Re: [PATCH bpf-next 2/3] bpf, libbpf: Support BPF_F_CPU for percpu_array map

[Leon Hwang <leon.hwang@linux.dev>]

On 2025/7/18 23:57, Andrii Nakryiko wrote:
> On Thu, Jul 17, 2025 at 12:38 PM Leon Hwang <leon.hwang@linux.dev> wrote:
>>
>> This patch adds libbpf support for the BPF_F_CPU flag in percpu_array maps,
>> introducing the following APIs:
>>
>> 1. bpf_map_update_elem_opts(): update with struct bpf_map_update_elem_opts
>> 2. bpf_map_lookup_elem_opts(): lookup with struct bpf_map_lookup_elem_opts
>> 3. bpf_map__update_elem_opts(): high-level wrapper with input validation
>> 4. bpf_map__lookup_elem_opts(): high-level wrapper with input validation
>>
>> Behavior:
>>
>> * If opts->cpu == (u32)~0, the update is applied to all CPUs.
>> * Otherwise, it applies only to the specified CPU.
>> * Lookup APIs retrieve values from the target CPU when BPF_F_CPU is used.
>>
>> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
>> ---
> 
> please use "libbpf: " prefix, not "bpf,libbpf:"
> 

Ack.

> 
> Then see my comment on flags vs separate field for cpu passing. If we
> go with just using flags, then I'd probably drop all the new libbpf
> APIs, because we already have bpf_map_lookup_elem_flags() and
> bpf_map_update_elem() (the latter accepts flags), so as far as
> low-level API we are good.
> 

All the new libbpf APIs are unnecessary when pass the cpu through flags.

> The comment describing the new BPF_F_CPU flag is good, so let's add
> it, but place it into bpf_map__lookup_elem() description (which, btw,
> also accepts flags, so no changes to API itself is necessary). Same
> for bpf_map__update_elem().
> 

No problem.

> validate_map_op() logic will stay, but just will extract cpu from flags, right?
> 

Yes, it is.

> So overall less API churn, but same possibilities for users (plus we
> get better documentation, which is always nice).
> 

When users want to use this feature, they should follow the libbpf API's
documentation, no matter whether the cpu is separated or flags-embedded.

Thanks,
Leon

>>  tools/lib/bpf/bpf.c           | 23 ++++++++++++++
>>  tools/lib/bpf/bpf.h           | 36 +++++++++++++++++++++-
>>  tools/lib/bpf/libbpf.c        | 56 +++++++++++++++++++++++++++++++----
>>  tools/lib/bpf/libbpf.h        | 53 ++++++++++++++++++++++++++++-----
>>  tools/lib/bpf/libbpf.map      |  5 ++++
>>  tools/lib/bpf/libbpf_common.h | 14 +++++++++
>>  6 files changed, 173 insertions(+), 14 deletions(-)
>>
> 
> [...]