Re: [PATCH v5 17/17] powerpc64/bpf: Add support for bpf trampolines

[Alexei Starovoitov]

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?