On Tue, May 19, 2015 at 4:59 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> 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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