Classifier: Track address prefixes.

Add a prefix tree (trie) structure for tracking the used address
space, enabling skipping classifier tables containing longer masks
than necessary for an address field value in a packet header being
classified.  This enables less unwildcarding for datapath flows in
parts of the address space without host routes.

Trie lookup is interwoven to the staged lookup, so that a trie is
searched only when the configured trie field becomes relevant
for the lookup.  The trie lookup results are retained so that each
trie is checked at most once for each classifier lookup.

This implementation tracks the number of rules at each address prefix
for the whole classifier.  More aggressive table skipping would be
possible by maintaining lists of tables that have prefixes at the
lengths encountered on tree traversal, or by maintaining separate
tries for subsets of rules separated by metadata fields.

Prefix tracking is configured via OVSDB.  A new column "prefixes" is
added to the database table "Flow_Table".  "prefixes" is a set of
string values listing the field names for which prefix lookup should
be used.

As of now, the fields for which prefix lookup can be enabled are:
- tun_id, tun_src, tun_dst
- nw_src, nw_dst (or aliases ip_src and ip_dst)
- ipv6_src, ipv6_dst

There is a maximum number of fields that can be enabled for any one
flow table.  Currently this limit is 3.

Examples:

ovs-vsctl set Bridge br0 flow_tables:0=@n1 -- \
 --id=@n1 create Flow_Table name=table0
ovs-vsctl set Bridge br0 flow_tables:1=@n1 -- \
 --id=@n1 create Flow_Table name=table1

ovs-vsctl set Flow_Table table0 prefixes=ip_dst,ip_src
ovs-vsctl set Flow_Table table1 prefixes=[]

Signed-off-by: Jarno Rajahalme <[email protected]>
Acked-by: Ben Pfaff <[email protected]>

NEWS

@@ -1,5 +1,29 @@
 Post-v2.0.0
 ---------------------
+
+   - Address prefix tracking support for flow tables.  New columns
+     "prefixes" in OVS-DB table "Flow_Table" controls which packet
+     header fields are used for address prefix tracking.  Prefix
+     tracking allows the classifier to skip rules with longer than
+     necessary prefixes, resulting in better wildcarding for datapath
+     flows.  Default configuration is to not use any fields for prefix
+     tracking.  However, if any flow tables contain both exact matches
+     and masked matches for IP address fields, OVS performance may be
+     increased by using this feature.
+     * As of now, the fields for which prefix lookup can be enabled
+       are: 'tun_id', 'tun_src', 'tun_dst', 'nw_src', 'nw_dst' (or
+       aliases 'ip_src' and 'ip_dst'), 'ipv6_src', and 'ipv6_dst'.
+       (Using this feature for 'tun_id' would only make sense if the
+       tunnel IDs have prefix structure similar to IP addresses.)
+     * There is a maximum number of fields that can be enabled for any
+       one flow table.  Currently this limit is 3.
+     * Examples:
+       $ ovs-vsctl set Bridge br0 flow_tables:0=@N1 -- \
+         --id=@N1 create Flow_Table name=table0
+       $ ovs-vsctl set Bridge br0 flow_tables:1=@N1 -- \
+         --id=@N1 create Flow_Table name=table1
+       $ ovs-vsctl set Flow_Table table0 prefixes=ip_dst,ip_src
+       $ ovs-vsctl set Flow_Table table1 prefixes=[]
    - TCP flags matching: OVS now supports matching of TCP flags.  This
      has an adverse performance impact when using OVS userspace 1.10
      or older (no megaflows support) together with the new OVS kernel