+ /* Returns the Netlink PID value to supply in OVS_ACTION_ATTR_USERSPACE
+ * actions as the OVS_USERSPACE_ATTR_PID attribute's value, for use in
+ * flows whose packets arrived on port 'port_no'.
+ *
+ * The return value only needs to be meaningful when DPIF_UC_ACTION has
+ * been enabled in the 'dpif''s listen mask, and it is allowed to change
+ * when DPIF_UC_ACTION is disabled and then re-enabled.
+ *
+ * A dpif provider that doesn't have meaningful Netlink PIDs can use NULL
+ * for this function. This is equivalent to always returning 0. */
+ uint32_t (*port_get_pid)(const struct dpif *dpif, uint16_t port_no);
+