On Wed, Apr 7, 2021 at 8:23 AM Yonghong Song <yhs@xxxxxx> wrote: > > > > On 4/6/21 8:01 PM, Sedat Dilek wrote: > > On Tue, Apr 6, 2021 at 6:13 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxx/ > >> (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@xxxxxxxxxxxxxxxx> wrote: > >>>> > >>>> On Thu, Apr 1, 2021 at 4:27 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxx/ > >>>>> [2] https://lore.kernel.org/bpf/20210331001623.2778934-1-yhs@xxxxxx/ > >>>>> > >>>>> Suggested-by: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> > >>>>> Signed-off-by: Yonghong Song <yhs@xxxxxx> > >>>> > >>>> LGTM thanks Yonghong! > >>>> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > >>>> > >>> > >>> Thanks for the patch. > >>> > >>> Feel free to add: > >>> > >>> Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx> # 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 - > >>> > [...]
