On Wed, Mar 31, 2021 at 6:24 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 type BUILD_COMPILER_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/ > > Signed-off-by: Yonghong Song <yhs@xxxxxx> > --- > include/linux/compiler.h | 8 ++++++++ > include/linux/elfnote.h | 1 + > init/version.c | 2 ++ > scripts/mod/modpost.c | 1 + > 4 files changed, 12 insertions(+) > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index df5b405e6305..b92930877277 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -245,6 +245,14 @@ static inline void *offset_to_ptr(const int *off) > */ > #define prevent_tail_call_optimization() mb() > > +#include <linux/elfnote.h> > + > +#ifdef CONFIG_LTO > +#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO, 1) > +#else > +#define BUILD_COMPILER_LTO_INFO ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_LTO, 0) > +#endif With this approach BUILD_COMPILER_LTO_INFO won't be available `#ifdef __ASSEMBLER__`; we don't need it today, and perhaps YAGNI, but I think I prefer how include/linux/build-salt.h defines LINUX_ELFNOTE_BUILD_SALT and keeps it isolated there. Similarly, I think it would be better to create a new header, say include/linux/elfnote-lto.h that is basically a copy of include/linux/build-salt.h, but with the relevant defines replaced with the LTO identifiers you add above. Then init/version.c and scripts/mod/modpost.c can include include/linux/elfnote-lto.h and you don't have to touch include/linux/build-salt.h and we can keep the elfnote "types" isolated to their respective headers (otherwise this approach reduces the usefulness of include/linux/build-salt.h even existing, IMO. Feels like it should just be merged into include/linux/elfnote.h entirely at that point). But, this is a much nicer approach! I forgot that elf notes were a thing! > + > #include <asm/rwonce.h> > > #endif /* __LINUX_COMPILER_H */ > diff --git a/include/linux/elfnote.h b/include/linux/elfnote.h > index 04af7ac40b1a..f5ec2b50ab7d 100644 > --- a/include/linux/elfnote.h > +++ b/include/linux/elfnote.h > @@ -100,5 +100,6 @@ > * The types for "Linux" owned notes. > */ > #define LINUX_ELFNOTE_BUILD_SALT 0x100 > +#define LINUX_ELFNOTE_BUILD_LTO 0x101 > > #endif /* _LINUX_ELFNOTE_H */ > diff --git a/init/version.c b/init/version.c > index 92afc782b043..a4f74b06fe78 100644 > --- a/init/version.c > +++ b/init/version.c > @@ -9,6 +9,7 @@ > > #include <generated/compile.h> > #include <linux/build-salt.h> > +#include <linux/compiler.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_COMPILER_LTO_INFO; > diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c > index 24725e50c7b4..713c0d5d5525 100644 > --- a/scripts/mod/modpost.c > +++ b/scripts/mod/modpost.c > @@ -2195,6 +2195,7 @@ static void add_header(struct buffer *b, struct module *mod) > buf_printf(b, "#include <linux/compiler.h>\n"); > buf_printf(b, "\n"); > buf_printf(b, "BUILD_SALT;\n"); > + buf_printf(b, "BUILD_COMPILER_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
