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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Sun, Sep 29, 2024 at 10:33 PM Hari Bathini <hbathini@linux.ibm.com> wrote:
>
>
>
> On 17/09/24 1:20 pm, Alexei Starovoitov wrote:
> > On Sun, Sep 15, 2024 at 10:58 PM Hari Bathini <hbathini@linux.ibm.com> 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 ?

> [1]
> https://lore.kernel.org/all/40b65ab2bb3a48837ab047a70887de3ccd70c56b.1474661927.git.naveen.n.rao@linux.vnet.ibm.com/
> [2] https://lore.kernel.org/all/20180503230824.3462-11-daniel@iogearbox.net/
>
> Thanks
> Hari