Re: [PATCH bpf-next v10 3/9] libbpf: Probe percpu data feature

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Jul 15, 2026 at 11:32 AM EDT, Leon Hwang 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.
>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  tools/lib/bpf/features.c        | 35 +++++++++++++++++++++++++++++++++
>  tools/lib/bpf/libbpf_internal.h |  2 ++
>  2 files changed, 37 insertions(+)
>
> 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 d5b7db703b3f..b786f1a35567 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,
>  };
>