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Subject: Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc:     bpf <bpf@vger.kernel.org>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
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        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>, Shuah Khan <shuah@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
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From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <48b99ccc-8ef6-4ba9-00f9-d7e71ae4fb5d@iogearbox.net>
Date:   Tue, 30 Mar 2021 23:25:47 +0200
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On 3/30/21 10:39 PM, Andrii Nakryiko wrote:
> On Sun, Mar 28, 2021 at 1:11 AM Kumar Kartikeya Dwivedi
> <memxor@gmail.com> wrote:
>> On Sun, Mar 28, 2021 at 10:12:40AM IST, Andrii Nakryiko wrote:
>>> Is there some succinct but complete enough documentation/tutorial/etc
>>> that I can reasonably read to understand kernel APIs provided by TC
>>> (w.r.t. BPF, of course). I'm trying to wrap my head around this and
>>> whether API makes sense or not. Please share links, if you have some.
>>
>> Hi Andrii,
>>
>> Unfortunately for the kernel API part, I couldn't find any when I was working
>> on this. So I had to read the iproute2 tc code (tc_filter.c, f_bpf.c,
>> m_action.c, m_bpf.c) and the kernel side bits (cls_api.c, cls_bpf.c, act_api.c,
>> act_bpf.c) to grok anything I didn't understand. There's also similar code in
>> libnl (lib/route/{act,cls}.c).
>>
>> Other than that, these resources were useful (perhaps you already went through
>> some/all of them):
>>
>> https://docs.cilium.io/en/latest/bpf/#tc-traffic-control
>> https://qmonnet.github.io/whirl-offload/2020/04/11/tc-bpf-direct-action/
>> tc(8), and tc-bpf(8) man pages
>>
>> I hope this is helpful!
> 
> Thanks! I'll take a look. Sorry, I'm a bit behind with all the stuff,
> trying to catch up.
> 
> I was just wondering if it would be more natural instead of having
> _dev _block variants and having to specify __u32 ifindex, __u32
> parent_id, __u32 protocol, to have some struct specifying TC
> "destination"? Maybe not, but I thought I'd bring this up early. So
> you'd have just bpf_tc_cls_attach(), and you'd so something like
> 
> bpf_tc_cls_attach(prog_fd, TC_DEV(ifindex, parent_id, protocol))
> 
> or
> 
> bpf_tc_cls_attach(prog_fd, TC_BLOCK(block_idx, protocol))
> 
> ? Or it's taking it too far?
> 
> But even if not, I think detaching can be unified between _dev and
> _block, can't it?

Do we even need the _block variant? I would rather prefer to take the chance
and make it as simple as possible, and only iff really needed extend with
other APIs, for example:

   bpf_tc_attach(prog_fd, ifindex, {INGRESS,EGRESS});

Internally, this will create the sch_clsact qdisc & cls_bpf filter instance
iff not present yet, and attach to a default prio 1 handle 1, and _always_ in
direct-action mode. This is /as simple as it gets/ and we don't need to bother
users with more complex tc/cls_bpf internals unless desired. For example,
extended APIs could add prio/parent so that multi-prog can be attached to a
single cls_bpf instance, but even that could be a second step, imho.

Thanks,
Daniel