On Tue, Oct 1, 2024 at 12:18 AM Hari Bathini <hbathini@xxxxxxxxxxxxx> wrote: > > > > On 30/09/24 6:25 pm, Alexei Starovoitov wrote: > > On Sun, Sep 29, 2024 at 10:33 PM Hari Bathini <hbathini@xxxxxxxxxxxxx> wrote: > >> > >> > >> > >> On 17/09/24 1:20 pm, Alexei Starovoitov wrote: > >>> On Sun, Sep 15, 2024 at 10:58 PM Hari Bathini <hbathini@xxxxxxxxxxxxx> wrote: > >>>> > >>>> + > >>>> + /* > >>>> + * Generated stack layout: > >>>> + * > >>>> + * func prev back chain [ back chain ] > >>>> + * [ ] > >>>> + * bpf prog redzone/tailcallcnt [ ... ] 64 bytes (64-bit powerpc) > >>>> + * [ ] -- > >>> ... > >>>> + > >>>> + /* Dummy frame size for proper unwind - includes 64-bytes red zone for 64-bit powerpc */ > >>>> + bpf_dummy_frame_size = STACK_FRAME_MIN_SIZE + 64; > >>> > >>> What is the goal of such a large "red zone" ? > >>> The kernel stack is a limited resource. > >>> Why reserve 64 bytes ? > >>> tail call cnt can probably be optional as well. > >> > >> Hi Alexei, thanks for reviewing. > >> FWIW, the redzone on ppc64 is 288 bytes. BPF JIT for ppc64 was using > >> a redzone of 80 bytes since tailcall support was introduced [1]. > >> It came down to 64 bytes thanks to [2]. The red zone is being used > >> to save NVRs and tail call count when a stack is not setup. I do > >> agree that we should look at optimizing it further. Do you think > >> the optimization should go as part of PPC64 trampoline enablement > >> being done here or should that be taken up as a separate item, maybe? > > > > The follow up is fine. > > It just odd to me that we currently have: > > > > [ unused red zone ] 208 bytes protected > > > > I simply don't understand why we need to waste this much stack space. > > Why can't it be zero today ? > > > > The ABI for ppc64 has a redzone of 288 bytes below the current > stack pointer that can be used as a scratch area until a new > stack frame is created. So, no wastage of stack space as such. > It is just red zone that can be used before a new stack frame > is created. The comment there is only to show how redzone is > being used in ppc64 BPF JIT. I think the confusion is with the > mention of "208 bytes" as protected. As not all of that scratch > area is used, it mentions the remaining as unused. Essentially > 288 bytes below current stack pointer is protected from debuggers > and interrupt code (red zone). Note that it should be 224 bytes > of unused red zone instead of 208 bytes as red zone usage in > ppc64 BPF JIT come down from 80 bytes to 64 bytes since [2]. > Hope that clears the misunderstanding.. I see. That makes sense. So it's similar to amd64 red zone, but there we have an issue with irqs, hence the kernel is compiled with -mno-red-zone. I guess ppc always has a different interrupt stack and it's not an issue?
