Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 14 Nov 2019 14:09:57 +0100

Daniel Borkmann <[email-masked]> writes:

> On 11/14/19 1:31 PM, Toke Høiland-Jørgensen wrote:
>> Björn Töpel <[email-masked]> writes:
>>> From: Björn Töpel <[email-masked]>
>>>
>>> The BPF dispatcher builds on top of the BPF trampoline ideas;
>>> Introduce bpf_arch_text_poke() and (re-)use the BPF JIT generate
>>> code. The dispatcher builds a dispatch table for XDP programs, for
>>> retpoline avoidance. The table is a simple binary search model, so
>>> lookup is O(log n). Here, the dispatch table is limited to four
>>> entries (for laziness reason -- only 1B relative jumps :-P). If the
>>> dispatch table is full, it will fallback to the retpoline path.
>> 
>> So it's O(log n) with n == 4? Have you compared the performance of just
>> doing four linear compare-and-jumps? Seems to me it may not be that big
>> of a difference for such a small N?
>
> Did you perform some microbenchmarks wrt search tree? Mainly wondering
> since for code emission for switch/case statements, clang/gcc turns off
> indirect calls entirely under retpoline, see [0] from back then.

Yes, this was exactly the example I had in mind :)

>>> An example: A module/driver allocates a dispatcher. The dispatcher is
>>> shared for all netdevs. Each netdev allocate a slot in the dispatcher
>>> and a BPF program. The netdev then uses the dispatcher to call the
>>> correct program with a direct call (actually a tail-call).
>> 
>> Is it really accurate to call it a tail call? To me, that would imply
>> that it increments the tail call limit counter and all that? Isn't this
>> just a direct jump using the trampoline stuff?
>
> Not meant in BPF context here, but more general [1].

Ah, right, that makes more sense.

> (For actual BPF tail calls I have a series close to ready for getting
> rid of most indirect calls which I'll post later today.)

Cool!

-Toke