netdev - Re: [RFC PATCH 3/3] bpf, riscv: added eBPF JIT for RV64G

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Message-ID: <CAJ+HfNjJ+Yy=+bpvBnAMugCQBjNNvaMBs=gBFfgE7U5dv37MRw@mail.gmail.com>
Date:   Wed, 16 Jan 2019 20:06:08 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     linux-riscv@...ts.infradead.org,
        Palmer Dabbelt <palmer@...ive.com>, davidlee@...ive.com,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [RFC PATCH 3/3] bpf, riscv: added eBPF JIT for RV64G

Den ons 16 jan. 2019 kl 16:41 skrev Daniel Borkmann <daniel@...earbox.net>:
>
[...]
>
> > For the fixed prologue: how does, say x86, deal with BPF stack usage
> > in the tail call case? If the caller doesn't use the bpf stack, but
> > the callee does. From a quick glance in the code, the x86 prologue
> > still uses aux->stack_depth. If the callee has a different stack usage
> > that the caller, and then the callee does a function call, wouldn't
> > this mess up the frame? (Yeah, obviously missing something! :-))
>
> Basically in this case verifier sets stack size to MAX_BPF_STACK when it
> finds a tail call in the prog, meaning the callee will be reusing <= stack
> size than the caller and then upon exit unwinds it via leave+ret.
>

Ugh, so for "dynamic" tail calls this would mean "more expensive
functions calls with maximum stack usage per call"? I.e. each tail
call consumes MAX_BPF_STACK plus regular pro-/epilogue plus depth
tracking. I'd still prefer optimizing to regular functions calls, than
tail calls -- or is that naive? What is most common in larger bpf
deployments, say, Katran or Cilium?

Cheers!
Björn


> Cheers,
> Daniel