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From: Andy Lutomirski <luto@amacapital.net>
Date: Tue, 19 May 2015 17:11:59 -0700
Message-ID: <CALCETrVZyB+m8Oaad1JGwrLx1rTUoL0F6Afrvg_hQWiYyTTang@mail.gmail.com>
Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: "David S. Miller" <davem@davemloft.net>, Ingo Molnar <mingo@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Michael Holzheu <holzheu@linux.vnet.ibm.com>,
        Zi Shen Lim <zlim.lnx@gmail.com>,
        Linux API <linux-api@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <ast@plumgrid.com> wrote:
> bpf_tail_call() arguments:
> ctx - context pointer
> jmp_table - one of BPF_MAP_TYPE_PROG_ARRAY maps used as the jump table
> index - index in the jump table
>
> In this implementation x64 JIT bypasses stack unwind and jumps into the
> callee program after prologue, so the callee program reuses the same stack.
>
> The logic can be roughly expressed in C like:
>
> u32 tail_call_cnt;
>
> void *jumptable[2] = { &&label1, &&label2 };
>
> int bpf_prog1(void *ctx)
> {
> label1:
>     ...
> }
>
> int bpf_prog2(void *ctx)
> {
> label2:
>     ...
> }
>
> int bpf_prog1(void *ctx)
> {
>     ...
>     if (tail_call_cnt++ < MAX_TAIL_CALL_CNT)
>         goto *jumptable[index]; ... and pass my 'ctx' to callee ...
>
>     ... fall through if no entry in jumptable ...
> }
>

What causes the stack pointer to be right?  Is there some reason that
the stack pointer is the same no matter where you are in the generated
code?

--Andy