Re: [PATCH] x86: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

["liuyuntao (F)" <liuyuntao12@huawei.com>]

On 2024/4/23 3:24, Nathan Chancellor wrote:
> Hi Yuntao,
> 
> Just a drive by review since I saw this patch via another CC in my
> inbox, I would wait for x86 maintainer thoughts before sending a v2.
> 
> On Mon, Apr 22, 2024 at 06:05:56AM +0000, Yuntao Liu wrote:
>> The current x86 architecture does not yet support the
>> HAVE_LD_DEAD_CODE_DATA_ELIMINATION feature. x86 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.
>>
>> Enabling CONFIG_UNWINDER_ORC or CONFIG_MITIGATION_RETPOLINE will enable
>> the objtool's --orc and --retpoline parameters, which will alter the
>> layout of the binary file, thereby preventing gc-sections from functioning
>> properly. Therefore, HAVE_LD_DEAD_CODE_DATA_ELIMINATION should only be
>> selected when they are not enabled.
>>
>> Enabling CONFIG_LTO_CLANG or CONFIG_X86_KERNEL_IBT will use vmlinux.o
>> instead of performing the slow LTO link again. This can also prevent
>> gc-sections from functioning properly. Therefore, using this optimization
>> when CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not enabled.
> 
> These two paragraphs indicate to me that this feature will be
> unselectable the vast majority of x86 configurations, why should the
> upstream kernel support it in that case?
> 

Just to refine the DEAD_CODE_DATA_ELIMINATION this feature, it might 
also offer users an extra choice in certain situations.

>> The size comparison of zImage is as follows:
> 
>                           ^ bzImage?
> 

Yes, it should be bzImage.

>> x86_def_defconfig  i386_defconfig    tinyconfig
>> 10892288           10826240          607232          no dce
>> 10748928           10719744          529408          dce
>> 1.3%               0.98%             12.8%           shrink
>>
>> When using smaller config file, there is a significant reduction in the
>> size of the zImage.
> 
> Same here.
> 
> What toolchain was this tested with? There have been behavior
> differences between the GNU and LLVM toolchains that have shown up when
> dead code elimination is enabled, such as with 32-bit ARM [1] and RISC-V
> [2]. While I am not saying there are any problems here, it would be good
> to qualify how well this has been tested and perhaps do some testing
> with other toolchains and versions, especially since you are touching
> areas guarded by CONFIG_LTO_CLANG. Does the resulting kernel boot and
> run properly?
> 
> [1]: https://lore.kernel.org/30b01c65-12f2-4ee0-81d5-c7a2da2c36b4@app.fastmail.com/
> [2]: https://lore.kernel.org/20230622215327.GA1135447@dev-arch.thelio-3990X/
> 

I use GNU toolchains, and the kernel boots well with x86_64_defconfig in 
qemu.
Using LLVM toolchains, I came acrossa link failure:
> ld.lld: error: undefined hidden symbol: __alt_reloc_selftest               
> referenced by alternative.c                                            
>                .thinlto-cache/llvmcache-6140C39409062E0AC950603FE9B6042154C497B6:(.altinstr_replacement+0x30)
I am still struggling with it.

>> ---
>>   arch/x86/Kconfig              |  1 +
>>   arch/x86/kernel/vmlinux.lds.S | 24 ++++++++++++------------
>>   scripts/link-vmlinux.sh       |  2 +-
>>   3 files changed, 14 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index a902680b6537..92dfbc8ee4e7 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -247,6 +247,7 @@ config X86
>>   	select HAVE_FUNCTION_ERROR_INJECTION
>>   	select HAVE_KRETPROBES
>>   	select HAVE_RETHOOK
>> +	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !CONFIG_UNWINDER_ORC && !CONFIG_MITIGATION_RETPOLINE
> 
> This is incorrect, it should be
> 
>      select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !UNWINDER_ORC && !MITIGATION_RETPOLINE
> 

It is my mistake.

>>   	select HAVE_LIVEPATCH			if X86_64
>>   	select HAVE_MIXED_BREAKPOINTS_REGS
>>   	select HAVE_MOD_ARCH_SPECIFIC
> 
> Cheers,
> Nathan