Re: [RFC PATCH bpf-next 6/8] libbpf: allow specifying map definitions using BTF

[Alexei Starovoitov <ast@fb.com>]

On 6/6/19 5:10 PM, Jakub Kicinski wrote:
> 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 :)

the solution we're discussing should solve BPF_ANNOTATE_KV_PAIR too.
That hack must go.

If I understood your objections to Andrii's format is that
you don't like pointer part of key/value while Andrii explained
why we picked the pointer, right?

So how about:

struct {
   int type;
   int max_entries;
   struct {
     __u32 key;
     struct my_value value;
   } types[];
} ...