[PATCH bpf-next v2 2/7] bpf/flow_dissector: document flags

[Stanislav Fomichev <sdf@google.com>]

Describe what each input flag does and who uses it.

Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Petar Penkov <ppenkov@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 Documentation/bpf/prog_flow_dissector.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
index ed343abe541e..0f3f380b2ce4 100644
--- a/Documentation/bpf/prog_flow_dissector.rst
+++ b/Documentation/bpf/prog_flow_dissector.rst
@@ -26,6 +26,7 @@ and output arguments.
   * ``nhoff`` - initial offset of the networking header
   * ``thoff`` - initial offset of the transport header, initialized to nhoff
   * ``n_proto`` - L3 protocol type, parsed out of L2 header
+  * ``flags`` - optional flags
 
 Flow dissector BPF program should fill out the rest of the ``struct
 bpf_flow_keys`` fields. Input arguments ``nhoff/thoff/n_proto`` should be
@@ -101,6 +102,23 @@ can be called for both cases and would have to be written carefully to
 handle both cases.
 
 
+Flags
+=====
+
+``flow_keys->flags`` might contain optional input flags that work as follows:
+
+* ``FLOW_DISSECTOR_F_PARSE_1ST_FRAG`` - tells BPF flow dissector to continue
+  parsing first fragment; the default expected behavior is that flow dissector
+  returns as soon as it finds out that the packet is fragmented;
+  used by ``eth_get_headlen`` to estimate length of all headers for GRO.
+* ``FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL`` - tells BPF flow dissector to stop
+  parsing as soon as it reaches IPv6 flow label; used by ``___skb_get_hash``
+  and ``__skb_get_hash_symmetric`` to get flow hash.
+* ``FLOW_DISSECTOR_F_STOP_AT_ENCAP`` - tells BPF flow dissector to stop
+  parsing as soon as it reaches encapsulated headers; used by routing
+  infrastructure.
+
+
 Reference Implementation
 ========================
 
-- 
2.22.0.657.g960e92d24f-goog