Re: BTF tag support in DWARF (notes for today's BPF Office Hours)

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2023-02-21 at 11:38 -0800, David Faust wrote:
[...]
> Very nice.
> Keeping the 0x6000 tag and instead changing the name sounds good to us.
> 
> From the GCC side, support for BTF tags will be new either way but
> conserving DWARF tag numbers is a good idea.

Great, thank you!

> > Both [1] and [2] are in a workable state, but [2] lacks support for
> > subroutine types and "void *" for now. If you are onboard with this change
> > I'll proceed with finalizing [1] and [2]. (Also, ":v2" suffix might be not
> > the best, I'm open to naming suggestions).
> 
> As for the name, I am not sure the ":v2" suffix is a good idea.
> 
> If we need a new name anyway, this could be a good opportunity to use
> something more generic. The annotation DIEs, especially with the new
> format, could be more widely useful than exclusively for producing BTF.
> 
> For example, some other tool may want to process these same user
> annotations which are now recorded in DWARF, but may not involve BPF/BTF
> at all. Tying "btf" into the name seems to unnecessarily discourage
> those use cases.
> 
> What do you think about something like "debug_type_tag" or 
> "debug_type_annotation" (and a similar update for the decl tags)?
> The translation into BTF records would be the same, but the DWARF info
> would stand on its own without being tied to BTF.
> 
> (Naming is a bit tricky since terms like 'tag' are already in use by
> DWARF, e.g. "type tag" in the context of DWARF DIEs makes me think of
> DW_TAG_xxxx_type...)
> 
> As far as I understand, early proposals for the tags were more generic
> but the LLVM reviewers wished for something more specific due to the
> relatively limited use of the tags at the time. Now that the tags and
> their DWARF format have matured I think a good case can be made to
> make these generic. We'd be happy to help push for such change.

On the other hand, BTF is a thing we are using this annotation for.
Any other tool can reuse DW_TAG_LLVM_annotation, but it will need a
way to distinguish it's annotations from BTF annotations. And this can
be done by using a different DW_AT_name. So, it seems logical to
retain "btf" in the DW_AT_name. What do you think?

> > As a somewhat orthogonal question, would it be possible for you to use the
> > same 0x6000 tag on GCC side? I looked at master branch of [3] but can't
> > find any mentions of btf_type_tag.
> 
> Yes, we plan to use the same 0x6000 in GCC. Support for btf_type_tag isn't
> committed in master yet; I originally worked on patches [1] last spring but
> they were not committed due to some of the issues we've now worked out
> (notably the attribute ordering/association problem). But 0x6000 is not
> currently in use in GCC and didn't come up as a problem for those patches,
> so I don't think it should be an issue.

Understood, thank you for the clarification.

Thanks,
Eduard