netdev - Re: [RFC PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and tailcall handling in JIT

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Date:   Fri, 10 Jul 2020 20:25:20 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>
Subject: Re: [RFC PATCH bpf-next 4/5] bpf, x64: rework pro/epilogue and
 tailcall handling in JIT

On Fri, Jul 10, 2020 at 8:20 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> Of course you are right.
> pop+nop+push is incorrect.
>
> How about the following instead:
> - during JIT:
> emit_jump(to_skip_below)  <- poke->tailcall_bypass
> pop_callee_regs
> emit_jump(to_tailcall_target) <- poke->tailcall_target
>
> - Transition from one target to another:
> text_poke(poke->tailcall_target, MOD_JMP, old_jmp, new_jmp)
> if (new_jmp != NULL)
>   text_poke(poke->tailcall_bypass, MOD jmp into nop);
> else
>   text_poke(poke->tailcall_bypass, MOD nop into jmp);

One more correction. I meant:

if (new_jmp != NULL) {
  text_poke(poke->tailcall_target, MOD_JMP, old_jmp, new_jmp)
  text_poke(poke->tailcall_bypass, MOD jmp into nop);
} else {
  text_poke(poke->tailcall_bypass, MOD nop into jmp);
}