Re: [PATCH kbuild v4] kbuild: add an elfnote for whether vmlinux is built with lto

[Yonghong Song <yhs@fb.com>]

On 4/6/21 8:01 PM, Sedat Dilek wrote:
> On Tue, Apr 6, 2021 at 6:13 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>> Masahiro and Michal,
>>
>> Friendly ping. Any comments on this patch?
>>
>> The addition LTO .notes information emitted by kernel is used by pahole
>> in the following patch:
>>      https://lore.kernel.org/bpf/20210401025825.2254746-1-yhs@fb.com/
>>      (dwarf_loader: check .notes section for lto build info)
>>
> 
> Hi Yonghong,
> 
> the above pahole patch has this define and comment:
> 
> -static bool cus__merging_cu(Dwarf *dw)
> +/* Match the define in linux:include/linux/elfnote.h */
> +#define LINUX_ELFNOTE_BUILD_LTO 0x101
> 
> ...and does not fit with the define and comment in this kernel patch:
> 
> +#include <linux/elfnote.h>
> +
> +#define LINUX_ELFNOTE_LTO_INFO 0x101

Thanks, Sedat. I am aware of this. I think we can wait in pahole
to make a change until the kernel patch is finalized and merged.
The kernel patch may still change as we haven't get
maintainer's comment. This will avoid unnecessary churn's
in pahole side.

> 
> Thanks.
> 
> - Sedat -
> 
> 
>> Thanks,
>>
>> Yonghong
>>
>> On 4/6/21 12:05 AM, Sedat Dilek wrote:
>>> On Fri, Apr 2, 2021 at 8:07 PM 'Nick Desaulniers' via Clang Built
>>> Linux <clang-built-linux@googlegroups.com> wrote:
>>>>
>>>> On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@fb.com> wrote:
>>>>>
>>>>> Currently, clang LTO built vmlinux won't work with pahole.
>>>>> LTO introduced cross-cu dwarf tag references and broke
>>>>> current pahole model which handles one cu as a time.
>>>>> The solution is to merge all cu's as one pahole cu as in [1].
>>>>> We would like to do this merging only if cross-cu dwarf
>>>>> references happens. The LTO build mode is a pretty good
>>>>> indication for that.
>>>>>
>>>>> In earlier version of this patch ([2]), clang flag
>>>>> -grecord-gcc-switches is proposed to add to compilation flags
>>>>> so pahole could detect "-flto" and then merging cu's.
>>>>> This will increate the binary size of 1% without LTO though.
>>>>>
>>>>> Arnaldo suggested to use a note to indicate the vmlinux
>>>>> is built with LTO. Such a cheap way to get whether the vmlinux
>>>>> is built with LTO or not helps pahole but is also useful
>>>>> for tracing as LTO may inline/delete/demote global functions,
>>>>> promote static functions, etc.
>>>>>
>>>>> So this patch added an elfnote with a new type LINUX_ELFNOTE_LTO_INFO.
>>>>> The owner of the note is "Linux".
>>>>>
>>>>> With gcc 8.4.1 and clang trunk, without LTO, I got
>>>>>     $ readelf -n vmlinux
>>>>>     Displaying notes found in: .notes
>>>>>       Owner                Data size        Description
>>>>>     ...
>>>>>       Linux                0x00000004       func
>>>>>        description data: 00 00 00 00
>>>>>     ...
>>>>> With "readelf -x ".notes" vmlinux", I can verify the above "func"
>>>>> with type code 0x101.
>>>>>
>>>>> With clang thin-LTO, I got the same as above except the following:
>>>>>        description data: 01 00 00 00
>>>>> which indicates the vmlinux is built with LTO.
>>>>>
>>>>>     [1] https://lore.kernel.org/bpf/20210325065316.3121287-1-yhs@fb.com/
>>>>>     [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@fb.com/
>>>>>
>>>>> Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
>>>>> Signed-off-by: Yonghong Song <yhs@fb.com>
>>>>
>>>> LGTM thanks Yonghong!
>>>> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
>>>>
>>>
>>> Thanks for the patch.
>>>
>>> Feel free to add:
>>>
>>> Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM/Clang v12.0.0-rc4 (x86-64)
>>>
>>> As a note for the pahole side:
>>> Recent patches require an adaptation of the define and its comment.
>>>
>>> 1. LINUX_ELFNOTE_BUILD_LTO -> LINUX_ELFNOTE_LTO_INFO
>>> 2. include/linux/elfnote.h -> include/linux/elfnote-lto.h
>>>
>>> - Sedat -
>>>
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