new qcow2 disk image format

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@2083 c046a42c-6fe2-441c-8c8c-71466251a162

1 files changed, 2236 insertions, 0 deletions

diff --git a/block-qcow2.c b/block-qcow2.c
new file mode 100644
index 000000000..f499b13be
--- /dev/null
+++ b/block-qcow2.c
@@ -0,0 +1,2236 @@
+/*
+ * Block driver for the QCOW version 2 format
+ * 
+ * Copyright (c) 2004-2006 Fabrice Bellard
+ * 
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+#include "vl.h"
+#include "block_int.h"
+#include <zlib.h>
+#include "aes.h"
+#include <assert.h>
+
+/*
+  Differences with QCOW:
+
+  - Support for multiple incremental snapshots.
+  - Memory management by reference counts.
+  - Clusters which have a reference count of one have the bit
+    QCOW_OFLAG_COPIED to optimize write performance.
+  - Size of compressed clusters is stored in sectors to reduce bit usage 
+    in the cluster offsets.
+  - Support for storing additional data (such as the VM state) in the
+    snapshots.  
+  - If a backing store is used, the cluster size is not constrained
+    (could be backported to QCOW).
+  - L2 tables have always a size of one cluster.
+*/
+
+//#define DEBUG_ALLOC
+//#define DEBUG_ALLOC2
+ 
+#define QCOW_MAGIC (('Q' << 24) | ('F' << 16) | ('I' << 8) | 0xfb)
+#define QCOW_VERSION 2
+
+#define QCOW_CRYPT_NONE 0
+#define QCOW_CRYPT_AES  1
+
+/* indicate that the refcount of the referenced cluster is exactly one. */
+#define QCOW_OFLAG_COPIED     (1LL << 63)
+/* indicate that the cluster is compressed (they never have the copied flag) */
+#define QCOW_OFLAG_COMPRESSED (1LL << 62)
+
+#define REFCOUNT_SHIFT 1 /* refcount size is 2 bytes */
+
+#ifndef offsetof
+#define offsetof(type, field) ((size_t) &((type *)0)->field)
+#endif
+
+typedef struct QCowHeader {
+    uint32_t magic;
+    uint32_t version;
+    uint64_t backing_file_offset;
+    uint32_t backing_file_size;
+    uint32_t cluster_bits;
+    uint64_t size; /* in bytes */
+    uint32_t crypt_method;
+    uint32_t l1_size; /* XXX: save number of clusters instead ? */
+    uint64_t l1_table_offset;
+    uint64_t refcount_table_offset;
+    uint32_t refcount_table_clusters;
+    uint32_t nb_snapshots;
+    uint64_t snapshots_offset;
+} QCowHeader;
+
+typedef struct __attribute__((packed)) QCowSnapshotHeader {
+    /* header is 8 byte aligned */
+    uint64_t l1_table_offset;
+
+    uint32_t l1_size;
+    uint16_t id_str_size;
+    uint16_t name_size;
+
+    uint32_t date_sec;
+    uint32_t date_nsec;
+
+    uint64_t vm_clock_nsec;
+
+    uint32_t vm_state_size;
+    uint32_t extra_data_size; /* for extension */
+    /* extra data follows */
+    /* id_str follows */
+    /* name follows  */
+} QCowSnapshotHeader;
+
+#define L2_CACHE_SIZE 16
+
+typedef struct QCowSnapshot {
+    uint64_t l1_table_offset;
+    uint32_t l1_size;
+    char *id_str;
+    char *name;
+    uint32_t vm_state_size;
+    uint32_t date_sec;
+    uint32_t date_nsec;
+    uint64_t vm_clock_nsec;
+} QCowSnapshot;
+
+typedef struct BDRVQcowState {
+    BlockDriverState *hd;
+    int cluster_bits;
+    int cluster_size;
+    int cluster_sectors;
+    int l2_bits;
+    int l2_size;
+    int l1_size;
+    int l1_vm_state_index;
+    int csize_shift;
+    int csize_mask;
+    uint64_t cluster_offset_mask;
+    uint64_t l1_table_offset;
+    uint64_t *l1_table;
+    uint64_t *l2_cache;
+    uint64_t l2_cache_offsets[L2_CACHE_SIZE];
+    uint32_t l2_cache_counts[L2_CACHE_SIZE];
+    uint8_t *cluster_cache;
+    uint8_t *cluster_data;
+    uint64_t cluster_cache_offset;
+
+    uint64_t *refcount_table;
+    uint64_t refcount_table_offset;
+    uint32_t refcount_table_size;
+    uint64_t refcount_block_cache_offset;
+    uint16_t *refcount_block_cache;
+    int64_t free_cluster_index;
+    int64_t free_byte_offset;
+
+    uint32_t crypt_method; /* current crypt method, 0 if no key yet */
+    uint32_t crypt_method_header;
+    AES_KEY aes_encrypt_key;
+    AES_KEY aes_decrypt_key;
+    uint64_t snapshots_offset;
+    int snapshots_size;
+    int nb_snapshots;
+    QCowSnapshot *snapshots;
+} BDRVQcowState;
+
+static int decompress_cluster(BDRVQcowState *s, uint64_t cluster_offset);
+static int qcow_read(BlockDriverState *bs, int64_t sector_num, 
+                     uint8_t *buf, int nb_sectors);
+static int qcow_read_snapshots(BlockDriverState *bs);
+static void qcow_free_snapshots(BlockDriverState *bs);
+static int refcount_init(BlockDriverState *bs);
+static void refcount_close(BlockDriverState *bs);
+static int get_refcount(BlockDriverState *bs, int64_t cluster_index);
+static int update_cluster_refcount(BlockDriverState *bs, 
+                                   int64_t cluster_index,
+                                   int addend);
+static void update_refcount(BlockDriverState *bs, 
+                            int64_t offset, int64_t length, 
+                            int addend);
+static int64_t alloc_clusters(BlockDriverState *bs, int64_t size);
+static int64_t alloc_bytes(BlockDriverState *bs, int size);
+static void free_clusters(BlockDriverState *bs, 
+                          int64_t offset, int64_t size);
+#ifdef DEBUG_ALLOC
+static void check_refcounts(BlockDriverState *bs);
+#endif
+
+static int qcow_probe(const uint8_t *buf, int buf_size, const char *filename)
+{
+    const QCowHeader *cow_header = (const void *)buf;
+    
+    if (buf_size >= sizeof(QCowHeader) &&
+        be32_to_cpu(cow_header->magic) == QCOW_MAGIC &&
+        be32_to_cpu(cow_header->version) == QCOW_VERSION) 
+        return 100;
+    else
+        return 0;
+}
+
+static int qcow_open(BlockDriverState *bs, const char *filename, int flags)
+{
+    BDRVQcowState *s = bs->opaque;
+    int len, i, shift, ret;
+    QCowHeader header;
+
+    ret = bdrv_file_open(&s->hd, filename, flags);
+    if (ret < 0)
+        return ret;
+    if (bdrv_pread(s->hd, 0, &header, sizeof(header)) != sizeof(header))
+        goto fail;
+    be32_to_cpus(&header.magic);
+    be32_to_cpus(&header.version);
+    be64_to_cpus(&header.backing_file_offset);
+    be32_to_cpus(&header.backing_file_size);
+    be64_to_cpus(&header.size);
+    be32_to_cpus(&header.cluster_bits);
+    be32_to_cpus(&header.crypt_method);
+    be64_to_cpus(&header.l1_table_offset);
+    be32_to_cpus(&header.l1_size);
+    be64_to_cpus(&header.refcount_table_offset);
+    be32_to_cpus(&header.refcount_table_clusters);
+    be64_to_cpus(&header.snapshots_offset);
+    be32_to_cpus(&header.nb_snapshots);
+    
+    if (header.magic != QCOW_MAGIC || header.version != QCOW_VERSION)
+        goto fail;
+    if (header.size <= 1 || 
+        header.cluster_bits < 9 || 
+        header.cluster_bits > 16)
+        goto fail;
+    if (header.crypt_method > QCOW_CRYPT_AES)
+        goto fail;
+    s->crypt_method_header = header.crypt_method;
+    if (s->crypt_method_header)
+        bs->encrypted = 1;
+    s->cluster_bits = header.cluster_bits;
+    s->cluster_size = 1 << s->cluster_bits;
+    s->cluster_sectors = 1 << (s->cluster_bits - 9);
+    s->l2_bits = s->cluster_bits - 3; /* L2 is always one cluster */
+    s->l2_size = 1 << s->l2_bits;
+    bs->total_sectors = header.size / 512;
+    s->csize_shift = (62 - (s->cluster_bits - 8));
+    s->csize_mask = (1 << (s->cluster_bits - 8)) - 1;
+    s->cluster_offset_mask = (1LL << s->csize_shift) - 1;
+    s->refcount_table_offset = header.refcount_table_offset;
+    s->refcount_table_size = 
+        header.refcount_table_clusters << (s->cluster_bits - 3);
+
+    s->snapshots_offset = header.snapshots_offset;
+    s->nb_snapshots = header.nb_snapshots;
+
+    /* read the level 1 table */
+    s->l1_size = header.l1_size;
+    shift = s->cluster_bits + s->l2_bits;
+    s->l1_vm_state_index = (header.size + (1LL << shift) - 1) >> shift;
+    /* the L1 table must contain at least enough entries to put
+       header.size bytes */
+    if (s->l1_size < s->l1_vm_state_index)
+        goto fail;
+    s->l1_table_offset = header.l1_table_offset;
+    s->l1_table = qemu_malloc(s->l1_size * sizeof(uint64_t));
+    if (!s->l1_table)
+        goto fail;
+    if (bdrv_pread(s->hd, s->l1_table_offset, s->l1_table, s->l1_size * sizeof(uint64_t)) != 
+        s->l1_size * sizeof(uint64_t))
+        goto fail;
+    for(i = 0;i < s->l1_size; i++) {
+        be64_to_cpus(&s->l1_table[i]);
+    }
+    /* alloc L2 cache */
+    s->l2_cache = qemu_malloc(s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t));
+    if (!s->l2_cache)
+        goto fail;
+    s->cluster_cache = qemu_malloc(s->cluster_size);
+    if (!s->cluster_cache)
+        goto fail;
+    /* one more sector for decompressed data alignment */
+    s->cluster_data = qemu_malloc(s->cluster_size + 512);
+    if (!s->cluster_data)
+        goto fail;
+    s->cluster_cache_offset = -1;
+    
+    if (refcount_init(bs) < 0)
+        goto fail;
+
+    /* read the backing file name */
+    if (header.backing_file_offset != 0) {
+        len = header.backing_file_size;
+        if (len > 1023)
+            len = 1023;
+        if (bdrv_pread(s->hd, header.backing_file_offset, bs->backing_file, len) != len)
+            goto fail;
+        bs->backing_file[len] = '\0';
+    }
+    if (qcow_read_snapshots(bs) < 0)
+        goto fail;
+
+#ifdef DEBUG_ALLOC
+    check_refcounts(bs);
+#endif
+    return 0;
+
+ fail:
+    qcow_free_snapshots(bs);
+    refcount_close(bs);
+    qemu_free(s->l1_table);
+    qemu_free(s->l2_cache);
+    qemu_free(s->cluster_cache);
+    qemu_free(s->cluster_data);
+    bdrv_delete(s->hd);
+    return -1;
+}
+
+static int qcow_set_key(BlockDriverState *bs, const char *key)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint8_t keybuf[16];
+    int len, i;
+    
+    memset(keybuf, 0, 16);
+    len = strlen(key);
+    if (len > 16)
+        len = 16;
+    /* XXX: we could compress the chars to 7 bits to increase
+       entropy */
+    for(i = 0;i < len;i++) {
+        keybuf[i] = key[i];
+    }
+    s->crypt_method = s->crypt_method_header;
+
+    if (AES_set_encrypt_key(keybuf, 128, &s->aes_encrypt_key) != 0)
+        return -1;
+    if (AES_set_decrypt_key(keybuf, 128, &s->aes_decrypt_key) != 0)
+        return -1;
+#if 0
+    /* test */
+    {
+        uint8_t in[16];
+        uint8_t out[16];
+        uint8_t tmp[16];
+        for(i=0;i<16;i++)
+            in[i] = i;
+        AES_encrypt(in, tmp, &s->aes_encrypt_key);
+        AES_decrypt(tmp, out, &s->aes_decrypt_key);
+        for(i = 0; i < 16; i++)
+            printf(" %02x", tmp[i]);
+        printf("\n");
+        for(i = 0; i < 16; i++)
+            printf(" %02x", out[i]);
+        printf("\n");
+    }
+#endif
+    return 0;
+}
+
+/* The crypt function is compatible with the linux cryptoloop
+   algorithm for < 4 GB images. NOTE: out_buf == in_buf is
+   supported */
+static void encrypt_sectors(BDRVQcowState *s, int64_t sector_num,
+                            uint8_t *out_buf, const uint8_t *in_buf,
+                            int nb_sectors, int enc,
+                            const AES_KEY *key)
+{
+    union {
+        uint64_t ll[2];
+        uint8_t b[16];
+    } ivec;
+    int i;
+
+    for(i = 0; i < nb_sectors; i++) {
+        ivec.ll[0] = cpu_to_le64(sector_num);
+        ivec.ll[1] = 0;
+        AES_cbc_encrypt(in_buf, out_buf, 512, key, 
+                        ivec.b, enc);
+        sector_num++;
+        in_buf += 512;
+        out_buf += 512;
+    }
+}
+
+static int copy_sectors(BlockDriverState *bs, uint64_t start_sect,
+                        uint64_t cluster_offset, int n_start, int n_end)
+{
+    BDRVQcowState *s = bs->opaque;
+    int n, ret;
+
+    n = n_end - n_start;
+    if (n <= 0)
+        return 0;
+    ret = qcow_read(bs, start_sect + n_start, s->cluster_data, n);
+    if (ret < 0)
+        return ret;
+    if (s->crypt_method) {
+        encrypt_sectors(s, start_sect + n_start, 
+                        s->cluster_data, 
+                        s->cluster_data, n, 1,
+                        &s->aes_encrypt_key);
+    }
+    ret = bdrv_write(s->hd, (cluster_offset >> 9) + n_start, 
+                     s->cluster_data, n);
+    if (ret < 0)
+        return ret;
+    return 0;
+}
+
+static void l2_cache_reset(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+
+    memset(s->l2_cache, 0, s->l2_size * L2_CACHE_SIZE * sizeof(uint64_t));
+    memset(s->l2_cache_offsets, 0, L2_CACHE_SIZE * sizeof(uint64_t));
+    memset(s->l2_cache_counts, 0, L2_CACHE_SIZE * sizeof(uint32_t));
+}
+
+static inline int l2_cache_new_entry(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint32_t min_count;
+    int min_index, i;
+
+    /* find a new entry in the least used one */
+    min_index = 0;
+    min_count = 0xffffffff;
+    for(i = 0; i < L2_CACHE_SIZE; i++) {
+        if (s->l2_cache_counts[i] < min_count) {
+            min_count = s->l2_cache_counts[i];
+            min_index = i;
+        }
+    }
+    return min_index;
+}
+
+static int64_t align_offset(int64_t offset, int n)
+{
+    offset = (offset + n - 1) & ~(n - 1);
+    return offset;
+}
+
+static int grow_l1_table(BlockDriverState *bs, int min_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_l1_size, new_l1_size2, ret, i;
+    uint64_t *new_l1_table;
+    uint64_t new_l1_table_offset;
+    uint64_t data64;
+    uint32_t data32;
+
+    new_l1_size = s->l1_size;
+    if (min_size <= new_l1_size)
+        return 0;
+    while (min_size > new_l1_size) {
+        new_l1_size = (new_l1_size * 3 + 1) / 2;
+    }
+#ifdef DEBUG_ALLOC2
+    printf("grow l1_table from %d to %d\n", s->l1_size, new_l1_size);
+#endif
+
+    new_l1_size2 = sizeof(uint64_t) * new_l1_size;
+    new_l1_table = qemu_mallocz(new_l1_size2);
+    if (!new_l1_table)
+        return -ENOMEM;
+    memcpy(new_l1_table, s->l1_table, s->l1_size * sizeof(uint64_t));
+
+    /* write new table (align to cluster) */
+    new_l1_table_offset = alloc_clusters(bs, new_l1_size2);
+    
+    for(i = 0; i < s->l1_size; i++)
+        new_l1_table[i] = cpu_to_be64(new_l1_table[i]);
+    ret = bdrv_pwrite(s->hd, new_l1_table_offset, new_l1_table, new_l1_size2);
+    if (ret != new_l1_size2)
+        goto fail;
+    for(i = 0; i < s->l1_size; i++)
+        new_l1_table[i] = be64_to_cpu(new_l1_table[i]);
+    
+    /* set new table */
+    data64 = cpu_to_be64(new_l1_table_offset);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_table_offset),
+                    &data64, sizeof(data64)) != sizeof(data64))
+        goto fail;
+    data32 = cpu_to_be32(new_l1_size);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_size),
+                    &data32, sizeof(data32)) != sizeof(data32))
+        goto fail;
+    qemu_free(s->l1_table);
+    free_clusters(bs, s->l1_table_offset, s->l1_size * sizeof(uint64_t));
+    s->l1_table_offset = new_l1_table_offset;
+    s->l1_table = new_l1_table;
+    s->l1_size = new_l1_size;
+    return 0;
+ fail:
+    qemu_free(s->l1_table);
+    return -EIO;
+}
+
+/* 'allocate' is:
+ *
+ * 0 not to allocate.
+ *
+ * 1 to allocate a normal cluster (for sector indexes 'n_start' to
+ * 'n_end')
+ *
+ * 2 to allocate a compressed cluster of size
+ * 'compressed_size'. 'compressed_size' must be > 0 and <
+ * cluster_size 
+ *
+ * return 0 if not allocated.
+ */
+static uint64_t get_cluster_offset(BlockDriverState *bs,
+                                   uint64_t offset, int allocate,
+                                   int compressed_size,
+                                   int n_start, int n_end)
+{
+    BDRVQcowState *s = bs->opaque;
+    int min_index, i, j, l1_index, l2_index, ret;
+    uint64_t l2_offset, *l2_table, cluster_offset, tmp, old_l2_offset;
+    
+    l1_index = offset >> (s->l2_bits + s->cluster_bits);
+    if (l1_index >= s->l1_size) {
+        /* outside l1 table is allowed: we grow the table if needed */
+        if (!allocate)
+            return 0;
+        if (grow_l1_table(bs, l1_index + 1) < 0)
+            return 0;
+    }
+    l2_offset = s->l1_table[l1_index];
+    if (!l2_offset) {
+        if (!allocate)
+            return 0;
+    l2_allocate:
+        old_l2_offset = l2_offset;
+        /* allocate a new l2 entry */
+        l2_offset = alloc_clusters(bs, s->l2_size * sizeof(uint64_t));
+        /* update the L1 entry */
+        s->l1_table[l1_index] = l2_offset | QCOW_OFLAG_COPIED;
+        tmp = cpu_to_be64(l2_offset | QCOW_OFLAG_COPIED);
+        if (bdrv_pwrite(s->hd, s->l1_table_offset + l1_index * sizeof(tmp), 
+                        &tmp, sizeof(tmp)) != sizeof(tmp))
+            return 0;
+        min_index = l2_cache_new_entry(bs);
+        l2_table = s->l2_cache + (min_index << s->l2_bits);
+
+        if (old_l2_offset == 0) {
+            memset(l2_table, 0, s->l2_size * sizeof(uint64_t));
+        } else {
+            if (bdrv_pread(s->hd, old_l2_offset, 
+                           l2_table, s->l2_size * sizeof(uint64_t)) !=
+                s->l2_size * sizeof(uint64_t))
+                return 0;
+        }
+        if (bdrv_pwrite(s->hd, l2_offset, 
+                        l2_table, s->l2_size * sizeof(uint64_t)) !=
+            s->l2_size * sizeof(uint64_t))
+            return 0;
+    } else {
+        if (!(l2_offset & QCOW_OFLAG_COPIED)) {
+            if (allocate) {
+                free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t));
+                goto l2_allocate;
+            }
+        } else {
+            l2_offset &= ~QCOW_OFLAG_COPIED;
+        }
+        for(i = 0; i < L2_CACHE_SIZE; i++) {
+            if (l2_offset == s->l2_cache_offsets[i]) {
+                /* increment the hit count */
+                if (++s->l2_cache_counts[i] == 0xffffffff) {
+                    for(j = 0; j < L2_CACHE_SIZE; j++) {
+                        s->l2_cache_counts[j] >>= 1;
+                    }
+                }
+                l2_table = s->l2_cache + (i << s->l2_bits);
+                goto found;
+            }
+        }
+        /* not found: load a new entry in the least used one */
+        min_index = l2_cache_new_entry(bs);
+        l2_table = s->l2_cache + (min_index << s->l2_bits);
+        if (bdrv_pread(s->hd, l2_offset, l2_table, s->l2_size * sizeof(uint64_t)) != 
+            s->l2_size * sizeof(uint64_t))
+            return 0;
+    }
+    s->l2_cache_offsets[min_index] = l2_offset;
+    s->l2_cache_counts[min_index] = 1;
+ found:
+    l2_index = (offset >> s->cluster_bits) & (s->l2_size - 1);
+    cluster_offset = be64_to_cpu(l2_table[l2_index]);
+    if (!cluster_offset) {
+        if (!allocate)
+            return cluster_offset;
+    } else if (!(cluster_offset & QCOW_OFLAG_COPIED)) {
+        if (!allocate)
+            return cluster_offset;
+        /* free the cluster */
+        if (cluster_offset & QCOW_OFLAG_COMPRESSED) {
+            int nb_csectors;
+            nb_csectors = ((cluster_offset >> s->csize_shift) & 
+                           s->csize_mask) + 1;
+            free_clusters(bs, (cluster_offset & s->cluster_offset_mask) & ~511,
+                          nb_csectors * 512);
+        } else {
+            free_clusters(bs, cluster_offset, s->cluster_size);
+        }
+    } else {
+        cluster_offset &= ~QCOW_OFLAG_COPIED;
+        return cluster_offset;
+    }
+    if (allocate == 1) {
+        /* allocate a new cluster */
+        cluster_offset = alloc_clusters(bs, s->cluster_size);
+
+        /* we must initialize the cluster content which won't be
+           written */
+        if ((n_end - n_start) < s->cluster_sectors) {
+            uint64_t start_sect;
+            
+            start_sect = (offset & ~(s->cluster_size - 1)) >> 9;
+            ret = copy_sectors(bs, start_sect,
+                               cluster_offset, 0, n_start);
+            if (ret < 0)
+                return 0;
+            ret = copy_sectors(bs, start_sect,
+                               cluster_offset, n_end, s->cluster_sectors);
+            if (ret < 0)
+                return 0;
+        }
+        tmp = cpu_to_be64(cluster_offset | QCOW_OFLAG_COPIED);
+    } else {
+        int nb_csectors;
+        cluster_offset = alloc_bytes(bs, compressed_size);
+        nb_csectors = ((cluster_offset + compressed_size - 1) >> 9) - 
+            (cluster_offset >> 9);
+        cluster_offset |= QCOW_OFLAG_COMPRESSED | 
+            ((uint64_t)nb_csectors << s->csize_shift);
+        /* compressed clusters never have the copied flag */
+        tmp = cpu_to_be64(cluster_offset);
+    }
+    /* update L2 table */
+    l2_table[l2_index] = tmp;
+    if (bdrv_pwrite(s->hd, 
+                    l2_offset + l2_index * sizeof(tmp), &tmp, sizeof(tmp)) != sizeof(tmp))
+        return 0;
+    return cluster_offset;
+}
+
+static int qcow_is_allocated(BlockDriverState *bs, int64_t sector_num, 
+                             int nb_sectors, int *pnum)
+{
+    BDRVQcowState *s = bs->opaque;
+    int index_in_cluster, n;
+    uint64_t cluster_offset;
+
+    cluster_offset = get_cluster_offset(bs, sector_num << 9, 0, 0, 0, 0);
+    index_in_cluster = sector_num & (s->cluster_sectors - 1);
+    n = s->cluster_sectors - index_in_cluster;
+    if (n > nb_sectors)
+        n = nb_sectors;
+    *pnum = n;
+    return (cluster_offset != 0);
+}
+
+static int decompress_buffer(uint8_t *out_buf, int out_buf_size,
+                             const uint8_t *buf, int buf_size)
+{
+    z_stream strm1, *strm = &strm1;
+    int ret, out_len;
+
+    memset(strm, 0, sizeof(*strm));
+
+    strm->next_in = (uint8_t *)buf;
+    strm->avail_in = buf_size;
+    strm->next_out = out_buf;
+    strm->avail_out = out_buf_size;
+
+    ret = inflateInit2(strm, -12);
+    if (ret != Z_OK)
+        return -1;
+    ret = inflate(strm, Z_FINISH);
+    out_len = strm->next_out - out_buf;
+    if ((ret != Z_STREAM_END && ret != Z_BUF_ERROR) ||
+        out_len != out_buf_size) {
+        inflateEnd(strm);
+        return -1;
+    }
+    inflateEnd(strm);
+    return 0;
+}
+                              
+static int decompress_cluster(BDRVQcowState *s, uint64_t cluster_offset)
+{
+    int ret, csize, nb_csectors, sector_offset;
+    uint64_t coffset;
+
+    coffset = cluster_offset & s->cluster_offset_mask;
+    if (s->cluster_cache_offset != coffset) {
+        nb_csectors = ((cluster_offset >> s->csize_shift) & s->csize_mask) + 1;
+        sector_offset = coffset & 511;
+        csize = nb_csectors * 512 - sector_offset;
+        ret = bdrv_read(s->hd, coffset >> 9, s->cluster_data, nb_csectors);
+        if (ret < 0) {
+            return -1;
+        }
+        if (decompress_buffer(s->cluster_cache, s->cluster_size,
+                              s->cluster_data + sector_offset, csize) < 0) {
+            return -1;
+        }
+        s->cluster_cache_offset = coffset;
+    }
+    return 0;
+}
+
+static int qcow_read(BlockDriverState *bs, int64_t sector_num, 
+                     uint8_t *buf, int nb_sectors)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, index_in_cluster, n;
+    uint64_t cluster_offset;
+    
+    while (nb_sectors > 0) {
+        cluster_offset = get_cluster_offset(bs, sector_num << 9, 0, 0, 0, 0);
+        index_in_cluster = sector_num & (s->cluster_sectors - 1);
+        n = s->cluster_sectors - index_in_cluster;
+        if (n > nb_sectors)
+            n = nb_sectors;
+        if (!cluster_offset) {
+            if (bs->backing_hd) {
+                /* read from the base image */
+                ret = bdrv_read(bs->backing_hd, sector_num, buf, n);
+                if (ret < 0)
+                    return -1;
+            } else {
+                memset(buf, 0, 512 * n);
+            }
+        } else if (cluster_offset & QCOW_OFLAG_COMPRESSED) {
+            if (decompress_cluster(s, cluster_offset) < 0)
+                return -1;
+            memcpy(buf, s->cluster_cache + index_in_cluster * 512, 512 * n);
+        } else {
+            ret = bdrv_pread(s->hd, cluster_offset + index_in_cluster * 512, buf, n * 512);
+            if (ret != n * 512) 
+                return -1;
+            if (s->crypt_method) {
+                encrypt_sectors(s, sector_num, buf, buf, n, 0, 
+                                &s->aes_decrypt_key);
+            }
+        }
+        nb_sectors -= n;
+        sector_num += n;
+        buf += n * 512;
+    }
+    return 0;
+}
+
+static int qcow_write(BlockDriverState *bs, int64_t sector_num, 
+                     const uint8_t *buf, int nb_sectors)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, index_in_cluster, n;
+    uint64_t cluster_offset;
+    
+    while (nb_sectors > 0) {
+        index_in_cluster = sector_num & (s->cluster_sectors - 1);
+        n = s->cluster_sectors - index_in_cluster;
+        if (n > nb_sectors)
+            n = nb_sectors;
+        cluster_offset = get_cluster_offset(bs, sector_num << 9, 1, 0, 
+                                            index_in_cluster, 
+                                            index_in_cluster + n);
+        if (!cluster_offset)
+            return -1;
+        if (s->crypt_method) {
+            encrypt_sectors(s, sector_num, s->cluster_data, buf, n, 1,
+                            &s->aes_encrypt_key);
+            ret = bdrv_pwrite(s->hd, cluster_offset + index_in_cluster * 512, 
+                              s->cluster_data, n * 512);
+        } else {
+            ret = bdrv_pwrite(s->hd, cluster_offset + index_in_cluster * 512, buf, n * 512);
+        }
+        if (ret != n * 512) 
+            return -1;
+        nb_sectors -= n;
+        sector_num += n;
+        buf += n * 512;
+    }
+    s->cluster_cache_offset = -1; /* disable compressed cache */
+    return 0;
+}
+
+typedef struct {
+    int64_t sector_num;
+    uint8_t *buf;
+    int nb_sectors;
+    int n;
+    uint64_t cluster_offset;
+    uint8_t *cluster_data; 
+    BlockDriverAIOCB *hd_aiocb;
+    BlockDriverAIOCB *backing_hd_aiocb;
+} QCowAIOCB;
+
+static void qcow_aio_delete(BlockDriverAIOCB *acb);
+
+static int qcow_aio_new(BlockDriverAIOCB *acb)
+{
+    BlockDriverState *bs = acb->bs;
+    BDRVQcowState *s = bs->opaque;
+    QCowAIOCB *acb1;
+    acb1 = qemu_mallocz(sizeof(QCowAIOCB));
+    if (!acb1)
+        return -1;
+    acb->opaque = acb1;
+    acb1->hd_aiocb = bdrv_aio_new(s->hd);
+    if (!acb1->hd_aiocb)
+        goto fail;
+    if (bs->backing_hd) {
+        acb1->backing_hd_aiocb = bdrv_aio_new(bs->backing_hd);
+        if (!acb1->backing_hd_aiocb)
+            goto fail;
+    }
+    return 0;
+ fail:
+    qcow_aio_delete(acb);
+    return -1;
+}
+
+static void qcow_aio_read_cb(void *opaque, int ret)
+{
+    BlockDriverAIOCB *acb = opaque;
+    BlockDriverState *bs = acb->bs;
+    BDRVQcowState *s = bs->opaque;
+    QCowAIOCB *acb1 = acb->opaque;
+    int index_in_cluster;
+
+    if (ret < 0) {
+    fail:
+        acb->cb(acb->cb_opaque, ret);
+        return;
+    }
+
+ redo:
+    /* post process the read buffer */
+    if (!acb1->cluster_offset) {
+        /* nothing to do */
+    } else if (acb1->cluster_offset & QCOW_OFLAG_COMPRESSED) {
+        /* nothing to do */
+    } else {
+        if (s->crypt_method) {
+            encrypt_sectors(s, acb1->sector_num, acb1->buf, acb1->buf, 
+                            acb1->n, 0, 
+                            &s->aes_decrypt_key);
+        }
+    }
+
+    acb1->nb_sectors -= acb1->n;
+    acb1->sector_num += acb1->n;
+    acb1->buf += acb1->n * 512;
+
+    if (acb1->nb_sectors == 0) {
+        /* request completed */
+        acb->cb(acb->cb_opaque, 0);
+        return;
+    }
+    
+    /* prepare next AIO request */
+    acb1->cluster_offset = get_cluster_offset(bs, 
+                                              acb1->sector_num << 9, 
+                                              0, 0, 0, 0);
+    index_in_cluster = acb1->sector_num & (s->cluster_sectors - 1);
+    acb1->n = s->cluster_sectors - index_in_cluster;
+    if (acb1->n > acb1->nb_sectors)
+        acb1->n = acb1->nb_sectors;
+
+    if (!acb1->cluster_offset) {
+        if (bs->backing_hd) {
+            /* read from the base image */
+            ret = bdrv_aio_read(acb1->backing_hd_aiocb, acb1->sector_num, 
+                                acb1->buf, acb1->n, qcow_aio_read_cb, acb);
+            if (ret < 0)
+                goto fail;
+        } else {
+            /* Note: in this case, no need to wait */
+            memset(acb1->buf, 0, 512 * acb1->n);
+            goto redo;
+        }
+    } else if (acb1->cluster_offset & QCOW_OFLAG_COMPRESSED) {
+        /* add AIO support for compressed blocks ? */
+        if (decompress_cluster(s, acb1->cluster_offset) < 0)
+            goto fail;
+        memcpy(acb1->buf, 
+               s->cluster_cache + index_in_cluster * 512, 512 * acb1->n);
+        goto redo;
+    } else {
+        if ((acb1->cluster_offset & 511) != 0) {
+            ret = -EIO;
+            goto fail;
+        }
+        ret = bdrv_aio_read(acb1->hd_aiocb, 
+                            (acb1->cluster_offset >> 9) + index_in_cluster, 
+                            acb1->buf, acb1->n, qcow_aio_read_cb, acb);
+        if (ret < 0)
+            goto fail;
+    }
+}
+
+static int qcow_aio_read(BlockDriverAIOCB *acb, int64_t sector_num, 
+                         uint8_t *buf, int nb_sectors)
+{
+    QCowAIOCB *acb1 = acb->opaque;
+    
+    acb1->sector_num = sector_num;
+    acb1->buf = buf;
+    acb1->nb_sectors = nb_sectors;
+    acb1->n = 0;
+    acb1->cluster_offset = 0;    
+
+    qcow_aio_read_cb(acb, 0);
+    return 0;
+}
+
+static void qcow_aio_write_cb(void *opaque, int ret)
+{
+    BlockDriverAIOCB *acb = opaque;
+    BlockDriverState *bs = acb->bs;
+    BDRVQcowState *s = bs->opaque;
+    QCowAIOCB *acb1 = acb->opaque;
+    int index_in_cluster;
+    uint64_t cluster_offset;
+    const uint8_t *src_buf;
+    
+    if (ret < 0) {
+    fail:
+        acb->cb(acb->cb_opaque, ret);
+        return;
+    }
+
+    acb1->nb_sectors -= acb1->n;
+    acb1->sector_num += acb1->n;
+    acb1->buf += acb1->n * 512;
+
+    if (acb1->nb_sectors == 0) {
+        /* request completed */
+        acb->cb(acb->cb_opaque, 0);
+        return;
+    }
+    
+    index_in_cluster = acb1->sector_num & (s->cluster_sectors - 1);
+    acb1->n = s->cluster_sectors - index_in_cluster;
+    if (acb1->n > acb1->nb_sectors)
+        acb1->n = acb1->nb_sectors;
+    cluster_offset = get_cluster_offset(bs, acb1->sector_num << 9, 1, 0, 
+                                        index_in_cluster, 
+                                        index_in_cluster + acb1->n);
+    if (!cluster_offset || (cluster_offset & 511) != 0) {
+        ret = -EIO;
+        goto fail;
+    }
+    if (s->crypt_method) {
+        if (!acb1->cluster_data) {
+            acb1->cluster_data = qemu_mallocz(s->cluster_size);
+            if (!acb1->cluster_data) {
+                ret = -ENOMEM;
+                goto fail;
+            }
+        }
+        encrypt_sectors(s, acb1->sector_num, acb1->cluster_data, acb1->buf, 
+                        acb1->n, 1, &s->aes_encrypt_key);
+        src_buf = acb1->cluster_data;
+    } else {
+        src_buf = acb1->buf;
+    }
+    ret = bdrv_aio_write(acb1->hd_aiocb, 
+                         (cluster_offset >> 9) + index_in_cluster, 
+                         src_buf, acb1->n, 
+                         qcow_aio_write_cb, acb);
+    if (ret < 0)
+        goto fail;
+}
+
+static int qcow_aio_write(BlockDriverAIOCB *acb, int64_t sector_num, 
+                          const uint8_t *buf, int nb_sectors)
+{
+    QCowAIOCB *acb1 = acb->opaque;
+    BlockDriverState *bs = acb->bs;
+    BDRVQcowState *s = bs->opaque;
+    
+    s->cluster_cache_offset = -1; /* disable compressed cache */
+
+    acb1->sector_num = sector_num;
+    acb1->buf = (uint8_t *)buf;
+    acb1->nb_sectors = nb_sectors;
+    acb1->n = 0;
+    
+    qcow_aio_write_cb(acb, 0);
+    return 0;
+}
+
+static void qcow_aio_cancel(BlockDriverAIOCB *acb)
+{
+    QCowAIOCB *acb1 = acb->opaque;
+    if (acb1->hd_aiocb)
+        bdrv_aio_cancel(acb1->hd_aiocb);
+    if (acb1->backing_hd_aiocb)
+        bdrv_aio_cancel(acb1->backing_hd_aiocb);
+}
+
+static void qcow_aio_delete(BlockDriverAIOCB *acb)
+{
+    QCowAIOCB *acb1 = acb->opaque;
+    if (acb1->hd_aiocb)
+        bdrv_aio_delete(acb1->hd_aiocb);
+    if (acb1->backing_hd_aiocb)
+        bdrv_aio_delete(acb1->backing_hd_aiocb);
+    qemu_free(acb1->cluster_data);
+    qemu_free(acb1);
+}
+
+static void qcow_close(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    qemu_free(s->l1_table);
+    qemu_free(s->l2_cache);
+    qemu_free(s->cluster_cache);
+    qemu_free(s->cluster_data);
+    refcount_close(bs);
+    bdrv_delete(s->hd);
+}
+
+/* XXX: use std qcow open function ? */
+typedef struct QCowCreateState {
+    int cluster_size;
+    int cluster_bits;
+    uint16_t *refcount_block;
+    uint64_t *refcount_table;
+    int64_t l1_table_offset;
+    int64_t refcount_table_offset;
+    int64_t refcount_block_offset;
+} QCowCreateState;
+
+static void create_refcount_update(QCowCreateState *s,
+                                   int64_t offset, int64_t size)
+{
+    int refcount;
+    int64_t start, last, cluster_offset;
+    uint16_t *p;
+
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + size - 1)  & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last; 
+        cluster_offset += s->cluster_size) {
+        p = &s->refcount_block[cluster_offset >> s->cluster_bits];
+        refcount = be16_to_cpu(*p);
+        refcount++;
+        *p = cpu_to_be16(refcount);
+    }
+}
+
+static int qcow_create(const char *filename, int64_t total_size,
+                      const char *backing_file, int flags)
+{
+    int fd, header_size, backing_filename_len, l1_size, i, shift, l2_bits;
+    QCowHeader header;
+    uint64_t tmp, offset;
+    QCowCreateState s1, *s = &s1;
+    
+    memset(s, 0, sizeof(*s));
+
+    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0644);
+    if (fd < 0)
+        return -1;
+    memset(&header, 0, sizeof(header));
+    header.magic = cpu_to_be32(QCOW_MAGIC);
+    header.version = cpu_to_be32(QCOW_VERSION);
+    header.size = cpu_to_be64(total_size * 512);
+    header_size = sizeof(header);
+    backing_filename_len = 0;
+    if (backing_file) {
+        header.backing_file_offset = cpu_to_be64(header_size);
+        backing_filename_len = strlen(backing_file);
+        header.backing_file_size = cpu_to_be32(backing_filename_len);
+        header_size += backing_filename_len;
+    }
+    s->cluster_bits = 12;  /* 4 KB clusters */
+    s->cluster_size = 1 << s->cluster_bits;
+    header.cluster_bits = cpu_to_be32(s->cluster_bits);
+    header_size = (header_size + 7) & ~7;
+    if (flags) {
+        header.crypt_method = cpu_to_be32(QCOW_CRYPT_AES);
+    } else {
+        header.crypt_method = cpu_to_be32(QCOW_CRYPT_NONE);
+    }
+    l2_bits = s->cluster_bits - 3;
+    shift = s->cluster_bits + l2_bits;
+    l1_size = (((total_size * 512) + (1LL << shift) - 1) >> shift);
+    offset = align_offset(header_size, s->cluster_size);
+    s->l1_table_offset = offset;
+    header.l1_table_offset = cpu_to_be64(s->l1_table_offset);
+    header.l1_size = cpu_to_be32(l1_size);
+    offset += align_offset(l1_size, s->cluster_size);
+
+    s->refcount_table = qemu_mallocz(s->cluster_size);
+    if (!s->refcount_table)
+        goto fail;
+    s->refcount_block = qemu_mallocz(s->cluster_size);
+    if (!s->refcount_block)
+        goto fail;
+
+    s->refcount_table_offset = offset;
+    header.refcount_table_offset = cpu_to_be64(offset);
+    header.refcount_table_clusters = cpu_to_be32(1);
+    offset += s->cluster_size;
+
+    s->refcount_table[0] = cpu_to_be64(offset);
+    s->refcount_block_offset = offset;
+    offset += s->cluster_size;
+
+    /* update refcounts */
+    create_refcount_update(s, 0, header_size);
+    create_refcount_update(s, s->l1_table_offset, l1_size);
+    create_refcount_update(s, s->refcount_table_offset, s->cluster_size);
+    create_refcount_update(s, s->refcount_block_offset, s->cluster_size);
+    
+    /* write all the data */
+    write(fd, &header, sizeof(header));
+    if (backing_file) {
+        write(fd, backing_file, backing_filename_len);
+    }
+    lseek(fd, s->l1_table_offset, SEEK_SET);
+    tmp = 0;
+    for(i = 0;i < l1_size; i++) {
+        write(fd, &tmp, sizeof(tmp));
+    }
+    lseek(fd, s->refcount_table_offset, SEEK_SET);
+    write(fd, s->refcount_table, s->cluster_size);
+    
+    lseek(fd, s->refcount_block_offset, SEEK_SET);
+    write(fd, s->refcount_block, s->cluster_size);
+
+    qemu_free(s->refcount_table);
+    qemu_free(s->refcount_block);
+    close(fd);
+    return 0;
+ fail:
+    qemu_free(s->refcount_table);
+    qemu_free(s->refcount_block);
+    close(fd);
+    return -ENOMEM;
+}
+
+static int qcow_make_empty(BlockDriverState *bs)
+{
+#if 0
+    /* XXX: not correct */
+    BDRVQcowState *s = bs->opaque;
+    uint32_t l1_length = s->l1_size * sizeof(uint64_t);
+    int ret;
+
+    memset(s->l1_table, 0, l1_length);
+    if (bdrv_pwrite(s->hd, s->l1_table_offset, s->l1_table, l1_length) < 0)
+	return -1;
+    ret = bdrv_truncate(s->hd, s->l1_table_offset + l1_length);
+    if (ret < 0)
+        return ret;
+    
+    l2_cache_reset(bs);
+#endif
+    return 0;
+}
+
+/* XXX: put compressed sectors first, then all the cluster aligned
+   tables to avoid losing bytes in alignment */
+static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num, 
+                                 const uint8_t *buf, int nb_sectors)
+{
+    BDRVQcowState *s = bs->opaque;
+    z_stream strm;
+    int ret, out_len;
+    uint8_t *out_buf;
+    uint64_t cluster_offset;
+
+    if (nb_sectors == 0) {
+        /* align end of file to a sector boundary to ease reading with
+           sector based I/Os */
+        cluster_offset = bdrv_getlength(s->hd);
+        cluster_offset = (cluster_offset + 511) & ~511;
+        bdrv_truncate(s->hd, cluster_offset);
+        return 0;
+    }
+
+    if (nb_sectors != s->cluster_sectors)
+        return -EINVAL;
+
+    out_buf = qemu_malloc(s->cluster_size + (s->cluster_size / 1000) + 128);
+    if (!out_buf)
+        return -ENOMEM;
+
+    /* best compression, small window, no zlib header */
+    memset(&strm, 0, sizeof(strm));
+    ret = deflateInit2(&strm, Z_DEFAULT_COMPRESSION,
+                       Z_DEFLATED, -12, 
+                       9, Z_DEFAULT_STRATEGY);
+    if (ret != 0) {
+        qemu_free(out_buf);
+        return -1;
+    }
+
+    strm.avail_in = s->cluster_size;
+    strm.next_in = (uint8_t *)buf;
+    strm.avail_out = s->cluster_size;
+    strm.next_out = out_buf;
+
+    ret = deflate(&strm, Z_FINISH);
+    if (ret != Z_STREAM_END && ret != Z_OK) {
+        qemu_free(out_buf);
+        deflateEnd(&strm);
+        return -1;
+    }
+    out_len = strm.next_out - out_buf;
+
+    deflateEnd(&strm);
+
+    if (ret != Z_STREAM_END || out_len >= s->cluster_size) {
+        /* could not compress: write normal cluster */
+        qcow_write(bs, sector_num, buf, s->cluster_sectors);
+    } else {
+        cluster_offset = get_cluster_offset(bs, sector_num << 9, 2, 
+                                            out_len, 0, 0);
+        cluster_offset &= s->cluster_offset_mask;
+        if (bdrv_pwrite(s->hd, cluster_offset, out_buf, out_len) != out_len) {
+            qemu_free(out_buf);
+            return -1;
+        }
+    }
+    
+    qemu_free(out_buf);
+    return 0;
+}
+
+static void qcow_flush(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    bdrv_flush(s->hd);
+}
+
+static int qcow_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
+{
+    BDRVQcowState *s = bs->opaque;
+    bdi->cluster_size = s->cluster_size;
+    bdi->vm_state_offset = (int64_t)s->l1_vm_state_index << 
+        (s->cluster_bits + s->l2_bits);
+    return 0;
+}
+
+/*********************************************************/
+/* snapshot support */
+
+/* update the refcounts of snapshots and the copied flag */
+static int update_snapshot_refcount(BlockDriverState *bs, 
+                                    int64_t l1_table_offset,
+                                    int l1_size,
+                                    int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2, l1_allocated;
+    int64_t old_offset, old_l2_offset;
+    int l2_size, i, j, l1_modified, l2_modified, nb_csectors, refcount;
+    
+    l2_cache_reset(bs);
+
+    l2_table = NULL;
+    l1_table = NULL;
+    l1_size2 = l1_size * sizeof(uint64_t);
+    l1_allocated = 0;
+    if (l1_table_offset != s->l1_table_offset) {
+        l1_table = qemu_malloc(l1_size2);
+        if (!l1_table)
+            goto fail;
+        l1_allocated = 1;
+        if (bdrv_pread(s->hd, l1_table_offset, 
+                       l1_table, l1_size2) != l1_size2)
+            goto fail;
+        for(i = 0;i < l1_size; i++)
+            be64_to_cpus(&l1_table[i]);
+    } else {
+        assert(l1_size == s->l1_size);
+        l1_table = s->l1_table;
+        l1_allocated = 0;
+    }
+    
+    l2_size = s->l2_size * sizeof(uint64_t);
+    l2_table = qemu_malloc(l2_size);
+    if (!l2_table)
+        goto fail;
+    l1_modified = 0;
+    for(i = 0; i < l1_size; i++) {
+        l2_offset = l1_table[i];
+        if (l2_offset) {
+            old_l2_offset = l2_offset;
+            l2_offset &= ~QCOW_OFLAG_COPIED;
+            l2_modified = 0;
+            if (bdrv_pread(s->hd, l2_offset, l2_table, l2_size) != l2_size)
+                goto fail;
+            for(j = 0; j < s->l2_size; j++) {
+                offset = be64_to_cpu(l2_table[j]);
+                if (offset != 0) {
+                    old_offset = offset;
+                    offset &= ~QCOW_OFLAG_COPIED;
+                    if (offset & QCOW_OFLAG_COMPRESSED) {
+                        nb_csectors = ((offset >> s->csize_shift) & 
+                                       s->csize_mask) + 1;
+                        if (addend != 0)
+                            update_refcount(bs, (offset & s->cluster_offset_mask) & ~511,
+                                            nb_csectors * 512, addend);
+                        /* compressed clusters are never modified */
+                        refcount = 2; 
+                    } else {
+                        if (addend != 0) {
+                            refcount = update_cluster_refcount(bs, offset >> s->cluster_bits, addend);
+                        } else {
+                            refcount = get_refcount(bs, offset >> s->cluster_bits);
+                        }
+                    }
+
+                    if (refcount == 1) {
+                        offset |= QCOW_OFLAG_COPIED;
+                    }
+                    if (offset != old_offset) {
+                        l2_table[j] = cpu_to_be64(offset);
+                        l2_modified = 1;
+                    }
+                }
+            }
+            if (l2_modified) {
+                if (bdrv_pwrite(s->hd, 
+                                l2_offset, l2_table, l2_size) != l2_size)
+                    goto fail;
+            }
+
+            if (addend != 0) {
+                refcount = update_cluster_refcount(bs, l2_offset >> s->cluster_bits, addend);
+            } else {
+                refcount = get_refcount(bs, l2_offset >> s->cluster_bits);
+            }
+            if (refcount == 1) {
+                l2_offset |= QCOW_OFLAG_COPIED;
+            }
+            if (l2_offset != old_l2_offset) {
+                l1_table[i] = l2_offset;
+                l1_modified = 1;
+            }
+        }
+    }
+    if (l1_modified) {
+        for(i = 0; i < l1_size; i++)
+            cpu_to_be64s(&l1_table[i]);
+        if (bdrv_pwrite(s->hd, l1_table_offset, l1_table, 
+                        l1_size2) != l1_size2)
+            goto fail;
+        for(i = 0; i < l1_size; i++)
+            be64_to_cpus(&l1_table[i]);
+    }
+    if (l1_allocated)
+        qemu_free(l1_table);
+    qemu_free(l2_table);
+    return 0;
+ fail:
+    if (l1_allocated)
+        qemu_free(l1_table);
+    qemu_free(l2_table);
+    return -EIO;
+}
+
+static void qcow_free_snapshots(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i;
+
+    for(i = 0; i < s->nb_snapshots; i++) {
+        qemu_free(s->snapshots[i].name);
+        qemu_free(s->snapshots[i].id_str);
+    }
+    qemu_free(s->snapshots);
+    s->snapshots = NULL;
+    s->nb_snapshots = 0;
+}
+
+static int qcow_read_snapshots(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshotHeader h;
+    QCowSnapshot *sn;
+    int i, id_str_size, name_size;
+    int64_t offset;
+    uint32_t extra_data_size;
+
+    offset = s->snapshots_offset;
+    s->snapshots = qemu_mallocz(s->nb_snapshots * sizeof(QCowSnapshot));
+    if (!s->snapshots)
+        goto fail;
+    for(i = 0; i < s->nb_snapshots; i++) {
+        offset = align_offset(offset, 8);
+        if (bdrv_pread(s->hd, offset, &h, sizeof(h)) != sizeof(h))
+            goto fail;
+        offset += sizeof(h);
+        sn = s->snapshots + i;
+        sn->l1_table_offset = be64_to_cpu(h.l1_table_offset);
+        sn->l1_size = be32_to_cpu(h.l1_size);
+        sn->vm_state_size = be32_to_cpu(h.vm_state_size);
+        sn->date_sec = be32_to_cpu(h.date_sec);
+        sn->date_nsec = be32_to_cpu(h.date_nsec);
+        sn->vm_clock_nsec = be64_to_cpu(h.vm_clock_nsec);
+        extra_data_size = be32_to_cpu(h.extra_data_size);
+
+        id_str_size = be16_to_cpu(h.id_str_size);
+        name_size = be16_to_cpu(h.name_size);
+
+        offset += extra_data_size;
+
+        sn->id_str = qemu_malloc(id_str_size + 1);
+        if (!sn->id_str)
+            goto fail;
+        if (bdrv_pread(s->hd, offset, sn->id_str, id_str_size) != id_str_size)
+            goto fail;
+        offset += id_str_size;
+        sn->id_str[id_str_size] = '\0';
+
+        sn->name = qemu_malloc(name_size + 1);
+        if (!sn->name)
+            goto fail;
+        if (bdrv_pread(s->hd, offset, sn->name, name_size) != name_size)
+            goto fail;
+        offset += name_size;
+        sn->name[name_size] = '\0';
+    }
+    s->snapshots_size = offset - s->snapshots_offset;
+    return 0;
+ fail:
+    qcow_free_snapshots(bs);
+    return -1;
+}
+
+/* add at the end of the file a new list of snapshots */
+static int qcow_write_snapshots(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+    QCowSnapshotHeader h;
+    int i, name_size, id_str_size, snapshots_size;
+    uint64_t data64;
+    uint32_t data32;
+    int64_t offset, snapshots_offset;
+
+    /* compute the size of the snapshots */
+    offset = 0;
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn = s->snapshots + i;
+        offset = align_offset(offset, 8);
+        offset += sizeof(h);
+        offset += strlen(sn->id_str);
+        offset += strlen(sn->name);
+    }
+    snapshots_size = offset;
+
+    snapshots_offset = alloc_clusters(bs, snapshots_size);
+    offset = snapshots_offset;
+    
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn = s->snapshots + i;
+        memset(&h, 0, sizeof(h));
+        h.l1_table_offset = cpu_to_be64(sn->l1_table_offset);
+        h.l1_size = cpu_to_be32(sn->l1_size);
+        h.vm_state_size = cpu_to_be32(sn->vm_state_size);
+        h.date_sec = cpu_to_be32(sn->date_sec);
+        h.date_nsec = cpu_to_be32(sn->date_nsec);
+        h.vm_clock_nsec = cpu_to_be64(sn->vm_clock_nsec);
+        
+        id_str_size = strlen(sn->id_str);
+        name_size = strlen(sn->name);
+        h.id_str_size = cpu_to_be16(id_str_size);
+        h.name_size = cpu_to_be16(name_size);
+        offset = align_offset(offset, 8);
+        if (bdrv_pwrite(s->hd, offset, &h, sizeof(h)) != sizeof(h))
+            goto fail;
+        offset += sizeof(h);
+        if (bdrv_pwrite(s->hd, offset, sn->id_str, id_str_size) != id_str_size)
+            goto fail;
+        offset += id_str_size;
+        if (bdrv_pwrite(s->hd, offset, sn->name, name_size) != name_size)
+            goto fail;
+        offset += name_size;
+    }
+
+    /* update the various header fields */
+    data64 = cpu_to_be64(snapshots_offset);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, snapshots_offset),
+                    &data64, sizeof(data64)) != sizeof(data64))
+        goto fail;
+    data32 = cpu_to_be32(s->nb_snapshots);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, nb_snapshots),
+                    &data32, sizeof(data32)) != sizeof(data32))
+        goto fail;
+
+    /* free the old snapshot table */
+    free_clusters(bs, s->snapshots_offset, s->snapshots_size);
+    s->snapshots_offset = snapshots_offset;
+    s->snapshots_size = snapshots_size;
+    return 0;
+ fail:
+    return -1;
+}
+
+static void find_new_snapshot_id(BlockDriverState *bs,
+                                 char *id_str, int id_str_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+    int i, id, id_max = 0;
+
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn = s->snapshots + i;
+        id = strtoul(sn->id_str, NULL, 10);
+        if (id > id_max)
+            id_max = id;
+    }
+    snprintf(id_str, id_str_size, "%d", id_max + 1);
+}
+
+static int find_snapshot_by_id(BlockDriverState *bs, const char *id_str)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i;
+
+    for(i = 0; i < s->nb_snapshots; i++) {
+        if (!strcmp(s->snapshots[i].id_str, id_str))
+            return i;
+    }
+    return -1;
+}
+
+static int find_snapshot_by_id_or_name(BlockDriverState *bs, const char *name)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i, ret;
+    
+    ret = find_snapshot_by_id(bs, name);
+    if (ret >= 0)
+        return ret;
+    for(i = 0; i < s->nb_snapshots; i++) {
+        if (!strcmp(s->snapshots[i].name, name))
+            return i;
+    }
+    return -1;
+}
+
+/* if no id is provided, a new one is constructed */
+static int qcow_snapshot_create(BlockDriverState *bs, 
+                                QEMUSnapshotInfo *sn_info)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *snapshots1, sn1, *sn = &sn1;
+    int i, ret;
+    uint64_t *l1_table = NULL;
+    
+    memset(sn, 0, sizeof(*sn));
+
+    if (sn_info->id_str[0] == '\0') {
+        /* compute a new id */
+        find_new_snapshot_id(bs, sn_info->id_str, sizeof(sn_info->id_str));
+    }
+
+    /* check that the ID is unique */
+    if (find_snapshot_by_id(bs, sn_info->id_str) >= 0)
+        return -ENOENT;
+
+    sn->id_str = qemu_strdup(sn_info->id_str);
+    if (!sn->id_str)
+        goto fail;
+    sn->name = qemu_strdup(sn_info->name);
+    if (!sn->name)
+        goto fail;
+    sn->vm_state_size = sn_info->vm_state_size;
+    sn->date_sec = sn_info->date_sec;
+    sn->date_nsec = sn_info->date_nsec;
+    sn->vm_clock_nsec = sn_info->vm_clock_nsec;
+
+    ret = update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 1);
+    if (ret < 0)
+        goto fail;
+
+    /* create the L1 table of the snapshot */
+    sn->l1_table_offset = alloc_clusters(bs, s->l1_size * sizeof(uint64_t));
+    sn->l1_size = s->l1_size;
+
+    l1_table = qemu_malloc(s->l1_size * sizeof(uint64_t));
+    if (!l1_table)
+        goto fail;
+    for(i = 0; i < s->l1_size; i++) {
+        l1_table[i] = cpu_to_be64(s->l1_table[i]);
+    }
+    if (bdrv_pwrite(s->hd, sn->l1_table_offset,
+                    l1_table, s->l1_size * sizeof(uint64_t)) != 
+        (s->l1_size * sizeof(uint64_t)))
+        goto fail;
+    qemu_free(l1_table);
+    l1_table = NULL;
+
+    snapshots1 = qemu_malloc((s->nb_snapshots + 1) * sizeof(QCowSnapshot));
+    if (!snapshots1)
+        goto fail;
+    memcpy(snapshots1, s->snapshots, s->nb_snapshots * sizeof(QCowSnapshot));
+    s->snapshots = snapshots1;
+    s->snapshots[s->nb_snapshots++] = *sn;
+
+    if (qcow_write_snapshots(bs) < 0)
+        goto fail;
+#ifdef DEBUG_ALLOC
+    check_refcounts(bs);
+#endif
+    return 0;
+ fail:
+    qemu_free(sn->name);
+    qemu_free(l1_table);
+    return -1;
+}
+
+/* copy the snapshot 'snapshot_name' into the current disk image */
+static int qcow_snapshot_goto(BlockDriverState *bs, 
+                              const char *snapshot_id)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+    int i, snapshot_index, l1_size2;
+
+    snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
+    if (snapshot_index < 0)
+        return -ENOENT;
+    sn = &s->snapshots[snapshot_index];
+
+    if (update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, -1) < 0)
+        goto fail;
+
+    if (grow_l1_table(bs, sn->l1_size) < 0)
+        goto fail;
+
+    s->l1_size = sn->l1_size;
+    l1_size2 = s->l1_size * sizeof(uint64_t);
+    /* copy the snapshot l1 table to the current l1 table */
+    if (bdrv_pread(s->hd, sn->l1_table_offset, 
+                   s->l1_table, l1_size2) != l1_size2)
+        goto fail;
+    if (bdrv_pwrite(s->hd, s->l1_table_offset,
+                    s->l1_table, l1_size2) != l1_size2)
+        goto fail;
+    for(i = 0;i < s->l1_size; i++) {
+        be64_to_cpus(&s->l1_table[i]);
+    }
+
+    if (update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 1) < 0)
+        goto fail;
+
+#ifdef DEBUG_ALLOC
+    check_refcounts(bs);
+#endif
+    return 0;
+ fail:
+    return -EIO;
+}
+
+static int qcow_snapshot_delete(BlockDriverState *bs, const char *snapshot_id)
+{
+    BDRVQcowState *s = bs->opaque;
+    QCowSnapshot *sn;
+    int snapshot_index, ret;
+    
+    snapshot_index = find_snapshot_by_id_or_name(bs, snapshot_id);
+    if (snapshot_index < 0)
+        return -ENOENT;
+    sn = &s->snapshots[snapshot_index];
+
+    ret = update_snapshot_refcount(bs, sn->l1_table_offset, sn->l1_size, -1);
+    if (ret < 0)
+        return ret;
+    /* must update the copied flag on the current cluster offsets */
+    ret = update_snapshot_refcount(bs, s->l1_table_offset, s->l1_size, 0);
+    if (ret < 0)
+        return ret;
+    free_clusters(bs, sn->l1_table_offset, sn->l1_size * sizeof(uint64_t));
+
+    qemu_free(sn->id_str);
+    qemu_free(sn->name);
+    memmove(sn, sn + 1, (s->nb_snapshots - snapshot_index - 1) * sizeof(*sn));
+    s->nb_snapshots--;
+    ret = qcow_write_snapshots(bs);
+    if (ret < 0) {
+        /* XXX: restore snapshot if error ? */
+        return ret;
+    }
+#ifdef DEBUG_ALLOC
+    check_refcounts(bs);
+#endif
+    return 0;
+}
+
+static int qcow_snapshot_list(BlockDriverState *bs, 
+                              QEMUSnapshotInfo **psn_tab)
+{
+    BDRVQcowState *s = bs->opaque;
+    QEMUSnapshotInfo *sn_tab, *sn_info;
+    QCowSnapshot *sn;
+    int i;
+
+    sn_tab = qemu_mallocz(s->nb_snapshots * sizeof(QEMUSnapshotInfo));
+    if (!sn_tab)
+        goto fail;
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn_info = sn_tab + i;
+        sn = s->snapshots + i;
+        pstrcpy(sn_info->id_str, sizeof(sn_info->id_str),
+                sn->id_str);
+        pstrcpy(sn_info->name, sizeof(sn_info->name),
+                sn->name);
+        sn_info->vm_state_size = sn->vm_state_size;
+        sn_info->date_sec = sn->date_sec;
+        sn_info->date_nsec = sn->date_nsec;
+        sn_info->vm_clock_nsec = sn->vm_clock_nsec;
+    }
+    *psn_tab = sn_tab;
+    return s->nb_snapshots;
+ fail:
+    qemu_free(sn_tab);
+    *psn_tab = NULL;
+    return -ENOMEM;
+}
+
+/*********************************************************/
+/* refcount handling */
+
+static int refcount_init(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret, refcount_table_size2, i;
+    
+    s->refcount_block_cache = qemu_malloc(s->cluster_size);
+    if (!s->refcount_block_cache)
+        goto fail;
+    refcount_table_size2 = s->refcount_table_size * sizeof(uint64_t);
+    s->refcount_table = qemu_malloc(refcount_table_size2);
+    if (!s->refcount_table)
+        goto fail;
+    if (s->refcount_table_size > 0) {
+        ret = bdrv_pread(s->hd, s->refcount_table_offset,
+                         s->refcount_table, refcount_table_size2);
+        if (ret != refcount_table_size2)
+            goto fail;
+        for(i = 0; i < s->refcount_table_size; i++)
+            be64_to_cpus(&s->refcount_table[i]);
+    }
+    return 0;
+ fail:
+    return -ENOMEM;
+}
+
+static void refcount_close(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    qemu_free(s->refcount_block_cache);
+    qemu_free(s->refcount_table);
+}
+
+
+static int load_refcount_block(BlockDriverState *bs, 
+                               int64_t refcount_block_offset)
+{
+    BDRVQcowState *s = bs->opaque;
+    int ret;
+    ret = bdrv_pread(s->hd, refcount_block_offset, s->refcount_block_cache, 
+                     s->cluster_size);
+    if (ret != s->cluster_size)
+        return -EIO;
+    s->refcount_block_cache_offset = refcount_block_offset;
+    return 0;
+}
+
+static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
+{
+    BDRVQcowState *s = bs->opaque;
+    int refcount_table_index, block_index;
+    int64_t refcount_block_offset;
+
+    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    if (refcount_table_index >= s->refcount_table_size)
+        return 0;
+    refcount_block_offset = s->refcount_table[refcount_table_index];
+    if (!refcount_block_offset)
+        return 0;
+    if (refcount_block_offset != s->refcount_block_cache_offset) {
+        /* better than nothing: return allocated if read error */
+        if (load_refcount_block(bs, refcount_block_offset) < 0)
+            return 1;
+    }
+    block_index = cluster_index & 
+        ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+    return be16_to_cpu(s->refcount_block_cache[block_index]);
+}
+
+/* return < 0 if error */
+static int64_t alloc_clusters_noref(BlockDriverState *bs, int64_t size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int i, nb_clusters;
+
+    nb_clusters = (size + s->cluster_size - 1) >> s->cluster_bits;
+    for(;;) {
+        if (get_refcount(bs, s->free_cluster_index) == 0) {
+            s->free_cluster_index++;
+            for(i = 1; i < nb_clusters; i++) {
+                if (get_refcount(bs, s->free_cluster_index) != 0)
+                    goto not_found;
+                s->free_cluster_index++;
+            }
+#ifdef DEBUG_ALLOC2
+            printf("alloc_clusters: size=%lld -> %lld\n",
+                   size, 
+                   (s->free_cluster_index - nb_clusters) << s->cluster_bits);
+#endif
+            return (s->free_cluster_index - nb_clusters) << s->cluster_bits;
+        } else {
+        not_found:
+            s->free_cluster_index++;
+        }
+    }
+}
+
+static int64_t alloc_clusters(BlockDriverState *bs, int64_t size)
+{
+    int64_t offset;
+
+    offset = alloc_clusters_noref(bs, size);
+    update_refcount(bs, offset, size, 1);
+    return offset;
+}
+
+/* only used to allocate compressed sectors. We try to allocate
+   contiguous sectors. size must be <= cluster_size */
+static int64_t alloc_bytes(BlockDriverState *bs, int size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t offset, cluster_offset;
+    int free_in_cluster;
+    
+    assert(size > 0 && size <= s->cluster_size);
+    if (s->free_byte_offset == 0) {
+        s->free_byte_offset = alloc_clusters(bs, s->cluster_size);
+    }
+ redo:
+    free_in_cluster = s->cluster_size - 
+        (s->free_byte_offset & (s->cluster_size - 1));
+    if (size <= free_in_cluster) {
+        /* enough space in current cluster */
+        offset = s->free_byte_offset;
+        s->free_byte_offset += size;
+        free_in_cluster -= size;
+        if (free_in_cluster == 0)
+            s->free_byte_offset = 0;
+        if ((offset & (s->cluster_size - 1)) != 0)
+            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
+    } else {
+        offset = alloc_clusters(bs, s->cluster_size);
+        cluster_offset = s->free_byte_offset & ~(s->cluster_size - 1);
+        if ((cluster_offset + s->cluster_size) == offset) {
+            /* we are lucky: contiguous data */
+            offset = s->free_byte_offset;
+            update_cluster_refcount(bs, offset >> s->cluster_bits, 1);
+            s->free_byte_offset += size;
+        } else {
+            s->free_byte_offset = offset;
+            goto redo;
+        }
+    }
+    return offset;
+}
+
+static void free_clusters(BlockDriverState *bs, 
+                          int64_t offset, int64_t size)
+{
+    update_refcount(bs, offset, size, -1);
+}
+
+static int grow_refcount_table(BlockDriverState *bs, int min_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int new_table_size, new_table_size2, refcount_table_clusters, i, ret;
+    uint64_t *new_table;
+    int64_t table_offset;
+    uint64_t data64;
+    uint32_t data32;
+
+    if (min_size <= s->refcount_table_size)
+        return 0;
+    /* compute new table size */
+    refcount_table_clusters = s->refcount_table_size >> (s->cluster_bits - 3);
+    for(;;) {
+        if (refcount_table_clusters == 0) {
+            refcount_table_clusters = 1;
+        } else {
+            refcount_table_clusters = (refcount_table_clusters * 3 + 1) / 2;
+        }
+        new_table_size = refcount_table_clusters << (s->cluster_bits - 3);
+        if (min_size <= new_table_size)
+            break;
+    }
+
+    new_table_size2 = new_table_size * sizeof(uint64_t);
+    new_table = qemu_mallocz(new_table_size2);
+    if (!new_table)
+        return -ENOMEM;
+    memcpy(new_table, s->refcount_table, 
+           s->refcount_table_size * sizeof(uint64_t));
+    for(i = 0; i < s->refcount_table_size; i++)
+        cpu_to_be64s(&new_table[i]);
+    /* Note: we cannot update the refcount now to avoid recursion */
+    table_offset = alloc_clusters_noref(bs, new_table_size2);
+    ret = bdrv_pwrite(s->hd, table_offset, new_table, new_table_size2);
+    if (ret != new_table_size2) 
+        goto fail;
+    for(i = 0; i < s->refcount_table_size; i++)
+        be64_to_cpus(&new_table[i]);
+
+    data64 = cpu_to_be64(table_offset);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, refcount_table_offset),
+                    &data64, sizeof(data64)) != sizeof(data64))
+        goto fail;
+    data32 = cpu_to_be32(refcount_table_clusters);
+    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, refcount_table_clusters),
+                    &data32, sizeof(data32)) != sizeof(data32))
+        goto fail;
+    qemu_free(s->refcount_table);
+    s->refcount_table = new_table;
+    s->refcount_table_size = new_table_size;
+
+    update_refcount(bs, table_offset, new_table_size2, 1);
+    return 0;
+ fail:
+    free_clusters(bs, table_offset, new_table_size2);
+    qemu_free(new_table);
+    return -EIO;
+}
+
+/* addend must be 1 or -1 */
+/* XXX: cache several refcount block clusters ? */
+static int update_cluster_refcount(BlockDriverState *bs, 
+                                   int64_t cluster_index,
+                                   int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t offset, refcount_block_offset;
+    int ret, refcount_table_index, block_index, refcount;
+    uint64_t data64;
+
+    refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
+    if (refcount_table_index >= s->refcount_table_size) {
+        if (addend < 0)
+            return -EINVAL;
+        ret = grow_refcount_table(bs, refcount_table_index + 1);
+        if (ret < 0)
+            return ret;
+    }
+    refcount_block_offset = s->refcount_table[refcount_table_index];
+    if (!refcount_block_offset) {
+        if (addend < 0)
+            return -EINVAL;
+        /* create a new refcount block */
+        /* Note: we cannot update the refcount now to avoid recursion */
+        offset = alloc_clusters_noref(bs, s->cluster_size);
+        memset(s->refcount_block_cache, 0, s->cluster_size);
+        ret = bdrv_pwrite(s->hd, offset, s->refcount_block_cache, s->cluster_size);
+        if (ret != s->cluster_size)
+            return -EINVAL;
+        s->refcount_table[refcount_table_index] = offset;
+        data64 = cpu_to_be64(offset);
+        ret = bdrv_pwrite(s->hd, s->refcount_table_offset + 
+                          refcount_table_index * sizeof(uint64_t), 
+                          &data64, sizeof(data64));
+        if (ret != sizeof(data64))
+            return -EINVAL;
+
+        refcount_block_offset = offset;
+        s->refcount_block_cache_offset = offset;
+        update_refcount(bs, offset, s->cluster_size, 1);
+    } else {
+        if (refcount_block_offset != s->refcount_block_cache_offset) {
+            if (load_refcount_block(bs, refcount_block_offset) < 0)
+                return -EIO;
+        }
+    }
+    /* we can update the count and save it */
+    block_index = cluster_index & 
+        ((1 << (s->cluster_bits - REFCOUNT_SHIFT)) - 1);
+    refcount = be16_to_cpu(s->refcount_block_cache[block_index]);
+    refcount += addend;
+    if (refcount < 0 || refcount > 0xffff)
+        return -EINVAL;
+    if (refcount == 0 && cluster_index < s->free_cluster_index) {
+        s->free_cluster_index = cluster_index;
+    }
+    s->refcount_block_cache[block_index] = cpu_to_be16(refcount);
+    if (bdrv_pwrite(s->hd, 
+                    refcount_block_offset + (block_index << REFCOUNT_SHIFT), 
+                    &s->refcount_block_cache[block_index], 2) != 2)
+        return -EIO;
+    return refcount;
+}
+
+static void update_refcount(BlockDriverState *bs, 
+                            int64_t offset, int64_t length, 
+                            int addend)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t start, last, cluster_offset;
+
+#ifdef DEBUG_ALLOC2
+    printf("update_refcount: offset=%lld size=%lld addend=%d\n", 
+           offset, length, addend);
+#endif
+    if (length <= 0)
+        return;
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + length - 1) & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last; 
+        cluster_offset += s->cluster_size) {
+        update_cluster_refcount(bs, cluster_offset >> s->cluster_bits, addend);
+    }
+}
+
+#ifdef DEBUG_ALLOC
+static void inc_refcounts(BlockDriverState *bs, 
+                          uint16_t *refcount_table, 
+                          int refcount_table_size,
+                          int64_t offset, int64_t size)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t start, last, cluster_offset;
+    int k;
+    
+    if (size <= 0)
+        return;
+
+    start = offset & ~(s->cluster_size - 1);
+    last = (offset + size - 1) & ~(s->cluster_size - 1);
+    for(cluster_offset = start; cluster_offset <= last; 
+        cluster_offset += s->cluster_size) {
+        k = cluster_offset >> s->cluster_bits;
+        if (k < 0 || k >= refcount_table_size) {
+            printf("ERROR: invalid cluster offset=0x%llx\n", cluster_offset);
+        } else {
+            if (++refcount_table[k] == 0) {
+                printf("ERROR: overflow cluster offset=0x%llx\n", cluster_offset);
+            }
+        }
+    }
+}
+
+static int check_refcounts_l1(BlockDriverState *bs, 
+                              uint16_t *refcount_table, 
+                              int refcount_table_size,
+                              int64_t l1_table_offset, int l1_size,
+                              int check_copied)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table, *l2_table, l2_offset, offset, l1_size2;
+    int l2_size, i, j, nb_csectors, refcount;
+
+    l2_table = NULL;
+    l1_size2 = l1_size * sizeof(uint64_t);
+
+    inc_refcounts(bs, refcount_table, refcount_table_size,
+                  l1_table_offset, l1_size2);
+
+    l1_table = qemu_malloc(l1_size2);
+    if (!l1_table)
+        goto fail;
+    if (bdrv_pread(s->hd, l1_table_offset, 
+                   l1_table, l1_size2) != l1_size2)
+        goto fail;
+    for(i = 0;i < l1_size; i++)
+        be64_to_cpus(&l1_table[i]);
+    
+    l2_size = s->l2_size * sizeof(uint64_t);
+    l2_table = qemu_malloc(l2_size);
+    if (!l2_table)
+        goto fail;
+    for(i = 0; i < l1_size; i++) {
+        l2_offset = l1_table[i];
+        if (l2_offset) {
+            if (check_copied) {
+                refcount = get_refcount(bs, (l2_offset & ~QCOW_OFLAG_COPIED) >> s->cluster_bits);
+                if ((refcount == 1) != ((l2_offset & QCOW_OFLAG_COPIED) != 0)) {
+                    printf("ERROR OFLAG_COPIED: l2_offset=%llx refcount=%d\n",
+                           l2_offset, refcount);
+                }
+            }
+            l2_offset &= ~QCOW_OFLAG_COPIED;
+            if (bdrv_pread(s->hd, l2_offset, l2_table, l2_size) != l2_size)
+                goto fail;
+            for(j = 0; j < s->l2_size; j++) {
+                offset = be64_to_cpu(l2_table[j]);
+                if (offset != 0) {
+                    if (offset & QCOW_OFLAG_COMPRESSED) {
+                        if (offset & QCOW_OFLAG_COPIED) {
+                            printf("ERROR: cluster %lld: copied flag must never be set for compressed clusters\n",
+                                   offset >> s->cluster_bits);
+                            offset &= ~QCOW_OFLAG_COPIED;
+                        }
+                        nb_csectors = ((offset >> s->csize_shift) & 
+                                       s->csize_mask) + 1;
+                        offset &= s->cluster_offset_mask;
+                        inc_refcounts(bs, refcount_table, 
+                                      refcount_table_size,
+                                      offset & ~511, nb_csectors * 512);
+                    } else {
+                        if (check_copied) {
+                            refcount = get_refcount(bs, (offset & ~QCOW_OFLAG_COPIED) >> s->cluster_bits);
+                            if ((refcount == 1) != ((offset & QCOW_OFLAG_COPIED) != 0)) {
+                                printf("ERROR OFLAG_COPIED: offset=%llx refcount=%d\n",
+                                       offset, refcount);
+                            }
+                        }
+                        offset &= ~QCOW_OFLAG_COPIED;
+                        inc_refcounts(bs, refcount_table, 
+                                      refcount_table_size,
+                                      offset, s->cluster_size);
+                    }
+                }
+            }
+            inc_refcounts(bs, refcount_table, 
+                          refcount_table_size,
+                          l2_offset,
+                          s->cluster_size);
+        }
+    }
+    qemu_free(l1_table);
+    qemu_free(l2_table);
+    return 0;
+ fail:
+    printf("ERROR: I/O error in check_refcounts_l1\n");
+    qemu_free(l1_table);
+    qemu_free(l2_table);
+    return -EIO;
+}
+
+static void check_refcounts(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t size;
+    int nb_clusters, refcount1, refcount2, i;
+    QCowSnapshot *sn;
+    uint16_t *refcount_table;
+
+    size = bdrv_getlength(s->hd);
+    nb_clusters = (size + s->cluster_size - 1) >> s->cluster_bits;
+    refcount_table = qemu_mallocz(nb_clusters * sizeof(uint16_t));
+    
+    /* header */
+    inc_refcounts(bs, refcount_table, nb_clusters,
+                  0, s->cluster_size);
+    
+    check_refcounts_l1(bs, refcount_table, nb_clusters,
+                       s->l1_table_offset, s->l1_size, 1);
+
+    /* snapshots */
+    for(i = 0; i < s->nb_snapshots; i++) {
+        sn = s->snapshots + i;
+        check_refcounts_l1(bs, refcount_table, nb_clusters,
+                           sn->l1_table_offset, sn->l1_size, 0);
+    }
+    inc_refcounts(bs, refcount_table, nb_clusters,
+                  s->snapshots_offset, s->snapshots_size);
+
+    /* refcount data */
+    inc_refcounts(bs, refcount_table, nb_clusters,
+                  s->refcount_table_offset, 
+                  s->refcount_table_size * sizeof(uint64_t));
+    for(i = 0; i < s->refcount_table_size; i++) {
+        int64_t offset;
+        offset = s->refcount_table[i];
+        if (offset != 0) {
+            inc_refcounts(bs, refcount_table, nb_clusters,
+                          offset, s->cluster_size);
+        }
+    }
+
+    /* compare ref counts */
+    for(i = 0; i < nb_clusters; i++) {
+        refcount1 = get_refcount(bs, i);
+        refcount2 = refcount_table[i];
+        if (refcount1 != refcount2)
+            printf("ERROR cluster %d refcount=%d reference=%d\n",
+                   i, refcount1, refcount2);
+    }
+
+    qemu_free(refcount_table);
+}
+
+#if 0
+static void dump_refcounts(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    int64_t nb_clusters, k, k1, size;
+    int refcount;
+
+    size = bdrv_getlength(s->hd);
+    nb_clusters = (size + s->cluster_size - 1) >> s->cluster_bits;
+    for(k = 0; k < nb_clusters;) {
+        k1 = k;
+        refcount = get_refcount(bs, k);
+        k++;
+        while (k < nb_clusters && get_refcount(bs, k) == refcount)
+            k++;
+        printf("%lld: refcount=%d nb=%lld\n", k, refcount, k - k1);
+    }
+}
+#endif
+#endif
+
+BlockDriver bdrv_qcow2 = {
+    "qcow2",
+    sizeof(BDRVQcowState),
+    qcow_probe,
+    qcow_open,
+    NULL,
+    NULL,
+    qcow_close,
+    qcow_create,
+    qcow_flush,
+    qcow_is_allocated,
+    qcow_set_key,
+    qcow_make_empty,
+
+    .bdrv_aio_new = qcow_aio_new,
+    .bdrv_aio_read = qcow_aio_read,
+    .bdrv_aio_write = qcow_aio_write,
+    .bdrv_aio_cancel = qcow_aio_cancel,
+    .bdrv_aio_delete = qcow_aio_delete,
+    .bdrv_write_compressed = qcow_write_compressed,
+
+    .bdrv_snapshot_create = qcow_snapshot_create,
+    .bdrv_snapshot_goto = qcow_snapshot_goto,
+    .bdrv_snapshot_delete = qcow_snapshot_delete,
+    .bdrv_snapshot_list = qcow_snapshot_list,
+    .bdrv_get_info = qcow_get_info,
+};