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

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Message-ID: <20190606180253.36f6d2ae@cakuba.netronome.com>
Date:   Thu, 6 Jun 2019 18:02:53 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Alexei Starovoitov <ast@...com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Stanislav Fomichev <sdf@...ichev.me>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>, Yonghong Song <yhs@...com>
Subject: Re: [RFC PATCH bpf-next 6/8] libbpf: allow specifying map
 definitions using BTF

On Fri, 7 Jun 2019 00:27:52 +0000, Alexei Starovoitov wrote:
> the solution we're discussing should solve BPF_ANNOTATE_KV_PAIR too.
> That hack must go.

I see.

> 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[];
> } ...

My objection is that k/v fields are never initialized, so they're 
"metafields", mixed with real fields which hold parameters - like 
type, max_entries etc.

But I thought about this 3 times now, and I see no better solution.
FWIW my best shot was relos:

extern struct my_key my_key;
extern int type_int;

struct map_def {
    int type;
    int max_entries;
    void *btf_key_ref;
    void *btf_val_ref;
} = {
    ...
    .btf_key_ref = &my_key,
    .btf_val_ref = &type_int,
};

The advantage being that map_def is no longer modified for each
instance and k/v combination.  And I get my assignment to k/v members :)

But really, I give up, I can't come up with anything better :)