netdev - Re: [PATCH bpf-next v8 07/10] bpf, net: switch to dynamic registration

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Date: Wed, 1 Nov 2023 18:32:57 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Kui-Feng Lee <sinquersw@...il.com>
Cc: kuifeng@...a.com, netdev@...r.kernel.org, bpf@...r.kernel.org,
 ast@...nel.org, song@...nel.org, kernel-team@...a.com, andrii@...nel.org,
 thinker.li@...il.com, drosen@...gle.com
Subject: Re: [PATCH bpf-next v8 07/10] bpf, net: switch to dynamic
 registration

On 11/1/23 5:59 PM, Kui-Feng Lee wrote:
> 
> 
> On 11/1/23 17:17, Martin KaFai Lau wrote:
>> On 10/31/23 5:19 PM, Kui-Feng Lee wrote:
>>>
>>>
>>> On 10/31/23 17:02, Martin KaFai Lau wrote:
>>>> On 10/31/23 4:34 PM, Kui-Feng Lee wrote:
>>>>>>> diff --git a/include/linux/btf.h b/include/linux/btf.h
>>>>>>> index a8813605f2f6..954536431e0b 100644
>>>>>>> --- a/include/linux/btf.h
>>>>>>> +++ b/include/linux/btf.h
>>>>>>> @@ -12,6 +12,8 @@
>>>>>>>   #include <uapi/linux/bpf.h>
>>>>>>>   #define BTF_TYPE_EMIT(type) ((void)(type *)0)
>>>>>>> +#define BTF_STRUCT_OPS_TYPE_EMIT(type) {((void)(struct type *)0);    \
>>>>>>
>>>>>> ((void)(struct type *)0); is new. Why is it needed?
>>>>>
>>>>> This is a trick of BTF to force compiler generate type info for
>>>>> the given type. Without trick, compiler may skip these types if these
>>>>> type are not used at all in the module.  For example, modules usually
>>>>> don't use value types of struct_ops directly.
>>>> It is not the value type and value type emit is understood. It is the 
>>>> struct_ops type itself and it is new addition in this patchset afaict. The 
>>>> value type emit is in the next line which was cut out from the context here.
>>>>
>>> I mean both of them are required.
>>> In the case of a dummy implementation, struct_ops type itself properly never 
>>> being used, only being declared by the module. Without this line,
>>
>> Other than bpf_dummy_ops, after reg(), the struct_ops->func() must be used 
>> somewhere in the kernel or module. Like tcp must be using the 
>> tcp_congestion_ops after reg(). bpf_dummy_ops is very special and probably 
>> should be moved out to bpf_testmod somehow but this is for later. Even 
>> bpf_dummy_ops does not have an issue now. Why it is needed after the kmod 
>> support change?
>>
>> or it is a preemptive addition to be future proof only?
>>
>> Addition is fine if it is required to work. I am trying to understand why this 
>> new addition is needed after the kmod support change. The reason why this is 
>> needed after the kmod support change is not obvious from looking at the code. 
>> The commit message didn't mention why and what broke after this kmod change. 
>> If someone wants to clean it up a few months later, we will need to figure out 
>> why it was added in the first place.
> 
> 
> It is a future proof.
> What do you think if I add a comment in the code?

If it is not required to work, I prefer not adding it to avoid confusion and 
avoid future cleanup temptation. Even the artificial bpf_dummy_ops does not need 
it, so not enough reason to introduce this code redundancy.

Switch topic.
While we are on a new macro topic, I think a new macro will be useful to emit 
the value type and register_bpf_struct_ops together. wdyt?

> 
>>
>>
>>> the module developer will fail to load a struct_ops map of the dummy
>>> type. This line is added to avoid this awful situation.
>>>
>>