Re: [PATCH bpf-next] bpf: add a few more options for GCC_BPF in selftests/bpf/Makefile

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Apr 26, 2024 at 7:41 AM Jose E. Marchesi
<jose.marchesi@oracle.com> wrote:
>
>
> > On Thu, Apr 25, 2024 at 5:32 AM Jose E. Marchesi
> > <jose.marchesi@oracle.com> wrote:
> >>
> >>
> >> > On Wed, Apr 24, 2024 at 2:30 PM Jose E. Marchesi
> >> > <jose.marchesi@oracle.com> wrote:
> >> >>
> >> >>
> >> >> Hi Yonghong.
> >> >>
> >> >> > On 4/24/24 1:41 AM, Jose E. Marchesi wrote:
> >> >> >> This little patch modifies selftests/bpf/Makefile so it passes the
> >> >> >> following extra options when invoking gcc-bpf:
> >> >> >>
> >> >> >>   -gbtf
> >> >> >>     This makes GCC to emit BTF debug info in .BTF and .BTF.ext.
> >> >> >
> >> >> > Could we do if '-g' is specified, for bpf program,
> >> >> > btf will be automatically generated?
> >> >>
> >> >> Hmm, in principle I wouldn't oppose for -g to mean -gbtf instead of
> >> >> -gdwarf.  DWARF can always be generated by using -gdwarf.
> >> >>
> >> >> Faust, Indu, WDYT?
> >> >>
> >> >> >>
> >> >> >>   -mco-re
> >> >> >>     This tells GCC to generate CO-RE relocations in .BTF.ext.
> >> >> >
> >> >> > Can we make this default? That is, remove -mco-re option. I
> >> >> > can imagine for any serious bpf program, co-re is a must.
> >> >>
> >> >> CO-RE depends on BTF.  So I understand the above as making -mco-re the
> >> >> default if BTF is generated, i.e. if -gbtf (or -g with the modification
> >> >> above) are specified.  Isn't that what clang does?  Am I interpreting
> >> >> correctly?
> >> >>
> >> >> >>
> >> >> >>   -masm=pseudoc
> >> >> >>     This tells GCC to emit BPF assembler using the pseudo-c syntax.
> >> >> >
> >> >> > Can we make it the other way round such that -masm=pseudoc is
> >> >> > the default? You can have an option e.g., -masm=non-pseudoc,
> >> >> > for the other format?
> >> >>
> >> >> We could add a configure-time build option:
> >> >>
> >> >>   --with-bpf-default-asm-syntax={pseudoc,normal}
> >> >>
> >> >> so that GCC can be built to use whatever selected syntax as default.
> >> >> Distros and people can then decide what to do.
> >> >
> >> > distros just ship stuff.
> >> > It's our job to pick good defaults.
> >>
> >> Yeah it was a rather dumb idea that would only complicate things for no
> >> good reason.
> >>
> >> The unfortunate fact is that at this point the kernel headers that
> >> almost all BPF programs use contain pseudo-C inline assembly and having
> >> the toolchain using the conventional assembly syntax by default would
> >> force users to specify the command-line option explicitly, which is a
> >> great PITA.  So I guess this is one of these situations where the worse
> >> option is indeed the best default, in practical terms.
> >>
> >> So ok, as much as it sucks we will make -masm=pseudoc the default in GCC
> >> for the sake of practicality.
> >>
> >> > I agree with Yonghong that -g should imply -gbtf for bpf target
> >> > and -mco-re doesn't need to be a flag at all.
> >>
> >> We like the idea of -g implying -gbtf rather than -gdwarf for the BPF
> >> target.  It makes sense.  Faust is already working on it.
> >>
> >> As for -mco-re, it is already the default with -gbtf, and now it will be
> >> the default for -g.
> >>
> >> > Compiler should do it when it sees those special attributes in C code.
> >> > -masm=pseudoc is a good default as well, since that's what
> >> > everyone in bpf community is used to.
> >>
> >> We will try to get all the changes above upstream before GCC 14 gets
> >> branched, which shall happen any day now.  Once they are in GCC the only
> >> extra option to be added to GCC_BPF_BUILD_RULE will be -g.  Will send an
> >> updated patch then.
> >
> > Awesome. This is all great to hear.
>
> The GCC 14 release branch was created today, but we managed to get the
> changes for -g and default to pseudo-C just in time.

Nice. Pls let us know when mirrors/packages are ready, so we teach CI
to start using gcc-bpf.