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

2024-08-13 23:38:59   220人阅读

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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.

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第一步：从数据包中提取出key

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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;

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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);

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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 = http://www.mamicode.com/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;

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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 = http://www.mamicode.com/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 = http://www.mamicode.com/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 = http://www.mamicode.com/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 = http://www.mamicode.com/htons(icmp->type);>
  35. ??????key->tp.dst = htons(icmp->code);
  36. ???} else {
  37. ??????memset(&key->tp, 0, sizeof(key->tp));
  38. ???}
  39. }

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第二步：根据key查找flow table

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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。

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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。

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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. }

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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。

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第三步：执行action

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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. }

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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。

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


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