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

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Date:   Fri, 15 Nov 2019 13:58:34 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        bpf <bpf@...r.kernel.org>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Edward Cree <ecree@...arflare.com>
Subject: Re: [RFC PATCH bpf-next 2/4] bpf: introduce BPF dispatcher

On Fri, Nov 15, 2019 at 08:56:59AM +0100, Björn Töpel wrote:
> On Fri, 15 Nov 2019 at 01:30, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> [...]
> >
> > Could you try optimizing emit_mov_imm64() to recognize s32 ?
> > iirc there was a single x86 insns that could move and sign extend.
> > That should cut down on bytecode size and probably make things a bit faster?
> > Another alternative is compare lower 32-bit only, since on x86-64 upper 32
> > should be ~0 anyway for bpf prog pointers.
> 
> Good ideas, thanks! I'll do the optimization, extend it to >4 entries
> (as Toke suggested), and do a non-RFC respin.
> 
> > Looking at bookkeeping code, I think I should be able to generalize bpf
> > trampoline a bit and share the code for bpf dispatch.
> 
> Ok, good!
> 
> > Could you also try aligning jmp target a bit by inserting nops?
> > Some x86 cpus are sensitive to jmp target alignment. Even without considering
> > JCC bug it could be helpful. Especially since we're talking about XDP/AF_XDP
> > here that will be pushing millions of calls through bpf dispatch.
> >
> 
> Yeah, I need to address the Jcc bug anyway, so that makes sense.
> 
> Another thought; I'm using the fentry nop as patch point, so it wont
> play nice with other users of fentry atm -- but the plan is to move to
> Steve's *_ftrace_direct work at some point, correct?

Yes. I'll start playing with reg/mod/unreg_ftrace_direct on Monday.
Steven has a bunch more in his tree for merging, so I cannot just pull
all of ftrace api features into bpf-next. So "be nice to other fentry users"
would have to be done during merge window or shortly after in bpf-next tree
after window closes. I think it's fine. In bpf dispatch case it's really
one dummy function we're talking about. If it was marked 'notrace'
from get go no one would blink. It's a dummy function not interesting
for ftrac-ing and not interesting from live patching pov.