On Thu, Apr 25, 2024 at 5:33 AM Jose E. Marchesi
<jose.marchesi@xxxxxxxxxx> wrote:
>
>
> > On Wed, Apr 24, 2024 at 2:30 PM Jose E. Marchesi
> > <jose.marchesi@xxxxxxxxxx> 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.
-g is already passed through common BPF_CFLAGS, see Clang rules, you
won't see explicit -g, but it's there (all those flags are passed as
$3 argument)
>
> Salud!
>
