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

[Michael Ellerman]

Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> writes:
> 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 assume that issue is that the interrupt entry unconditionally writes
some data below the stack pointer, disregarding the red zone?

> I guess ppc always has a different interrupt stack and
> it's not an issue?

No, the interrupt entry allocates a frame that is big enough to cover
the red zone as well as the space it needs to save registers.

See STACK_INT_FRAME_SIZE which includes KERNEL_REDZONE_SIZE:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/include/asm/ptrace.h?commit=8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b#n165

Which is renamed to INT_FRAME_SIZE in asm-offsets.c and then is used in
the interrupt entry here:

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/exceptions-64s.S?commit=8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b#n497

cheers