Add sections explaining KF_IMPLICIT_ARGS kfunc flag. Mark __prog
annotation as deprecated.

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 Documentation/bpf/kfuncs.rst | 44 +++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 3eb59a8f9f34..b849598271d2 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -232,7 +232,7 @@ Or::
                 ...
         }
 
-2.3.6 __prog Annotation
+2.3.6 __prog Annotation (deprecated, use KF_IMPLICIT_ARGS instead)
 ---------------------------
 This annotation is used to indicate that the argument needs to be fixed up to
 the bpf_prog_aux of the caller BPF program. Any value passed into this argument
@@ -381,6 +381,48 @@ encouraged to make their use-cases known as early as possible, and participate
 in upstream discussions regarding whether to keep, change, deprecate, or remove
 those kfuncs if and when such discussions occur.
 
+2.5.9 KF_IMPLICIT_ARGS flag
+------------------------------------
+
+The KF_IMPLICIT_ARGS flag is used to indicate that the BPF signature
+of the kfunc is different from it's kernel signature, and the values
+for implicit arguments are provided at load time by the verifier.
+
+Only arguments of specific types are implicit.
+Currently only ``struct bpf_prog_aux *`` type is supported.
+
+A kfunc with KF_IMPLICIT_ARGS flag therefore has two types in BTF: one
+function matching the kernel declaration (with _impl suffix in the
+name by convention), and another matching the intended BPF API.
+
+Verifier only allows calls to the non-_impl version of a kfunc, that
+uses a signature without the implicit arguments.
+
+Example declaration:
+
+.. code-block:: c
+
+	__bpf_kfunc int bpf_task_work_schedule_signal(struct task_struct *task, struct bpf_task_work *tw,
+						      void *map__map, bpf_task_work_callback_t callback,
+						      struct bpf_prog_aux *aux) { ... }
+
+Example usage in BPF program:
+
+.. code-block:: c
+
+	/* note that the last argument is omitted */
+        bpf_task_work_schedule_signal(task, &work->tw, &arrmap, task_work_callback);
+
+An exception to this are kfuncs that use __prog argument, and were
+implemented before KF_IMPLICIT_ARGS mechanism was introduced:
+  * bpf_stream_vprintk_impl
+  * bpf_task_work_schedule_resume_impl
+  * bpf_task_work_schedule_signal_impl
+  * bpf_wq_set_callback_impl
+
+These are allowed for backwards compatibility, however BPF programs
+should use newer API that omits implicit arguments in BPF.
+
 2.6 Registering the kfuncs
 --------------------------
 
-- 
2.52.0