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

时间：2016-09-19 22:22:06 阅读：253 评论：0 收藏：0 [点我收藏+]

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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.

技术分享

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。

key->phy.priority = skb->priority;
key->phy.in_port = OVS_CB(skb)->input_vport->port_no;
key->phy.skb_mark = skb->mark;
ovs_ct_fill_key(skb, key);
key->ovs_flow_hash = 0;
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

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

提取网络层

struct iphdr *nh;
__be16 offset;
error = check_iphdr(skb);
if (unlikely(error)) {
???memset(&key->ip, 0, sizeof(key->ip));
???memset(&key->ipv4, 0, sizeof(key->ipv4));
???if (error == -EINVAL) {
??????skb->transport_header = skb->network_header;
??????error = 0;
???}
???return error;
}
nh = ip_hdr(skb);
key->ipv4.addr.src = nh->saddr;
key->ipv4.addr.dst = nh->daddr;
key->ip.proto = nh->protocol;
key->ip.tos = nh->tos;
key->ip.ttl = nh->ttl;
offset = nh->frag_off & htons(IP_OFFSET);
if (offset) {
???key->ip.frag = OVS_FRAG_TYPE_LATER;
???return 0;
}
if (nh->frag_off & htons(IP_MF) ||
???skb_shinfo(skb)->gso_type & SKB_GSO_UDP)
???key->ip.frag = OVS_FRAG_TYPE_FIRST;
else
???key->ip.frag = OVS_FRAG_TYPE_NONE;

提取传输层

/* Transport layer. */
if (key->ip.proto == IPPROTO_TCP) {
???if (tcphdr_ok(skb)) {
??????struct tcphdr *tcp = tcp_hdr(skb);
??????key->tp.src = tcp->source;
??????key->tp.dst = tcp->dest;
??????key->tp.flags = TCP_FLAGS_BE16(tcp);
???} else {
??????memset(&key->tp, 0, sizeof(key->tp));
???}
} else if (key->ip.proto == IPPROTO_UDP) {
???if (udphdr_ok(skb)) {
??????struct udphdr *udp = udp_hdr(skb);
??????key->tp.src = udp->source;
??????key->tp.dst = udp->dest;
???} else {
??????memset(&key->tp, 0, sizeof(key->tp));
???}
} else if (key->ip.proto == IPPROTO_SCTP) {
???if (sctphdr_ok(skb)) {
??????struct sctphdr *sctp = sctp_hdr(skb);
??????key->tp.src = sctp->source;
??????key->tp.dst = sctp->dest;
???} else {
??????memset(&key->tp, 0, sizeof(key->tp));
???}
} else if (key->ip.proto == IPPROTO_ICMP) {
???if (icmphdr_ok(skb)) {
??????struct icmphdr *icmp = icmp_hdr(skb);
??????/* The ICMP type and code fields use the 16-bit
???????* transport port fields, so we need to store
???????* them in 16-bit network byte order.
???????*/
??????key->tp.src = htons(icmp->type);
??????key->tp.dst = htons(icmp->code);
???} else {
??????memset(&key->tp, 0, sizeof(key->tp));
???}
}

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。

技术分享

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

static struct sw_flow *masked_flow_lookup(struct table_instance *ti,
?????????????????const struct sw_flow_key *unmasked,
?????????????????const struct sw_flow_mask *mask,
?????????????????u32 *n_mask_hit)
{
???struct sw_flow *flow;
???struct hlist_head *head;
???u32 hash;
???struct sw_flow_key masked_key;
?
???ovs_flow_mask_key(&masked_key, unmasked, false, mask);
???hash = flow_hash(&masked_key, &mask->range);
???head = find_bucket(ti, hash);
???(*n_mask_hit)++;
???hlist_for_each_entry_rcu(flow, head, flow_table.node[ti->node_ver]) {
??????if (flow->mask == mask && flow->flow_table.hash == hash &&
??????????flow_cmp_masked_key(flow, &masked_key, &mask->range))
?????????return flow;
???}
???return NULL;
}

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。

static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
???????????????struct sw_flow_key *key,
???????????????const struct nlattr *attr, int len)
{
???/* Every output action needs a separate clone of ‘skb‘, but the common
????* case is just a single output action, so that doing a clone and
????* then freeing the original skbuff is wasteful. So the following code
????* is slightly obscure just to avoid that.
????*/
???int prev_port = -1;
???const struct nlattr *a;
???int rem;
?
???for (a = attr, rem = len; rem > 0;
????????a = nla_next(a, &rem)) {
??????int err = 0;
?
??????if (unlikely(prev_port != -1)) {
?????????struct sk_buff *out_skb = skb_clone(skb, GFP_ATOMIC);
?
?????????if (out_skb)
????????????do_output(dp, out_skb, prev_port, key);
?
?????????prev_port = -1;
??????}
?
??????switch (nla_type(a)) {
??????case OVS_ACTION_ATTR_OUTPUT:
?????????prev_port = nla_get_u32(a);
?????????break;
?
??????case OVS_ACTION_ATTR_USERSPACE:
?????????output_userspace(dp, skb, key, a, attr, len);
?????????break;
?
??????case OVS_ACTION_ATTR_HASH:
?????????execute_hash(skb, key, a);
?????????break;
?
??????case OVS_ACTION_ATTR_PUSH_MPLS:
?????????err = push_mpls(skb, key, nla_data(a));
?????????break;
?
??????case OVS_ACTION_ATTR_POP_MPLS:
?????????err = pop_mpls(skb, key, nla_get_be16(a));
?????????break;
?
??????case OVS_ACTION_ATTR_PUSH_VLAN:
?????????err = push_vlan(skb, key, nla_data(a));
?????????break;
?
??????case OVS_ACTION_ATTR_POP_VLAN:
?????????err = pop_vlan(skb, key);
?????????break;
?
??????case OVS_ACTION_ATTR_RECIRC:
?????????err = execute_recirc(dp, skb, key, a, rem);
?????????if (nla_is_last(a, rem)) {
????????????/* If this is the last action, the skb has
?????????????* been consumed or freed.
?????????????* Return immediately.
?????????????*/
????????????return err;
?????????}
?????????break;
?
??????case OVS_ACTION_ATTR_SET:
?????????err = execute_set_action(skb, key, nla_data(a));
?????????break;
?
??????case OVS_ACTION_ATTR_SET_MASKED:
??????case OVS_ACTION_ATTR_SET_TO_MASKED:
?????????err = execute_masked_set_action(skb, key, nla_data(a));
?????????break;
?
??????case OVS_ACTION_ATTR_SAMPLE:
?????????err = sample(dp, skb, key, a, attr, len);
?????????break;
?
??????case OVS_ACTION_ATTR_CT:
?????????if (!is_flow_key_valid(key)) {
????????????err = ovs_flow_key_update(skb, key);
????????????if (err)
???????????????return err;
?????????}
?
?????????err = ovs_ct_execute(ovs_dp_get_net(dp), skb, key,
????????????????????nla_data(a));
?
?????????/* Hide stolen IP fragments from user space. */
?????????if (err)
????????????return err == -EINPROGRESS ? 0 : err;
?????????break;
??????}
?
??????if (unlikely(err)) {
?????????kfree_skb(skb);
?????????return err;
??????}
???}
?
???if (prev_port != -1)
??????do_output(dp, skb, prev_port, key);
???else
??????consume_skb(skb);
?
???return 0;
}

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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原文地址：http://www.cnblogs.com/popsuper1982/p/5886819.html

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