/* * QEMU Enhanced Disk Format L2 Cache * * Copyright IBM, Corp. 2010 * * Authors: * Anthony Liguori <aliguori@us.ibm.com> * * This work is licensed under the terms of the GNU LGPL, version 2 or later. * See the COPYING.LIB file in the top-level directory. * */ /* * L2 table cache usage is as follows: * * An open image has one L2 table cache that is used to avoid accessing the * image file for recently referenced L2 tables. * * Cluster offset lookup translates the logical offset within the block device * to a cluster offset within the image file. This is done by indexing into * the L1 and L2 tables which store cluster offsets. It is here where the L2 * table cache serves up recently referenced L2 tables. * * If there is a cache miss, that L2 table is read from the image file and * committed to the cache. Subsequent accesses to that L2 table will be served * from the cache until the table is evicted from the cache. * * L2 tables are also committed to the cache when new L2 tables are allocated * in the image file. Since the L2 table cache is write-through, the new L2 * table is first written out to the image file and then committed to the * cache. * * Multiple I/O requests may be using an L2 table cache entry at any given * time. That means an entry may be in use across several requests and * reference counting is needed to free the entry at the correct time. In * particular, an entry evicted from the cache will only be freed once all * references are dropped. * * An in-flight I/O request will hold a reference to a L2 table cache entry for * the period during which it needs to access the L2 table. This includes * cluster offset lookup, L2 table allocation, and L2 table update when a new * data cluster has been allocated. * * An interesting case occurs when two requests need to access an L2 table that * is not in the cache. Since the operation to read the table from the image * file takes some time to complete, both requests may see a cache miss and * start reading the L2 table from the image file. The first to finish will * commit its L2 table into the cache. When the second tries to commit its * table will be deleted in favor of the existing cache entry. */ #include "qemu/osdep.h" #include "trace.h" #include "qed.h" /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */ #define MAX_L2_CACHE_SIZE 50 /** * Initialize the L2 cache */ void qed_init_l2_cache(L2TableCache *l2_cache) { QTAILQ_INIT(&l2_cache->entries); l2_cache->n_entries = 0; } /** * Free the L2 cache */ void qed_free_l2_cache(L2TableCache *l2_cache) { CachedL2Table *entry, *next_entry; QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { qemu_vfree(entry->table); g_free(entry); } } /** * Allocate an uninitialized entry from the cache * * The returned entry has a reference count of 1 and is owned by the caller. * The caller must allocate the actual table field for this entry and it must * be freeable using qemu_vfree(). */ CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) { CachedL2Table *entry; entry = g_malloc0(sizeof(*entry)); entry->ref++; trace_qed_alloc_l2_cache_entry(l2_cache, entry); return entry; } /** * Decrease an entry's reference count and free if necessary when the reference * count drops to zero. * * Called with table_lock held. */ void qed_unref_l2_cache_entry(CachedL2Table *entry) { if (!entry) { return; } entry->ref--; trace_qed_unref_l2_cache_entry(entry, entry->ref); if (entry->ref == 0) { qemu_vfree(entry->table); g_free(entry); } } /** * Find an entry in the L2 cache. This may return NULL and it's up to the * caller to satisfy the cache miss. * * For a cached entry, this function increases the reference count and returns * the entry. * * Called with table_lock held. */ CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) { CachedL2Table *entry; QTAILQ_FOREACH(entry, &l2_cache->entries, node) { if (entry->offset == offset) { trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); entry->ref++; return entry; } } return NULL; } /** * Commit an L2 cache entry into the cache. This is meant to be used as part of * the process to satisfy a cache miss. A caller would allocate an entry which * is not actually in the L2 cache and then once the entry was valid and * present on disk, the entry can be committed into the cache. * * Since the cache is write-through, it's important that this function is not * called until the entry is present on disk and the L1 has been updated to * point to the entry. * * N.B. This function steals a reference to the l2_table from the caller so the * caller must obtain a new reference by issuing a call to * qed_find_l2_cache_entry(). * * Called with table_lock held. */ void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table) { CachedL2Table *entry; entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); if (entry) { qed_unref_l2_cache_entry(entry); qed_unref_l2_cache_entry(l2_table); return; } /* Evict an unused cache entry so we have space. If all entries are in use * we can grow the cache temporarily and we try to shrink back down later. */ if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) { CachedL2Table *next; QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) { if (entry->ref > 1) { continue; } QTAILQ_REMOVE(&l2_cache->entries, entry, node); l2_cache->n_entries--; qed_unref_l2_cache_entry(entry); /* Stop evicting when we've shrunk back to max size */ if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) { break; } } } l2_cache->n_entries++; QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); }