Subject: Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper

From: Alexei Starovoitov <[email-masked]>
Date: Tue, 19 May 2015 17:14:33 -0700

On 5/19/15 5:11 PM, Andy Lutomirski wrote:
> On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <[email-masked]> 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.