Openvswitch原理与代码分析(5): 内核中的流表flow table操作

2021-11-16 00:49:12

 

当一个数据包到达网卡的时候，首先要经过内核Openvswitch.ko，流表Flow Table在内核中有一份，通过key查找内核中的flow table，即可以得到action，然后执行action之后，直接发送这个包，只有在内核无法查找到流表项的时候，才会到用户态查找用户态的流表。仅仅查找内核中flow table的情况被称为fast path.

 

Openvswitch原理与代码分析(5): 内核中的流表flow table操作

 

第一步：从数据包中提取出key

 

实现函数为int ovs_flow_key_extract(const struct ip_tunnel_info *tun_info, struct sk_buff *skb, struct sw_flow_key *key)

在这个函数中，首先提取的是物理层的信息，主要是从哪个网口进入的。

  1. key->phy.priority = skb->priority;
  2. key->phy.in_port = OVS_CB(skb)->input_vport->port_no;
  3. key->phy.skb_mark = skb->mark;
  4. ovs_ct_fill_key(skb, key);
  5. key->ovs_flow_hash = 0;
  6. key->recirc_id = 0;

 

然后调用函数static int key_extract(struct sk_buff *skb, struct sw_flow_key *key)提取其他的key

提取MAC层

  1. /* Link layer. We are guaranteed to have at least the 14 byte Ethernet
  2.  * header in the linear data area.
  3.  */
  4. eth = eth_hdr(skb);
  5. ether_addr_copy(key->eth.src, eth->h_source);
  6. ether_addr_copy(key->eth.dst, eth->h_dest);
  7. __skb_pull(skb, 2 * ETH_ALEN);
  8. /* We are going to push all headers that we pull, so no need to
  9.  * update skb->csum here.
  10.  */
  11. key->eth.tci = 0;
  12. if (skb_vlan_tag_present(skb))
  13.    key->eth.tci = htons(vlan_get_tci(skb));
  14. else
    if (eth->h_proto == htons(ETH_P_8021Q))
  15.    if (unlikely(parse_vlan(skb, key)))
  16.       return -ENOMEM;
  17. key->eth.type = parse_ethertype(skb);

 

提取网络层

  1. struct iphdr *nh;
  2. __be16 offset;
  3. error = check_iphdr(skb);
  4. if (unlikely(error)) {
  5.    memset(&key->ip, 0, sizeof(key->ip));
  6.    memset(&key->ipv4, 0, sizeof(key->ipv4));
  7.    if (error == -EINVAL) {
  8.       skb->transport_header = skb->network_header;
  9.       error = 0;
  10.    }
  11.    return error;
  12. }
  13. nh = ip_hdr(skb);
  14. key->ipv4.addr.src = nh->saddr;
  15. key->ipv4.addr.dst = nh->daddr;
  16. key->ip.proto = nh->protocol;
  17. key->ip.tos = nh->tos;
  18. key->ip.ttl = nh->ttl;
  19. offset = nh->frag_off & htons(IP_OFFSET);
  20. if (offset) {
  21.    key->ip.frag = OVS_FRAG_TYPE_LATER;
  22.    return 0;
  23. }
  24. if (nh->frag_off & htons(IP_MF) ||
  25.    skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
  26.    key->ip.frag = OVS_FRAG_TYPE_FIRST;
  27. else
  28.    key->ip.frag = OVS_FRAG_TYPE_NONE;

 

提取传输层

  1. /* Transport layer. */
  2. if (key->ip.proto == IPPROTO_TCP) {
  3.    if (tcphdr_ok(skb)) {
  4.       struct tcphdr *tcp = tcp_hdr(skb);
  5.       key->tp.src = tcp->source;
  6.       key->tp.dst = tcp->dest;
  7.       key->tp.flags = TCP_FLAGS_BE16(tcp);
  8.    } else {
  9.       memset(&key->tp, 0, sizeof(key->tp));
  10.    }
  11. } else
    if (key->ip.proto == IPPROTO_UDP) {
  12.    if (udphdr_ok(skb)) {
  13.       struct udphdr *udp = udp_hdr(skb);
  14.       key->tp.src = udp->source;
  15.       key->tp.dst = udp->dest;
  16.    } else {
  17.       memset(&key->tp, 0, sizeof(key->tp));
  18.    }
  19. } else
    if (key->ip.proto == IPPROTO_SCTP) {
  20.    if (sctphdr_ok(skb)) {
  21.       struct sctphdr *sctp = sctp_hdr(skb);
  22.       key->tp.src = sctp->source;
  23.       key->tp.dst = sctp->dest;
  24.    } else {
  25.       memset(&key->tp, 0, sizeof(key->tp));
  26.    }
  27. } else
    if (key->ip.proto == IPPROTO_ICMP) {
  28.    if (icmphdr_ok(skb)) {
  29.       struct icmphdr *icmp = icmp_hdr(skb);
  30.       /* The ICMP type and code fields use the 16-bit
  31.        * transport port fields, so we need to store
  32.        * them in 16-bit network byte order.
  33.        */
  34.       key->tp.src = htons(icmp->type);
  35.       key->tp.dst = htons(icmp->code);
  36.    } else {
  37.       memset(&key->tp, 0, sizeof(key->tp));
  38.    }
  39. }

 

第二步：根据key查找flow table

 

调用struct sw_flow *ovs_flow_tbl_lookup_stats(struct flow_table *tbl, const struct sw_flow_key *key, u32 skb_hash, u32 *n_mask_hit)进行查找。

 

Openvswitch原理与代码分析(5): 内核中的流表flow table操作

 

在内核中，flow table的数据结构如上图所示。

每个虚拟交换机对应一个datapath，每个datapath有一个flow table，每个flow table分成N个桶，根据key进行哈希，不同的key分布在不同的桶里面。

每个桶的大小是一个内存页的大小，在内存页的头部保存了保存了多少个元素，每个元素的大小。每个元素都是sw_flow，里面有key，也有action。

 

ovs_flow_tbl_lookup_stats会调用static struct sw_flow *flow_lookup(struct flow_table *tbl, struct table_instance *ti, const struct mask_array *ma, const struct sw_flow_key *key, u32 *n_mask_hit, u32 *index)

会调用masked_flow_lookup如下

  1. static
    struct sw_flow *masked_flow_lookup(struct table_instance *ti,
  2.                  const
    struct sw_flow_key *unmasked,
  3.                  const
    struct sw_flow_mask *mask,
  4.                  u32 *n_mask_hit)
  5. {
  6.    struct sw_flow *flow;
  7.    struct hlist_head *head;
  8.    u32 hash;
  9.    struct sw_flow_key masked_key;
  10.  
  11.    ovs_flow_mask_key(&masked_key, unmasked, false, mask);
  12.    hash = flow_hash(&masked_key, &mask->range);
  13.    head = find_bucket(ti, hash);
  14.    (*n_mask_hit)++;
  15.    hlist_for_each_entry_rcu(flow, head, flow_table.node[ti->node_ver]) {
  16.       if (flow->mask == mask && flow->flow_table.hash == hash &&
  17.           flow_cmp_masked_key(flow, &masked_key, &mask->range))
  18.          return flow;
  19.    }
  20.    return NULL;
  21. }

 

其中flow_hash计算哈希值，find_bucket根据哈希值查找桶，然后就是一个循环，逐个比较key是否相等，相等则返回flow。

 

第三步：执行action

 

调用int ovs_execute_actions(struct datapath *dp, struct sk_buff *skb, const struct sw_flow_actions *acts,struct sw_flow_key *key)

调用static int do_execute_actions(struct datapath *dp, struct sk_buff *skb, struct sw_flow_key *key, const struct nlattr *attr, int len)

在这个函数中，通过case语句，不同的action进行不同的操作。

  1. static
    int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
  2.                struct sw_flow_key *key,
  3.                const
    struct nlattr *attr, int len)
  4. {
  5.    /* Every output action needs a separate clone of 'skb', but the common
  6.     * case is just a single output action, so that doing a clone and
  7.     * then freeing the original skbuff is wasteful. So the following code
  8.     * is slightly obscure just to avoid that.
  9.     */
  10.    int prev_port = -1;
  11.    const
    struct nlattr *a;
  12.    int rem;
  13.  
  14.    for (a = attr, rem = len; rem > 0;
  15.         a = nla_next(a, &rem)) {
  16.       int err = 0;
  17.  
  18.       if (unlikely(prev_port != -1)) {
  19.          struct sk_buff *out_skb = skb_clone(skb, GFP_ATOMIC);
  20.  
  21.          if (out_skb)
  22.             do_output(dp, out_skb, prev_port, key);
  23.  
  24.          prev_port = -1;
  25.       }
  26.  
  27.       switch (nla_type(a)) {
  28.       case OVS_ACTION_ATTR_OUTPUT:
  29.          prev_port = nla_get_u32(a);
  30.          break;
  31.  
  32.       case OVS_ACTION_ATTR_USERSPACE:
  33.          output_userspace(dp, skb, key, a, attr, len);
  34.          break;
  35.  
  36.       case OVS_ACTION_ATTR_HASH:
  37.          execute_hash(skb, key, a);
  38.          break;
  39.  
  40.       case OVS_ACTION_ATTR_PUSH_MPLS:
  41.          err = push_mpls(skb, key, nla_data(a));
  42.          break;
  43.  
  44.       case OVS_ACTION_ATTR_POP_MPLS:
  45.          err = pop_mpls(skb, key, nla_get_be16(a));
  46.          break;
  47.  
  48.       case OVS_ACTION_ATTR_PUSH_VLAN:
  49.          err = push_vlan(skb, key, nla_data(a));
  50.          break;
  51.  
  52.       case OVS_ACTION_ATTR_POP_VLAN:
  53.          err = pop_vlan(skb, key);
  54.          break;
  55.  
  56.       case OVS_ACTION_ATTR_RECIRC:
  57.          err = execute_recirc(dp, skb, key, a, rem);
  58.          if (nla_is_last(a, rem)) {
  59.             /* If this is the last action, the skb has
  60.              * been consumed or freed.
  61.              * Return immediately.
  62.              */
  63.             return err;
  64.          }
  65.          break;
  66.  
  67.       case OVS_ACTION_ATTR_SET:
  68.          err = execute_set_action(skb, key, nla_data(a));
  69.          break;
  70.  
  71.       case OVS_ACTION_ATTR_SET_MASKED:
  72.       case OVS_ACTION_ATTR_SET_TO_MASKED:
  73.          err = execute_masked_set_action(skb, key, nla_data(a));
  74.          break;
  75.  
  76.       case OVS_ACTION_ATTR_SAMPLE:
  77.          err = sample(dp, skb, key, a, attr, len);
  78.          break;
  79.  
  80.       case OVS_ACTION_ATTR_CT:
  81.          if (!is_flow_key_valid(key)) {
  82.             err = ovs_flow_key_update(skb, key);
  83.             if (err)
  84.                return err;
  85.          }
  86.  
  87.          err = ovs_ct_execute(ovs_dp_get_net(dp), skb, key,
  88.                     nla_data(a));
  89.  
  90.          /* Hide stolen IP fragments from user space. */
  91.          if (err)
  92.             return err == -EINPROGRESS ? 0 : err;
  93.          break;
  94.       }
  95.  
  96.       if (unlikely(err)) {
  97.          kfree_skb(skb);
  98.          return err;
  99.       }
  100.    }
  101.  
  102.    if (prev_port != -1)
  103.       do_output(dp, skb, prev_port, key);
  104.    else
  105.       consume_skb(skb);
  106.  
  107.    return 0;
  108. }

 

如果可以直接输出，则调用static void do_output(struct datapath *dp, struct sk_buff *skb, int out_port, struct sw_flow_key *key)他调用void ovs_vport_send(struct vport *vport, struct sk_buff *skb)进行发送。

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