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

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Date:   Tue, 14 Jul 2020 03:00:45 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 08:25:20PM -0700, Alexei Starovoitov wrote:
> 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

That's the jump to the instruction right after the poke->tailcall_target.

> > pop_callee_regs
> > emit_jump(to_tailcall_target) <- poke->tailcall_target

During JIT there's no tailcall_target so this will be nop5, right?

> >
> > - 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)

Problem with having the old_jmp here is that you could have the
tailcall_target removed followed by the new program being inserted. So for
that case old_jmp is NULL but we decided to not poke the
poke->tailcall_target when removing the program, only the tailcall_bypass
is poked back to jmp from nop. IOW old_jmp is not equal to what
poke->tailcall_target currently stores. This means that
bpf_arch_text_poke() would not be successful for this update and that is
the reason of faking it in this patch.

>   text_poke(poke->tailcall_bypass, MOD jmp into nop);
> } else {
>   text_poke(poke->tailcall_bypass, MOD nop into jmp);
> }

I think that's what we currently (mostly) have. map_poke_run() is skipping
the poke of poke->tailcall_target if new bpf_prog is NULL, just like
you're proposing above. Of course I can rename the members in poke
descriptor to names you're suggesting. I also assume that by text_poke you
meant the bpf_arch_text_poke?

I've been able to hide the nop5 detection within the bpf_arch_text_poke so
map_poke_run() is arch-independent in that approach. My feeling is that
we don't need the old bpf_prog at all.

Some bits might change here due to the jump target alignment that I'm
trying to introduce.