Re: [PATCH dwarves v2] btf: Add support for the floating-point types

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Tue, 2021-03-09 at 08:48 -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Mar 09, 2021 at 12:59:13AM +0100, Ilya Leoshkevich escreveu:
> > Some BPF programs compiled on s390 fail to load, because s390
> > arch-specific linux headers contain float and double types.
> > 
> > Fix as follows:
> > 
> > - Make the DWARF loader fill base_type.float_type.
> > 
> > - Introduce libbpf compatibility level command-line parameter, so
> > that
> >   pahole could be used to build both the older and the newer
> > kernels.
> > 
> > - libbpf introduced the support for the floating-point types in
> > commit
> >   986962fade5, so update the libbpf submodule to that version and
> > use
> >   the new btf__add_float() function in order to emit the floating-
> > point
> >   types when not in the compatibility mode and base_type.float_type
> > is
> >   set.
> > 
> > - Make the BTF loader recognize the new BTF kind.
> > 
> > Example of the resulting entry in the vmlinux BTF:
> > 
> >     [7164] FLOAT 'double' size=8
> > 
> > when building with:
> > 
> >     LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} --libbpf_compat=0.4.0
> 
> I'm testing it now, and added as a followup patch the man page entry,
> please check that the wording is appropriate.
> 
> Thanks,
> 
> - Arnaldo
> 
> [acme@five pahole]$ vim man-pages/pahole.1
> [acme@five pahole]$ git diff
> diff --git a/man-pages/pahole.1 b/man-pages/pahole.1
> index 352bb5e45f319da4..787771753d1933b1 100644
> --- a/man-pages/pahole.1
> +++ b/man-pages/pahole.1
> @@ -199,6 +199,12 @@ Path to the base BTF file, for instance: vmlinux
> when encoding kernel module BTF
>  This may be inferred when asking for a /sys/kernel/btf/MODULE, when
> it will be autoconfigured
>  to "/sys/kernel/btf/vmlinux".
> 
> +.TP
> +.B \-\-libbpf_compat=LIBBPF_VERSION
> +Produce output compatible with this libbpf version. For instance,
> specifying 0.4.0 as
> +the version would encode BTF_KIND_FLOAT entries in systems where the
> vmlinux DWARF
> +information has float types.
> +
>  .TP
>  .B \-l, \-\-show_first_biggest_size_base_type_member
>  Show first biggest size base_type member.
> [acme@five pahole]$

The wording matches what I had in mind for this new flag, thanks!