Re: [PATCH bpf-next v7 05/11] libbpf: Probe percpu data feature

[Andrii Nakryiko]

On Mon, Jun 22, 2026 at 7:37 AM Leon Hwang <leon.hwang@xxxxxxxxx> wrote:
>
> libbpf needs a reliable way to distinguish kernels that can support
> global percpu data from those that cannot.
>
> Add a dedicated feature probe, so libbpf can make capability decisions
> early and fail predictably when global percpu data is unavailable.
>
> Signed-off-by: Leon Hwang <leon.hwang@xxxxxxxxx>
> ---
>  tools/lib/bpf/features.c        | 35 +++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf_internal.h |  2 ++
>  2 files changed, 37 insertions(+)
>

lgtm

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>


> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index b7e388f99d0b..ef9581c11303 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c
> @@ -620,6 +620,38 @@ static int probe_bpf_syscall_common_attrs(int token_fd)
>         return probe_sys_bpf_ext();
>  }
>
> +static int probe_kern_percpu_data(int token_fd)
> +{
> +       struct bpf_insn insns[] = {
> +               BPF_LD_MAP_VALUE(BPF_REG_1, 0, 0),
> +               BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, 0),
> +               BPF_EXIT_INSN(),
> +       };
> +       LIBBPF_OPTS(bpf_map_create_opts, map_opts,
> +               .token_fd = token_fd,
> +               .map_flags = token_fd ? BPF_F_TOKEN_FD : 0,
> +       );
> +       LIBBPF_OPTS(bpf_prog_load_opts, prog_opts,
> +               .token_fd = token_fd,
> +               .prog_flags = token_fd ? BPF_F_TOKEN_FD : 0,
> +       );
> +       int ret, map, insn_cnt = ARRAY_SIZE(insns);
> +
> +       map = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "libbpf_percpu", sizeof(int), 8, 1,
> +                            &map_opts);
> +       if (map < 0) {
> +               pr_warn("Error in %s(): %s. Couldn't create simple percpu_array map.\n",
> +                       __func__, errstr(map));
> +               return map;
> +       }
> +
> +       insns[0].imm = map;
> +
> +       ret = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, NULL, "GPL", insns, insn_cnt, &prog_opts);
> +       close(map);
> +       return probe_fd(ret);
> +}
> +
>  typedef int (*feature_probe_fn)(int /* token_fd */);
>
>  static struct kern_feature_cache feature_cache;
> @@ -707,6 +739,9 @@ static struct kern_feature_desc {
>         [FEAT_BPF_SYSCALL_COMMON_ATTRS] = {
>                 "BPF syscall common attributes support", probe_bpf_syscall_common_attrs,
>         },
> +       [FEAT_PERCPU_DATA] = {
> +               "kernel supports percpu data", probe_kern_percpu_data,
> +       },
>  };
>
>  bool feat_supported(struct kern_feature_cache *cache, enum kern_feature_id feat_id)
> diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
> index 04cd303fb5a8..47ae39125f68 100644
> --- a/tools/lib/bpf/libbpf_internal.h
> +++ b/tools/lib/bpf/libbpf_internal.h
> @@ -401,6 +401,8 @@ enum kern_feature_id {
>         FEAT_BTF_LAYOUT,
>         /* Kernel supports BPF syscall common attributes */
>         FEAT_BPF_SYSCALL_COMMON_ATTRS,
> +       /* Kernel supports percpu data */
> +       FEAT_PERCPU_DATA,
>         __FEAT_CNT,
>  };
>
> --
> 2.54.0
>