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

[David Faust <david.faust@oracle.com>]

On 2/21/23 14:57, Eduard Zingerman wrote:
> 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?

OK I can understand keeping it BTF specific.

Other than that, I don't come up with any significantly different idea 
than to use the ":v2" suffix, so let's go with "btf_type_tag:v2"?

Thanks

> 
>>> 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