ax", %progbits
> +       .reloc  .text, R_ARM_NONE, .
>          W(b)    vector_rst
>          W(b)    vector_und
> @@ -1079,4 +1080,5 @@
>   #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
> @@ -1091,4 +1093,5 @@
> 
>          .section .vectors.bhb.bpiall, "ax", %progbits
> +       .reloc  .text, R_ARM_NONE, .
>          W(b)    vector_rst
>          W(b)    vector_bhb_bpiall_und

I used `.reloc  ., R_ARM_NONE, .vectors` to KEEP .vectors section, but 
it failed. It seems now that I did not use the reloc directive correctly.
Thanks Ard, and your approach is concise and effctive.
and, could I submit a v3 patch to apply these new changes?