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/*

 * 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 "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);

        qemu_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 = qemu_mallocz(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.

 */

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);

        qemu_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.

 */

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().

 */

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;

    }

    if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {

        entry = QTAILQ_FIRST(&l2_cache->entries);

        QTAILQ_REMOVE(&l2_cache->entries, entry, node);

        l2_cache->n_entries--;

        qed_unref_l2_cache_entry(entry);

    }

    l2_cache->n_entries++;

    QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node);

}