Re: XDP bpf_tail_call_redirect(): yea or nay?

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Björn Töpel <bjorn.topel@gmail.com> writes:

> On Thu, 7 May 2020 at 20:08, John Fastabend <john.fastabend@gmail.com> wrote:
>>
> []
>>
>> I'm wondering if we can teach the verifier to recognize tail calls,
>>
>> int xdp_prog1(struct xdp_md *ctx)
>> {
>>         return xdp_do_redirect(ctx, &xsks_map, 0);
>> }
>>
>> This would be useful for normal calls as well. I guess the question here
>> is would a tail call be sufficient for above case or do you need the
>> 'return XDP_PASS' at the end? If so maybe we could fold it into the
>> helper somehow.
>>
>
> No, that was just for handling the "failed call", bpf_tail_call() style.
>
>> I think it would also address Toke's concerns, no new action so
>> bpf developers can just develope like normal but "smart" developers
>> will try do calls as tail calls. Not sure it can be done without
>> driver changes though.
>>
>
> Take me though this. So, the new xdp_do_redirect() would return
> XDP_REDIRECT? If the call is a tail call, we can "consume" (perform
> the REDIRECT action) in the helper, set a "we're done/tail call
> performed" flag in bpf_redirect_info and the xdp_do_redirect() checks
> this flag and returns directly. If the call is *not* a tail call, the
> regular REDIRECT path is performed. Am I following that correctly? So
> we would be able to detect if the optimization has been performed, so
> the "consume" semantics can be done.

Yeah, that was my understanding. And what I meant with the 'new flag'
bit was that you could prototype this by just adding a new flag to
bpf_redirect_map() which would trigger this consume behaviour. That
would allow you to get performance numbers without waiting for the
verifier to learn about tail calls... :)

-Toke