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

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Message-ID: <7893c97d-3d3f-35cc-4ea0-ac34d3d84dbc@iogearbox.net>
Date:   Thu, 14 Nov 2019 14:03:34 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Toke Høiland-Jørgensen <thoiland@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        netdev@...r.kernel.org, ast@...nel.org
Cc:     Björn Töpel <bjorn.topel@...el.com>,
        bpf@...r.kernel.org, magnus.karlsson@...il.com,
        magnus.karlsson@...el.com, jonathan.lemon@...il.com
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On 11/14/19 1:31 PM, Toke Høiland-Jørgensen wrote:
> Björn Töpel <bjorn.topel@...il.com> writes:
>> From: Björn Töpel <bjorn.topel@...el.com>
>>
>> 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.

>> 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].

(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.)

Best,
Daniel

   [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a9d57ef15cbe327fe54416dd194ee0ea66ae53a4
   [1] https://en.wikipedia.org/wiki/Tail_call