Re: [PATCH bpf-next 3/5] libbpf: add low level TC-BPF API

[Alexei Starovoitov]

On Tue, Mar 30, 2021 at 2:26 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
>
> On 3/30/21 10:39 PM, Andrii Nakryiko wrote:
> > On Sun, Mar 28, 2021 at 1:11 AM Kumar Kartikeya Dwivedi
> > <memxor@xxxxxxxxx> 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.

+1 to support sched_cls in direct-action mode only.