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 - > 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 - > > > >>> --- > >>> include/linux/elfnote-lto.h | 14 ++++++++++++++ > >>> init/version.c | 2 ++ > >>> scripts/mod/modpost.c | 2 ++ > >>> 3 files changed, 18 insertions(+) > >>> create mode 100644 include/linux/elfnote-lto.h > >>> > >>> Changelogs: > >>> v3 -> v4: > >>> . put new lto note in its own header file similar to > >>> build-salt.h. (Nick) > >>> v2 -> v3: > >>> . abandoned the approach of adding -grecord-gcc-switches, > >>> instead create a note to indicate whether it is a lto build > >>> or not. The note definition is in compiler.h. (Arnaldo) > >>> v1 -> v2: > >>> . limited to add -grecord-gcc-switches for LTO_CLANG > >>> instead of all clang build > >>> > >>> diff --git a/include/linux/elfnote-lto.h b/include/linux/elfnote-lto.h > >>> new file mode 100644 > >>> index 000000000000..d4635a3ecc4f > >>> --- /dev/null > >>> +++ b/include/linux/elfnote-lto.h > >>> @@ -0,0 +1,14 @@ > >>> +#ifndef __ELFNOTE_LTO_H > >>> +#define __ELFNOTE_LTO_H > >>> + > >>> +#include <linux/elfnote.h> > >>> + > >>> +#define LINUX_ELFNOTE_LTO_INFO 0x101 > >>> + > >>> +#ifdef CONFIG_LTO > >>> +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 1) > >>> +#else > >>> +#define BUILD_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_LTO_INFO, 0) > >>> +#endif > >>> + > >>> +#endif /* __ELFNOTE_LTO_H */ > >>> diff --git a/init/version.c b/init/version.c > >>> index 92afc782b043..1a356f5493e8 100644 > >>> --- a/init/version.c > >>> +++ b/init/version.c > >>> @@ -9,6 +9,7 @@ > >>> > >>> #include <generated/compile.h> > >>> #include <linux/build-salt.h> > >>> +#include <linux/elfnote-lto.h> > >>> #include <linux/export.h> > >>> #include <linux/uts.h> > >>> #include <linux/utsname.h> > >>> @@ -45,3 +46,4 @@ const char linux_proc_banner[] = > >>> " (" LINUX_COMPILER ") %s\n"; > >>> > >>> BUILD_SALT; > >>> +BUILD_LTO_INFO; > >>> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c > >>> index 24725e50c7b4..98fb2bb024db 100644 > >>> --- a/scripts/mod/modpost.c > >>> +++ b/scripts/mod/modpost.c > >>> @@ -2191,10 +2191,12 @@ static void add_header(struct buffer *b, struct module *mod) > >>> */ > >>> buf_printf(b, "#define INCLUDE_VERMAGIC\n"); > >>> buf_printf(b, "#include <linux/build-salt.h>\n"); > >>> + buf_printf(b, "#include <linux/elfnote-lto.h>\n"); > >>> buf_printf(b, "#include <linux/vermagic.h>\n"); > >>> buf_printf(b, "#include <linux/compiler.h>\n"); > >>> buf_printf(b, "\n"); > >>> buf_printf(b, "BUILD_SALT;\n"); > >>> + buf_printf(b, "BUILD_LTO_INFO;\n"); > >>> buf_printf(b, "\n"); > >>> buf_printf(b, "MODULE_INFO(vermagic, VERMAGIC_STRING);\n"); > >>> buf_printf(b, "MODULE_INFO(name, KBUILD_MODNAME);\n"); > >>> -- > >>> 2.30.2 > >>> > >> > >> > >> -- > >> Thanks, > >> ~Nick Desaulniers > >> > >> -- > >> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > >> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > >> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAKwvOdmX8d3XTzJFk5rN_PnOQYJ8bXMrh8DrhzqN=UBNdQiO3g@xxxxxxxxxxxxxx .
