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

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Message-ID: <5a8520dd-0dd6-4d51-9e4a-6eebcf7e792d@linux.dev>
Date: Tue, 31 Oct 2023 17:02:56 -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 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.