Messages in this thread | | | Date | Tue, 19 May 2015 17:14:33 -0700 | From | Alexei Starovoitov <> | Subject | Re: [PATCH net-next 2/4] x86: bpf_jit: implement bpf_tail_call() helper |
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On 5/19/15 5:11 PM, Andy Lutomirski wrote: > 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?
that's why I said 'it's _roughly_ expressed in C' this way. Stack pointer doesn't change. It uses the same stack frame.
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