处理upcall的整体框架是：

1.由函数handle_upcalls（）批量处理（in batches）的是由内核传上来的dpif_upcalls，会解析出upcall的类型。这里主要看在内核中匹配流表失败的MISS_UPCALL。处理完成后会得到多个flow_miss。

结构体dpif_upcall代表的是由内核传到用户空间的一个包，包括上传原因，packet data，以及以netlink attr形式存在的键值。

struct dpif_upcall {
    /* All types. */
    enum dpif_upcall_type type;
    struct ofpbuf *packet;      /* Packet data. */
    struct nlattr *key;         /* Flow key. */
    size_t key_len;             /* Length of ‘key‘ in bytes. */

    /* DPIF_UC_ACTION only. */
    uint64_t userdata;          /* Argument to OVS_ACTION_ATTR_USERSPACE. */
};

2. 接下来，函数handle_miss_upcalls（）会依次遍历这个flow_misses数组，完成的工作有：1）得到odp_key_fitness （也就是内核层/用户层在流匹配上的一致程度）；2）从packet data中析取出流信息miss->flow；3）然后对miss->flow进行哈希，如果不存在则插入到TO-DO-List中；4）将这个upcall->packet插入到相应的节点上。

3.然后对于TO-DO-List中的每个元素，调用handle_flow_miss（）函数，它会从这个flow_miss中构造得到flow_miss_op，具体的过程是：1）查询ofproto的facet表ofproto->facets看针对这个flow的facet是否已存在；2）从ofproto的分类表中查找与这个flow对应的分类规则，对于第一个进入系统的包，还没有建立起cls_rule，此时返回ofproto->miss_rule（是如何初始化的呢？）；3）构造一个facet，和当前的flow和rule_dpif关联起来；4）这时候与flow_miss 匹配的facet也有了，接着呼叫函数 handle_flow_miss_with_facet（）可能会增加需要的操作到flow_miss_op中，具体过程是：先是通过内核传上来的key找subfacet是否存在，如果不存在就构建一个；然后针对每个连接到这个flow_miss中的packet进行分别处理；handle_flow_miss_common（）会判断如果rule->up.cr.priority = FAIL_OPEN_PRIORITY的话就会发送一个packetin到SDN Controller；对于刚创建的subfacet，其actions为空，所以函数subfacet_make_actions（）会根据subfacet中的rule来创建datapath action，存储在odp_actions中；如果upcall的类型是DPIF_UC_MISS，就创建一个DPIF_OP_FLOW_PUT类型的flow_miss_op（即dpif_flow_put），然后compose_slow_path（）会构建一个用户空间的user_action_cookie，它的类型是USER_ACTION_COOKIE_SLOW_PATH 表示这个流得到了用户空间的处理，然后-> odp_put_userspace_action() 会增加一个OVS_ACTION_ATTR_USERSPACE action到odp_actions中，属性值包括netlink pid 和 刚才的cookie。

结构体facet是openflow flow的完全匹配（ exact-match）的实例抽象，它与"struct flow"关联，代表OVS用户空间对于exact match flow的观点，有一个或多个subfacet，每个subfacet追踪着内核层datapath对于这个exact-match flow 的观点。当内核层和用户空间对一个flow key观点一致的时候，就只有一个subfacet（通常如此）。更多理解参考[]。

struct facet {
    /* Owners. */
    struct hmap_node hmap_node;  /* In owning ofproto‘s ‘facets‘ hmap. */
    struct list list_node;       /* In owning rule‘s ‘facets‘ list. */
    struct rule_dpif *rule;      /* Owning rule. */

    /* Owned data. */
    struct list subfacets;
    long long int used;         /* Time last used; time created if not used. */

    /* Key. */
    struct flow flow;

   // 接下来是 一些统计字段；

    /* Storage for a single subfacet, to reduce malloc() time and space
     * overhead.  (A facet always has at least one subfacet and in the common
     * case has exactly one subfacet.) */
    struct subfacet one_subfacet;
};

struct rule_dpif {
    struct rule up;

    uint64_t packet_count;       /* Number of packets received. */
    uint64_t byte_count;         /* Number of bytes received. */

    tag_type tag;                /* Caches rule_calculate_tag() result. */

    struct list facets;          /* List of "struct facet"s. */
};

/* An OpenFlow flow within a "struct ofproto".
*
* With few exceptions, ofproto implementations may look at these fields but
* should not modify them. */
struct rule {
    struct list ofproto_node;    /* Owned by ofproto base code. */
    struct ofproto *ofproto;     /* The ofproto that contains this rule. */
    struct cls_rule cr;          /* In owning ofproto‘s classifier. */

    struct ofoperation *pending; /* Operation now in progress, if nonnull. */

    ovs_be64 flow_cookie;        /* Controller-issued identifier. */

    long long int created;       /* Creation time. */
    long long int modified;      /* Time of last modification. */
    long long int used;          /* Last use; time created if never used. */
    uint16_t hard_timeout;       /* In seconds from ->modified. */
    uint16_t idle_timeout;       /* In seconds from ->used. */
    uint8_t table_id;            /* Index in ofproto‘s ‘tables‘ array. */
    bool send_flow_removed;      /* Send a flow removed message? */

    /* Eviction groups. */
    bool evictable;              /* If false, prevents eviction. */
    struct heap_node evg_node;   /* In eviction_group‘s "rules" heap. */
    struct eviction_group *eviction_group; /* NULL if not in any group. */

    struct ofpact *ofpacts;      /* Sequence of "struct ofpacts". */
    unsigned int ofpacts_len;    /* Size of ‘ofpacts‘, in bytes. */

    /* Flow monitors. */
    enum nx_flow_monitor_flags monitor_flags;
    uint64_t add_seqno;         /* Sequence number when added. */
    uint64_t modify_seqno;      /* Sequence number when changed. */
};

struct subfacet {
    /* Owners. */
    struct hmap_node hmap_node; /* In struct ofproto_dpif ‘subfacets‘ list. */
    struct list list_node;      /* In struct facet‘s ‘facets‘ list. */
    struct facet *facet;        /* Owning facet. */

    /* Key.
     *
     * To save memory in the common case, ‘key‘ is NULL if ‘key_fitness‘ is
     * ODP_FIT_PERFECT, that is, odp_flow_key_from_flow() can accurately
     * regenerate the ODP flow key from ->facet->flow. */
    enum odp_key_fitness key_fitness;
    struct nlattr *key;
    int key_len;

    long long int used;         /* Time last used; time created if not used. */

    uint64_t dp_packet_count;   /* Last known packet count in the datapath. */
    uint64_t dp_byte_count;     /* Last known byte count in the datapath. */

    /* Datapath actions.
     *
     * These should be essentially identical for every subfacet in a facet, but
     * may differ in trivial ways due to VLAN splinters. */
    size_t actions_len;         /* Number of bytes in actions[]. */
    struct nlattr *actions;     /* Datapath actions. */

    enum slow_path_reason slow; /* 0 if fast path may be used. */
    enum subfacet_path path;    /* Installed in datapath? */
};

枚举体slow_path_reason 列举的是packet没有在内核层被转发的原因（也就是说这个packet是fast path）。
enum slow_path_reason {
    /* These reasons are mutually exclusive. */
    SLOW_CFM = 1 << 0,          /* CFM packets need per-packet processing. */
    SLOW_LACP = 1 << 1,         /* LACP packets need per-packet processing. */
    SLOW_STP = 1 << 2,          /* STP packets need per-packet processing. */
    SLOW_IN_BAND = 1 << 3,      /* In-band control needs every packet. */

    // 和 SLOW_CFM, SLOW_LACP, SLOW_STP互斥，可以和SLOW_IN_BAND组合。
    SLOW_CONTROLLER = 1 << 4,   /* Packets must go to OpenFlow controller. */
};

枚举体subfacet_path列举的是其可能的当前状态：1）SF_NOT_INSTALLED表示没有安装在datapath中，这种情况出现在这个subfacet构建之后，销毁之前，或者当我们在安装一个subfacet到datapath时出错。因为subfacet中对应的有action，所以这里的facet install指的是datapath执行了由用户空间下发的具体action。2）SF_FAST_PATH说明相应的action已经得到了执行，packets可以在内核层直接转发；3）SF_SLOW_PATH是流规则指定了要发往用户空间。

enum subfacet_path {
    SF_NOT_INSTALLED,           /* No datapath flow for this subfacet. */
    SF_FAST_PATH,               /* Full actions are installed. */
    SF_SLOW_PATH,               /* Send-to-userspace action is installed. */
};

4. 通过上面的操作，flow_miss_op数组就得到了，接下来调用函数 dpif_operate() 依次对dpif执行这些operation。

 for (i = 0; i < n_ops; i++) {
        struct dpif_op *op = ops[i];

        switch (op->type) {
        case DPIF_OP_FLOW_PUT:
            op->error = dpif_flow_put__(dpif, &op->u.flow_put);
            break;

        case DPIF_OP_FLOW_DEL:
            op->error = dpif_flow_del__(dpif, &op->u.flow_del);
            break;

        case DPIF_OP_EXECUTE:
            op->error = dpif_execute__(dpif, &op->u.execute);
            break;

        default:
            NOT_REACHED();

}

这里就看flow put的情况，用户空间会通过genl把相应的动作下发给内核datapath，并且接收响应。