Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 19 May 2015 17:11:59 -0700 | Subject | Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper |
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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
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