2 * Copyright (c) 2008, 2011, 2012, 2013 Nicira, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at:
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
28 * A 'tag' represents an arbitrary category. Currently, tags are used to
29 * represent categories of flows and in particular the value of the 64-bit
30 * "metadata" field in the flow. The universe of possible categories is very
31 * large (2**64). The number of categories in use at a given time can also be
32 * large. This means that keeping track of category membership via
33 * conventional means (lists, bitmaps, etc.) is likely to be expensive.
35 * Tags are actually implemented via a "superimposed coding", as discussed in
36 * Knuth TAOCP v.3 section 6.5 "Retrieval on Secondary Keys". A tag is an
37 * unsigned integer in which exactly 2 bits are set to 1 and the rest set to 0.
38 * For 32-bit integers (as currently used) there are 32 * 31 / 2 = 496 unique
39 * tags; for 64-bit integers there are 64 * 63 / 2 = 2,016.
41 * Because there is a small finite number of unique tags, tags must collide
42 * after some number of them have been created. In practice we generally
43 * create tags by choosing bits randomly or based on a hash function.
45 * The key property of tags is that we can combine them without increasing the
46 * amount of data required using bitwise-OR, since the result has the 1-bits
47 * from both tags set. The necessary tradeoff is that the result is even more
48 * ambiguous: if combining two tags yields a value with 4 bits set to 1, then
49 * the result value will test as having 4 * 3 / 2 = 6 unique tags, not just the
50 * two tags that we combined.
52 * The upshot is this: a value that is the bitwise-OR combination of a number
53 * of tags will always include the tags that were combined, but it may contain
54 * any number of additional tags as well. This is acceptable for our use,
55 * since we want to be sure that we check every classifier table that contains
56 * a rule with a given metadata value, but it is OK if we check a few extra
59 * If we combine too many tags, then the result will have every bit set, so
60 * that it will test as including every tag. This can happen, but we hope that
61 * this is not the common case.
64 /* Represents a tag, or the combination of 0 or more tags. */
65 typedef uint32_t tag_type;
67 #define N_TAG_BITS (CHAR_BIT * sizeof(tag_type))
68 BUILD_ASSERT_DECL(IS_POW2(N_TAG_BITS));
70 /* A 'tag_type' value that intersects every tag. */
71 #define TAG_ALL UINT32_MAX
73 /* An arbitrary tag. */
74 #define TAG_ARBITRARY UINT32_C(3)
76 tag_type tag_create_deterministic(uint32_t seed);
77 static inline bool tag_intersects(tag_type, tag_type);
79 /* Returns true if 'a' and 'b' have at least one tag in common,
80 * false if their set of tags is disjoint. */
82 tag_intersects(tag_type a, tag_type b)
85 return (x & (x - 1)) != 0;
88 /* Adding tags is easy, but subtracting is hard because you can't tell whether
89 * a bit was set only by the tag you're removing or by multiple tags. The
90 * tag_tracker data structure counts the number of tags that set each bit,
91 * which allows for efficient subtraction. */
93 unsigned int counts[N_TAG_BITS];
96 void tag_tracker_init(struct tag_tracker *);
97 void tag_tracker_add(struct tag_tracker *, tag_type *, tag_type);
98 void tag_tracker_subtract(struct tag_tracker *, tag_type *, tag_type);