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> > > > --- > > 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) That is a bit smarter (and smaller) than v3. Queued up and building a new clang-lto kernel... Will report later. - Sedat - > > 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%3DUBNdQiO3g%40mail.gmail.com.
