Subject: Re: [PATCH bpf-next v3 1/5] bpf: Support chain calling multiple BPF programs after each other

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 14 Oct 2019 14:35:45 +0200

Alexei Starovoitov <[email-masked]> writes:

> On Wed, Oct 09, 2019 at 10:03:43AM +0200, Toke Høiland-Jørgensen wrote:
>> Alexei Starovoitov <[email-masked]> writes:
>> 
>> > Please implement proper indirect calls and jumps.
>> 
>> I am still not convinced this will actually solve our problem; but OK, I
>> can give it a shot.
>
> If you're not convinced let's talk about it first.
>
> Indirect calls is a building block for debugpoints.
> Let's not call them tracepoints, because Linus banned any discusion
> that includes that name.
> The debugpoints is a way for BPF program to insert points in its
> code to let external facility to do tracing and debugging.
>
> void (*debugpoint1)(struct xdp_buff *, int code);
> void (*debugpoint2)(struct xdp_buff *);
> void (*debugpoint3)(int len);

So how would these work? Similar to global variables (i.e., the loader
creates a single-entry PROG_ARRAY map for each one)? Presumably with
some BTF to validate the argument types?

So what would it take to actually support this? It doesn't quite sound
trivial to add?

> Essentially it's live debugging (tracing) of cooperative bpf programs
> that added debugpoints to their code.

Yup, certainly not disputing that this would be useful for debugging;
although it'll probably be a while before its use becomes widespread
enough that it'll be a reliable tool for people deploying XDP programs...

> Obviously indirect calls can be used for a ton of other things
> including proper chaing of progs, but I'm convinced that
> you don't need chaining to solve your problem.
> You need debugging.

Debugging is certainly also an area that I want to improve. However, I
think that focusing on debugging as the driver for chaining programs was
a mistake on my part; rudimentary debugging (using a tool such as
xdpdump) is something that falls out of program chaining, but it's not
the main driver for it.

> If you disagree please explain _your_ problem again.
> Saying that fb katran is a use case for chaining is, hrm, not correct.

I never said Katran was the driver for this. I just used Katran as one
of the "prior art" examples for my "how are people solving running
multiple programs on the same interface" survey.

What I want to achieve is simply the ability to run multiple independent
XDP programs on the same interface, without having to put any
constraints on the programs themselves. I'm not disputing that this is
*possible* to do completely in userspace, I just don't believe the
resulting solution will be very good. Proper kernel support for indirect
calls (or just "tail calls that return") may change that; but in any
case I think I need to go write some userspace code to have some more
concrete examples to discuss from. So we can come back to the
particulars once I've done that :)

-Toke