netdev - Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on BTF

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Date:   Fri, 5 Jun 2020 09:58:26 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     David Ahern <dsahern@...il.com>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Daniel Borkmann <borkmann@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on BTF

On Fri, Jun 5, 2020 at 1:23 AM Jesper Dangaard Brouer <brouer@...hat.com> wrote:
>
> Great. If we can remove this requirement of -1 init (and let zero mean
> feature isn't used), then I'm all for exposing expose in uapi/bpf.h.

Not having it in bpf.h doesn't magically make it invisible.
It's uapi because user space C sources rely on its fixed format.
vmlinux.h contains all kernel types. both uapi and kernel internal.
devmap selftest taking uapi 'struct bpf_devmap_val' from vmlinux.h is
an awful hack.
I prefer to keep vmlinux.h usage to bpf programs only.
User space C code should interface with kernel via proper uapi headers.
When vmlinux.h is used by bpf C program it's completely different from
user space C code doing the same, because llvm emits relocations for
bpf prog and libbpf adjusts them.
So doing 'foo->bar' in bpf C is specific to target kernel, whereas
user C code 'foo->bar' is a hard constant which bakes it into uapi
that kernel has to keep backwards compatible.
If in some distant future we teach both gcc and clang to do bpf-style
relocations for x86 and teach ld.so to adjust them then we can dream
about very differently looking kernel/user interfaces.
Right now any struct used by user C code and passed into kernel is uapi.