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

[Alexei Starovoitov]

On 5/19/15 5:11 PM, Andy Lutomirski wrote:
> 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?

that's why I said 'it's _roughly_ expressed in C' this way.
Stack pointer doesn't change. It uses the same stack frame.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/