linux-kernel - Re: [PATCH-next v2] arm32: enable HAVE_LD_DEAD_CODE_DATA

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Message-ID: <cbf5f847-a5c7-443d-9c6f-57a9678a8d2e@huawei.com>
Date: Sat, 9 Mar 2024 14:56:46 +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)
> 
After applying .reloc .text, R_ARM_NONE, ., the resulting kernel boots 
well in QEMU. I tested it with the latest linux-next master branch and 
the mainline master branch.
By the way, I used vexpress_defconfig for testing, it worked.