> 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.
Salud!
