Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 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 | |
Peter Maydell | 80c71a2 | 2016-01-18 18:01:42 +0000 | [diff] [blame] | 53 | #include "qemu/osdep.h" |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 54 | #include "trace.h" |
| 55 | #include "qed.h" |
| 56 | |
| 57 | /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */ |
| 58 | #define MAX_L2_CACHE_SIZE 50 |
| 59 | |
| 60 | /** |
| 61 | * Initialize the L2 cache |
| 62 | */ |
| 63 | void qed_init_l2_cache(L2TableCache *l2_cache) |
| 64 | { |
| 65 | QTAILQ_INIT(&l2_cache->entries); |
| 66 | l2_cache->n_entries = 0; |
| 67 | } |
| 68 | |
| 69 | /** |
| 70 | * Free the L2 cache |
| 71 | */ |
| 72 | void qed_free_l2_cache(L2TableCache *l2_cache) |
| 73 | { |
| 74 | CachedL2Table *entry, *next_entry; |
| 75 | |
| 76 | QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { |
| 77 | qemu_vfree(entry->table); |
Anthony Liguori | 7267c09 | 2011-08-20 22:09:37 -0500 | [diff] [blame] | 78 | g_free(entry); |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 79 | } |
| 80 | } |
| 81 | |
| 82 | /** |
| 83 | * Allocate an uninitialized entry from the cache |
| 84 | * |
| 85 | * The returned entry has a reference count of 1 and is owned by the caller. |
| 86 | * The caller must allocate the actual table field for this entry and it must |
| 87 | * be freeable using qemu_vfree(). |
| 88 | */ |
| 89 | CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) |
| 90 | { |
| 91 | CachedL2Table *entry; |
| 92 | |
Anthony Liguori | 7267c09 | 2011-08-20 22:09:37 -0500 | [diff] [blame] | 93 | entry = g_malloc0(sizeof(*entry)); |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 94 | entry->ref++; |
| 95 | |
| 96 | trace_qed_alloc_l2_cache_entry(l2_cache, entry); |
| 97 | |
| 98 | return entry; |
| 99 | } |
| 100 | |
| 101 | /** |
| 102 | * Decrease an entry's reference count and free if necessary when the reference |
| 103 | * count drops to zero. |
Paolo Bonzini | 1f01e50 | 2017-06-29 15:27:47 +0200 | [diff] [blame] | 104 | * |
| 105 | * Called with table_lock held. |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 106 | */ |
| 107 | void qed_unref_l2_cache_entry(CachedL2Table *entry) |
| 108 | { |
| 109 | if (!entry) { |
| 110 | return; |
| 111 | } |
| 112 | |
| 113 | entry->ref--; |
| 114 | trace_qed_unref_l2_cache_entry(entry, entry->ref); |
| 115 | if (entry->ref == 0) { |
| 116 | qemu_vfree(entry->table); |
Anthony Liguori | 7267c09 | 2011-08-20 22:09:37 -0500 | [diff] [blame] | 117 | g_free(entry); |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 118 | } |
| 119 | } |
| 120 | |
| 121 | /** |
| 122 | * Find an entry in the L2 cache. This may return NULL and it's up to the |
| 123 | * caller to satisfy the cache miss. |
| 124 | * |
| 125 | * For a cached entry, this function increases the reference count and returns |
| 126 | * the entry. |
Paolo Bonzini | 1f01e50 | 2017-06-29 15:27:47 +0200 | [diff] [blame] | 127 | * |
| 128 | * Called with table_lock held. |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 129 | */ |
| 130 | CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) |
| 131 | { |
| 132 | CachedL2Table *entry; |
| 133 | |
| 134 | QTAILQ_FOREACH(entry, &l2_cache->entries, node) { |
| 135 | if (entry->offset == offset) { |
| 136 | trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); |
| 137 | entry->ref++; |
| 138 | return entry; |
| 139 | } |
| 140 | } |
| 141 | return NULL; |
| 142 | } |
| 143 | |
| 144 | /** |
| 145 | * Commit an L2 cache entry into the cache. This is meant to be used as part of |
| 146 | * the process to satisfy a cache miss. A caller would allocate an entry which |
| 147 | * is not actually in the L2 cache and then once the entry was valid and |
| 148 | * present on disk, the entry can be committed into the cache. |
| 149 | * |
| 150 | * Since the cache is write-through, it's important that this function is not |
| 151 | * called until the entry is present on disk and the L1 has been updated to |
| 152 | * point to the entry. |
| 153 | * |
| 154 | * N.B. This function steals a reference to the l2_table from the caller so the |
| 155 | * caller must obtain a new reference by issuing a call to |
| 156 | * qed_find_l2_cache_entry(). |
Paolo Bonzini | 1f01e50 | 2017-06-29 15:27:47 +0200 | [diff] [blame] | 157 | * |
| 158 | * Called with table_lock held. |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 159 | */ |
| 160 | void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table) |
| 161 | { |
| 162 | CachedL2Table *entry; |
| 163 | |
| 164 | entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); |
| 165 | if (entry) { |
| 166 | qed_unref_l2_cache_entry(entry); |
| 167 | qed_unref_l2_cache_entry(l2_table); |
| 168 | return; |
| 169 | } |
| 170 | |
Stefan Hajnoczi | 14fe292 | 2012-02-27 13:16:01 +0000 | [diff] [blame] | 171 | /* Evict an unused cache entry so we have space. If all entries are in use |
| 172 | * we can grow the cache temporarily and we try to shrink back down later. |
| 173 | */ |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 174 | if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) { |
Stefan Hajnoczi | 14fe292 | 2012-02-27 13:16:01 +0000 | [diff] [blame] | 175 | CachedL2Table *next; |
| 176 | QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) { |
| 177 | if (entry->ref > 1) { |
| 178 | continue; |
| 179 | } |
| 180 | |
| 181 | QTAILQ_REMOVE(&l2_cache->entries, entry, node); |
| 182 | l2_cache->n_entries--; |
| 183 | qed_unref_l2_cache_entry(entry); |
| 184 | |
| 185 | /* Stop evicting when we've shrunk back to max size */ |
| 186 | if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) { |
| 187 | break; |
| 188 | } |
| 189 | } |
Stefan Hajnoczi | 298800c | 2010-12-06 16:08:01 +0000 | [diff] [blame] | 190 | } |
| 191 | |
| 192 | l2_cache->n_entries++; |
| 193 | QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); |
| 194 | } |