usb-bsd: convert to QOM
[qemu.git] / block / qed-l2-cache.c
1 /*
2 * QEMU Enhanced Disk Format L2 Cache
3 *
4 * Copyright IBM, Corp. 2010
5 *
6 * Authors:
7 * Anthony Liguori <aliguori@us.ibm.com>
8 *
9 * This work is licensed under the terms of the GNU LGPL, version 2 or later.
10 * See the COPYING.LIB file in the top-level directory.
11 *
12 */
13
14 /*
15 * L2 table cache usage is as follows:
16 *
17 * An open image has one L2 table cache that is used to avoid accessing the
18 * image file for recently referenced L2 tables.
19 *
20 * Cluster offset lookup translates the logical offset within the block device
21 * to a cluster offset within the image file. This is done by indexing into
22 * the L1 and L2 tables which store cluster offsets. It is here where the L2
23 * table cache serves up recently referenced L2 tables.
24 *
25 * If there is a cache miss, that L2 table is read from the image file and
26 * committed to the cache. Subsequent accesses to that L2 table will be served
27 * from the cache until the table is evicted from the cache.
28 *
29 * L2 tables are also committed to the cache when new L2 tables are allocated
30 * in the image file. Since the L2 table cache is write-through, the new L2
31 * table is first written out to the image file and then committed to the
32 * cache.
33 *
34 * Multiple I/O requests may be using an L2 table cache entry at any given
35 * time. That means an entry may be in use across several requests and
36 * reference counting is needed to free the entry at the correct time. In
37 * particular, an entry evicted from the cache will only be freed once all
38 * references are dropped.
39 *
40 * An in-flight I/O request will hold a reference to a L2 table cache entry for
41 * the period during which it needs to access the L2 table. This includes
42 * cluster offset lookup, L2 table allocation, and L2 table update when a new
43 * data cluster has been allocated.
44 *
45 * An interesting case occurs when two requests need to access an L2 table that
46 * is not in the cache. Since the operation to read the table from the image
47 * file takes some time to complete, both requests may see a cache miss and
48 * start reading the L2 table from the image file. The first to finish will
49 * commit its L2 table into the cache. When the second tries to commit its
50 * table will be deleted in favor of the existing cache entry.
51 */
52
53 #include "trace.h"
54 #include "qed.h"
55
56 /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */
57 #define MAX_L2_CACHE_SIZE 50
58
59 /**
60 * Initialize the L2 cache
61 */
62 void qed_init_l2_cache(L2TableCache *l2_cache)
63 {
64 QTAILQ_INIT(&l2_cache->entries);
65 l2_cache->n_entries = 0;
66 }
67
68 /**
69 * Free the L2 cache
70 */
71 void qed_free_l2_cache(L2TableCache *l2_cache)
72 {
73 CachedL2Table *entry, *next_entry;
74
75 QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) {
76 qemu_vfree(entry->table);
77 g_free(entry);
78 }
79 }
80
81 /**
82 * Allocate an uninitialized entry from the cache
83 *
84 * The returned entry has a reference count of 1 and is owned by the caller.
85 * The caller must allocate the actual table field for this entry and it must
86 * be freeable using qemu_vfree().
87 */
88 CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache)
89 {
90 CachedL2Table *entry;
91
92 entry = g_malloc0(sizeof(*entry));
93 entry->ref++;
94
95 trace_qed_alloc_l2_cache_entry(l2_cache, entry);
96
97 return entry;
98 }
99
100 /**
101 * Decrease an entry's reference count and free if necessary when the reference
102 * count drops to zero.
103 */
104 void qed_unref_l2_cache_entry(CachedL2Table *entry)
105 {
106 if (!entry) {
107 return;
108 }
109
110 entry->ref--;
111 trace_qed_unref_l2_cache_entry(entry, entry->ref);
112 if (entry->ref == 0) {
113 qemu_vfree(entry->table);
114 g_free(entry);
115 }
116 }
117
118 /**
119 * Find an entry in the L2 cache. This may return NULL and it's up to the
120 * caller to satisfy the cache miss.
121 *
122 * For a cached entry, this function increases the reference count and returns
123 * the entry.
124 */
125 CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset)
126 {
127 CachedL2Table *entry;
128
129 QTAILQ_FOREACH(entry, &l2_cache->entries, node) {
130 if (entry->offset == offset) {
131 trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref);
132 entry->ref++;
133 return entry;
134 }
135 }
136 return NULL;
137 }
138
139 /**
140 * Commit an L2 cache entry into the cache. This is meant to be used as part of
141 * the process to satisfy a cache miss. A caller would allocate an entry which
142 * is not actually in the L2 cache and then once the entry was valid and
143 * present on disk, the entry can be committed into the cache.
144 *
145 * Since the cache is write-through, it's important that this function is not
146 * called until the entry is present on disk and the L1 has been updated to
147 * point to the entry.
148 *
149 * N.B. This function steals a reference to the l2_table from the caller so the
150 * caller must obtain a new reference by issuing a call to
151 * qed_find_l2_cache_entry().
152 */
153 void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table)
154 {
155 CachedL2Table *entry;
156
157 entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset);
158 if (entry) {
159 qed_unref_l2_cache_entry(entry);
160 qed_unref_l2_cache_entry(l2_table);
161 return;
162 }
163
164 if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {
165 entry = QTAILQ_FIRST(&l2_cache->entries);
166 QTAILQ_REMOVE(&l2_cache->entries, entry, node);
167 l2_cache->n_entries--;
168 qed_unref_l2_cache_entry(entry);
169 }
170
171 l2_cache->n_entries++;
172 QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node);
173 }