netdev - Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail

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Message-ID: <CALCETrV8CStuwvDXDPp5zsZw5FsSpYWBDXMYjLh6Qq703a=cgQ@mail.gmail.com>
Date:	Wed, 20 May 2015 09:05:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper

On Tue, May 19, 2015 at 5:14 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> On 5/19/15 5:11 PM, Andy Lutomirski wrote:
>>
>> On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <ast@...mgrid.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?
>
>
> that's why I said 'it's _roughly_ expressed in C' this way.
> Stack pointer doesn't change. It uses the same stack frame.
>

I think the more relevant point is that (I think) eBPF never changes
the stack pointer after the prologue (i.e. the stack depth is truly
constant).

--Andy
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