On Mon, Jun 22, 2026 at 7:37 AM Leon Hwang <[email protected]> wrote:
>
> Introduce global percpu data, inspired by the commit
> 6316f78306c1 ("Merge branch 'support-global-data'"). It enables the
> definition of global percpu variables in BPF, similar to the
> include/linux/percpu-defs.h::DEFINE_PER_CPU() macro.
>
> For example, in BPF, it is able to define a global percpu variable like:
>
> int data SEC(".percpu");
>
> With this patch, tools like retsnoop [1] and bpfsnoop [2] can simplify
> their BPF code for handling LBRs. The code can be updated from
>
> static struct perf_branch_entry lbrs[1][MAX_LBR_ENTRIES] SEC(".data.lbrs");
>
> to
>
> static struct perf_branch_entry lbrs[MAX_LBR_ENTRIES] SEC(".percpu.lbrs");
>
> This eliminates the need to retrieve the CPU ID using the
> bpf_get_smp_processor_id() helper.
>
> Additionally, by reusing global percpu data map, sharing information
> between tail callers and callees or freplace callers and callees becomes
> simpler compared to reusing percpu_array maps.
>
> Links:
> [1] https://github.com/anakryiko/retsnoop
> [2] https://github.com/bpfsnoop/bpfsnoop
>
> Signed-off-by: Leon Hwang <[email protected]>
> ---
> kernel/bpf/arraymap.c | 39 +++++++++++++++++++++++++++++++++++++--
> kernel/bpf/const_fold.c | 1 -
> kernel/bpf/fixups.c | 34 ++++++++++++++++++++++++++++++++++
> kernel/bpf/verifier.c | 15 +++++++++++++++
> 4 files changed, 86 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 248b4818178c..876aa2604f41 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -259,6 +259,38 @@ static void *percpu_array_map_lookup_elem(struct bpf_map
> *map, void *key)
> return this_cpu_ptr(array->pptrs[index & array->index_mask]);
> }
>
> +static int percpu_array_map_direct_value_addr(const struct bpf_map *map, u64
> *imm, u32 off)
> +{
> + struct bpf_array *array = container_of(map, struct bpf_array, map);
> +
> + if (map->max_entries != 1)
> + return -EOPNOTSUPP;
> + if (off >= map->value_size)
> + return -EINVAL;
> + if (!bpf_jit_supports_percpu_insn())
> + return -EOPNOTSUPP;
> +
> + *imm = (u64)(__force unsigned long) array->pptrs[0];
> + return 0;
> +}
> +
> +static int percpu_array_map_direct_value_meta(const struct bpf_map *map, u64
> imm, u32 *off)
> +{
> + struct bpf_array *array = container_of(map, struct bpf_array, map);
> + u64 base = (u64)(__force unsigned long) array->pptrs[0];
> + u64 range = array->elem_size;
> +
> + if (map->max_entries != 1)
> + return -EOPNOTSUPP;
> + if (imm < base || imm >= base + range)
nit: not sure what's the value of range local variable, single place
where you use array->elem_size
> + return -ENOENT;
> + if (!bpf_jit_supports_percpu_insn())
> + return -EOPNOTSUPP;
> +
> + *off = imm - base;
> + return 0;
> +}
> +
> /* emit BPF instructions equivalent to C code of
> percpu_array_map_lookup_elem() */
> static int percpu_array_map_gen_lookup(struct bpf_map *map, struct bpf_insn
> *insn_buf)
> {
> @@ -551,9 +583,10 @@ static int array_map_check_btf(struct bpf_map *map,
> const struct btf_type *key_type,
> const struct btf_type *value_type)
> {
> - /* One exception for keyless BTF: .bss/.data/.rodata map */
> + /* One exception for keyless BTF: .bss/.data/.rodata/.percpu map */
> if (btf_type_is_void(key_type)) {
> - if (map->map_type != BPF_MAP_TYPE_ARRAY ||
> + if ((map->map_type != BPF_MAP_TYPE_ARRAY &&
> + map->map_type != BPF_MAP_TYPE_PERCPU_ARRAY) ||
> map->max_entries != 1)
> return -EINVAL;
>
> @@ -832,6 +865,8 @@ const struct bpf_map_ops percpu_array_map_ops = {
> .map_get_next_key = bpf_array_get_next_key,
> .map_lookup_elem = percpu_array_map_lookup_elem,
> .map_gen_lookup = percpu_array_map_gen_lookup,
> + .map_direct_value_addr = percpu_array_map_direct_value_addr,
> + .map_direct_value_meta = percpu_array_map_direct_value_meta,
> .map_update_elem = array_map_update_elem,
> .map_delete_elem = array_map_delete_elem,
> .map_lookup_percpu_elem = percpu_array_map_lookup_percpu_elem,
> diff --git a/kernel/bpf/const_fold.c b/kernel/bpf/const_fold.c
> index b2a19acadb91..5787246bef30 100644
> --- a/kernel/bpf/const_fold.c
> +++ b/kernel/bpf/const_fold.c
> @@ -182,7 +182,6 @@ static void const_reg_xfer(struct bpf_verifier_env *env,
> struct const_arg_info *
> u64 val = 0;
>
> if (!bpf_map_is_rdonly(map) ||
> !map->ops->map_direct_value_addr ||
> - map->map_type == BPF_MAP_TYPE_INSN_ARRAY ||
can you please elaborate on why this change is needed?
> off < 0 || off + size > map->value_size ||
> bpf_map_direct_read(map, off, size, &val, is_ldsx)) {
> *dst = unknown;
[...]
