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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 26 May 2020 22:09:11 -0700
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Subject: Re: [PATCH bpf-next] libbpf: Export bpf_object__load_vmlinux_btf
To:     Daniel Xu <dxu@dxuuu.xyz>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Yonghong Song <yhs@fb.com>, Andrii Nakryiko <andriin@fb.com>,
        john fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        bpf <bpf@vger.kernel.org>, Networking <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
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On Tue, May 26, 2020 at 7:09 PM Daniel Xu <dxu@dxuuu.xyz> wrote:
>
> Right now the libbpf model encourages loading the entire object at once.
> In this model, libbpf handles loading BTF from vmlinux for us. However,
> it can be useful to selectively load certain maps and programs inside an
> object without loading everything else.

There is no way to selectively load or not load a map. All maps are
created, unless they are reusing map FD or pinned instances. See
below, I'd like to understand the use case better.

>
> In the latter model, there was perviously no way to load BTF on-demand.
> This commit exports the bpf_object__load_vmlinux_btf such that we are
> able to load BTF on demand.
>

Let's start with the real problem, not a solution. Do you have
specific use case where you need bpf_object__load_vmlinux_btf()? It
might not do anything if none of BPF programs in the object requires
BTF, because it's very much tightly coupled with loading bpf_object as
a whole model. I'd like to understand what you are after with this,
before exposing internal implementation details as an API.

> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>  tools/lib/bpf/libbpf.c   | 2 +-
>  tools/lib/bpf/libbpf.h   | 1 +
>  tools/lib/bpf/libbpf.map | 1 +
>  3 files changed, 3 insertions(+), 1 deletion(-)
>

[...]