netdev - Re: [PATCH bpf-next v6 01/13] bpf: Introduce BTF ID flags and 8-byte BTF set

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Date:   Tue, 19 Jul 2022 11:37:45 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Florian Westphal <fw@...len.de>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH bpf-next v6 01/13] bpf: Introduce BTF ID flags and 8-byte
 BTF set

On Tue, Jul 19, 2022 at 03:24:18PM +0200, Kumar Kartikeya Dwivedi wrote:
>  
> +#define ____BTF_ID_FLAGS_LIST(_0, _1, _2, _3, _4, _5, N, ...) _1##_##_2##_##_3##_##_4##_##_5##__
> +#define __BTF_ID_FLAGS_LIST(...) ____BTF_ID_FLAGS_LIST(0x0, ##__VA_ARGS__, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0)
> +
> +#define __FLAGS(prefix, ...) \
> +	__PASTE(prefix, __BTF_ID_FLAGS_LIST(__VA_ARGS__))
> +
> +#define BTF_ID_FLAGS(prefix, name, ...) \
> +	BTF_ID(prefix, name)		\
> +	__BTF_ID(__ID(__FLAGS(__BTF_ID__flags__, ##__VA_ARGS__)))
> +
>  /*
>   * The BTF_ID_LIST macro defines pure (unsorted) list
>   * of BTF IDs, with following layout:
> @@ -145,10 +164,53 @@ asm(							\
>  ".popsection;                                 \n");	\
>  extern struct btf_id_set name;
>  
> +/*
> + * The BTF_SET8_START/END macros pair defines sorted list of
> + * BTF IDs and their flags plus its members count, with the
> + * following layout:
> + *
> + * BTF_SET8_START(list)
> + * BTF_ID_FLAGS(type1, name1, flags...)
> + * BTF_ID_FLAGS(type2, name2, flags...)
> + * BTF_SET8_END(list)
> + *
> + * __BTF_ID__set8__list:
> + * .zero 8
> + * list:
> + * __BTF_ID__type1__name1__3:
> + * .zero 4
> + * __BTF_ID__flags__0x0_0x0_0x0_0x0_0x0__4:
> + * .zero 4

Overall looks great,
but why encode flags into a name?
Why reuse ____BTF_ID for flags and complicate resolve_btfid?
Instead of .zero 4 insert the actual flags as .word ?

The usage will be slightly different.
Instead of:
BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE, KF_RET_NULL)
it will be
BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL)