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diff --git a/docs/developer/corearchitecture/featurearcs.rst b/docs/developer/corearchitecture/featurearcs.rst new file mode 100644 index 00000000000..89c50e38dce --- /dev/null +++ b/docs/developer/corearchitecture/featurearcs.rst @@ -0,0 +1,225 @@ +Feature Arcs +============ + +A significant number of vpp features are configurable on a per-interface +or per-system basis. Rather than ask feature coders to manually +construct the required graph arcs, we built a general mechanism to +manage these mechanics. + +Specifically, feature arcs comprise ordered sets of graph nodes. Each +feature node in an arc is independently controlled. Feature arc nodes +are generally unaware of each other. Handing a packet to “the next +feature node” is quite inexpensive. + +The feature arc implementation solves the problem of creating graph arcs +used for steering. + +At the beginning of a feature arc, a bit of setup work is needed, but +only if at least one feature is enabled on the arc. + +On a per-arc basis, individual feature definitions create a set of +ordering dependencies. Feature infrastructure performs a topological +sort of the ordering dependencies, to determine the actual feature +order. Missing dependencies **will** lead to runtime disorder. See +https://gerrit.fd.io/r/#/c/12753 for an example. + +If no partial order exists, vpp will refuse to run. Circular dependency +loops of the form “a then b, b then c, c then a” are impossible to +satisfy. + +Adding a feature to an existing feature arc +------------------------------------------- + +To nobody’s great surprise, we set up feature arcs using the typical +“macro -> constructor function -> list of declarations” pattern: + +.. code:: c + + VNET_FEATURE_INIT (mactime, static) = + { + .arc_name = "device-input", + .node_name = "mactime", + .runs_before = VNET_FEATURES ("ethernet-input"), + }; + +This creates a “mactime” feature on the “device-input” arc. + +Once per frame, dig up the vnet_feature_config_main_t corresponding to +the “device-input” feature arc: + +.. code:: c + + vnet_main_t *vnm = vnet_get_main (); + vnet_interface_main_t *im = &vnm->interface_main; + u8 arc = im->output_feature_arc_index; + vnet_feature_config_main_t *fcm; + + fcm = vnet_feature_get_config_main (arc); + +Note that in this case, we’ve stored the required arc index - assigned +by the feature infrastructure - in the vnet_interface_main_t. Where to +put the arc index is a programmer’s decision when creating a feature +arc. + +Per packet, set next0 to steer packets to the next node they should +visit: + +.. code:: c + + vnet_get_config_data (&fcm->config_main, + &b0->current_config_index /* value-result */, + &next0, 0 /* # bytes of config data */); + +Configuration data is per-feature arc, and is often unused. Note that +it’s normal to reset next0 to divert packets elsewhere; often, to drop +them for cause: + +.. code:: c + + next0 = MACTIME_NEXT_DROP; + b0->error = node->errors[DROP_CAUSE]; + +Creating a feature arc +---------------------- + +Once again, we create feature arcs using constructor macros: + +.. code:: c + + VNET_FEATURE_ARC_INIT (ip4_unicast, static) = + { + .arc_name = "ip4-unicast", + .start_nodes = VNET_FEATURES ("ip4-input", "ip4-input-no-checksum"), + .arc_index_ptr = &ip4_main.lookup_main.ucast_feature_arc_index, + }; + +In this case, we configure two arc start nodes to handle the +“hardware-verified ip checksum or not” cases. During initialization, the +feature infrastructure stores the arc index as shown. + +In the head-of-arc node, do the following to send packets along the +feature arc: + +.. code:: c + + ip_lookup_main_t *lm = &im->lookup_main; + arc = lm->ucast_feature_arc_index; + +Once per packet, initialize packet metadata to walk the feature arc: + +.. code:: c + + vnet_feature_arc_start (arc, sw_if_index0, &next, b0); + +Enabling / Disabling features +----------------------------- + +Simply call vnet_feature_enable_disable to enable or disable a specific +feature: + +.. code:: c + + vnet_feature_enable_disable ("device-input", /* arc name */ + "mactime", /* feature name */ + sw_if_index, /* Interface sw_if_index */ + enable_disable, /* 1 => enable */ + 0 /* (void *) feature_configuration */, + 0 /* feature_configuration_nbytes */); + +The feature_configuration opaque is seldom used. + +If you wish to make a feature a *de facto* system-level concept, pass +sw_if_index=0 at all times. Sw_if_index 0 is always valid, and +corresponds to the “local” interface. + +Related “show” commands +----------------------- + +To display the entire set of features, use “show features [verbose]”. +The verbose form displays arc indices, and feature indicies within the +arcs + +:: + + $ vppctl show features verbose + Available feature paths + <snip> + [14] ip4-unicast: + [ 0]: nat64-out2in-handoff + [ 1]: nat64-out2in + [ 2]: nat44-ed-hairpin-dst + [ 3]: nat44-hairpin-dst + [ 4]: ip4-dhcp-client-detect + [ 5]: nat44-out2in-fast + [ 6]: nat44-in2out-fast + [ 7]: nat44-handoff-classify + [ 8]: nat44-out2in-worker-handoff + [ 9]: nat44-in2out-worker-handoff + [10]: nat44-ed-classify + [11]: nat44-ed-out2in + [12]: nat44-ed-in2out + [13]: nat44-det-classify + [14]: nat44-det-out2in + [15]: nat44-det-in2out + [16]: nat44-classify + [17]: nat44-out2in + [18]: nat44-in2out + [19]: ip4-qos-record + [20]: ip4-vxlan-gpe-bypass + [21]: ip4-reassembly-feature + [22]: ip4-not-enabled + [23]: ip4-source-and-port-range-check-rx + [24]: ip4-flow-classify + [25]: ip4-inacl + [26]: ip4-source-check-via-rx + [27]: ip4-source-check-via-any + [28]: ip4-policer-classify + [29]: ipsec-input-ip4 + [30]: vpath-input-ip4 + [31]: ip4-vxlan-bypass + [32]: ip4-lookup + <snip> + +Here, we learn that the ip4-unicast feature arc has index 14, and that +e.g. ip4-inacl is the 25th feature in the generated partial order. + +To display the features currently active on a specific interface, use +“show interface features”: + +:: + + $ vppctl show interface GigabitEthernet3/0/0 features + Feature paths configured on GigabitEthernet3/0/0... + <snip> + ip4-unicast: + nat44-out2in + <snip> + +Table of Feature Arcs +--------------------- + +Simply search for name-strings to track down the arc definition, +location of the arc index, etc. + +:: + + | Arc Name | + |------------------| + | device-input | + | ethernet-output | + | interface-output | + | ip4-drop | + | ip4-local | + | ip4-multicast | + | ip4-output | + | ip4-punt | + | ip4-unicast | + | ip6-drop | + | ip6-local | + | ip6-multicast | + | ip6-output | + | ip6-punt | + | ip6-unicast | + | mpls-input | + | mpls-output | + | nsh-output | |