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

[Sedat Dilek <sedat.dilek@gmail.com>]

On Wed, Apr 7, 2021 at 8:23 AM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> 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.
>

I am OK with that.

- Sedat -

> >
> > 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 -
> >>>
> [...]