On Thu, 6 Jun 2019 23:27:36 +0000, Alexei Starovoitov wrote:
> On 6/6/19 4:02 PM, Andrii Nakryiko wrote:
> >> struct {
> >> int type;
> >> int max_entries;
> >> } my_map __attribute__((map(int,struct my_value))) = {
> >> .type = BPF_MAP_TYPE_ARRAY,
> >> .max_entries = 16,
> >> };
> >>
> >> Of course this would need BPF backend support, but at least that approach
> >> would be more C like. Thus this would define types where we can automatically
> > I guess it's technically possible (not a compiler guru, but I don't
> > see why it wouldn't be possible). But it will require at least two
> > things:
> > 1. Compiler support, obviously, as you mentioned.
>
> every time we're doing llvm common change it takes many months.
> Adding BTF took 6 month, though the common changes were trivial.
> Now we're already 1+ month into adding 4 intrinsics to support CO-RE.
>
> In the past I was very much in favor of extending __attribute__
> with bpf specific stuff. Now not so much.
> __attribute__((map(int,struct my_value))) cannot be done as strings.
> clang has to process the types, create new objects inside debug info.
> It's not clear to me how this modified debug info will be associated
> with the variable my_map.
> So I suspect doing __attribute__ with actual C type inside (())
> will not be possible.
> I think in the future we might still add string based attributes,
> but it's not going to be easy.
> So... Unless somebody in the community who is doing full time llvm work
> will not step in right now and says "I will code the above attr stuff",
> we should not count on such clang+llvm feature.
If nobody has resources to commit to this, perhaps we can just stick
to BPF_ANNOTATE_KV_PAIR()?
Apologies, but I think I missed the memo on why that's considered
a hack. Could someone point me to the relevant discussion?
We could conceivably add BTF-based map_def for other features, and
solve the K/V problem once a clean solution becomes apparent and
tractable? BPF_ANNOTATE_KV_PAIR() is not great, but we kinda already
have it..
Perhaps I'm not thinking clearly about this and I should stay quiet :)
