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

[Hari Bathini <hbathini@linux.ibm.com>]

On 01/10/24 8:23 pm, Alexei Starovoitov wrote:
> On Tue, Oct 1, 2024 at 12:18 AM Hari Bathini <hbathini@linux.ibm.com> wrote:
>>
>>
>>
>> On 30/09/24 6:25 pm, Alexei Starovoitov wrote:
>>> 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 ?
>>>
>>
>> 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?

Yeah. On ppc64, kernel also uses redzone.
Interrupts use a different stack..

Thanks
Hari