Archipelago » qemu

/*

 * 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 "qemu-common.h"

#include "block_int.h"

#include "module.h"

#include <zlib.h>

#include "aes.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 DEBUG_EXT

#define QCOW_MAGIC (('Q' << 24) | ('F' << 16) | ('I' << 8) | 0xfb)

#define QCOW_VERSION 2

#define QCOW_CRYPT_NONE 0

#define QCOW_CRYPT_AES  1

#define QCOW_MAX_CRYPT_CLUSTERS 32

/* 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 */

#define MIN_CLUSTER_BITS 9

#define MAX_CLUSTER_BITS 16

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 {

    uint32_t magic;

    uint32_t len;

} QCowExtension;

#define  QCOW_EXT_MAGIC_END 0

#define  QCOW_EXT_MAGIC_BACKING_FORMAT 0xE2792ACA

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);

static int check_refcounts(BlockDriverState *bs);

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;

}

/* 

 * read qcow2 extension and fill bs

 * start reading from start_offset

 * finish reading upon magic of value 0 or when end_offset reached

 * unknown magic is skipped (future extension this version knows nothing about)

 * return 0 upon success, non-0 otherwise

 */

static int qcow_read_extensions(BlockDriverState *bs, uint64_t start_offset,

                                uint64_t end_offset)

{

    BDRVQcowState *s = bs->opaque;

    QCowExtension ext;

    uint64_t offset;

#ifdef DEBUG_EXT

    printf("qcow_read_extensions: start=%ld end=%ld\n", start_offset, end_offset);

#endif

    offset = start_offset;

    while (offset < end_offset) {

#ifdef DEBUG_EXT

        /* Sanity check */

        if (offset > s->cluster_size)

            printf("qcow_handle_extension: suspicious offset %lu\n", offset);

        printf("attemting to read extended header in offset %lu\n", offset);

#endif

        if (bdrv_pread(s->hd, offset, &ext, sizeof(ext)) != sizeof(ext)) {

            fprintf(stderr, "qcow_handle_extension: ERROR: pread fail from offset %llu\n",

                    (unsigned long long)offset);

            return 1;

        }

        be32_to_cpus(&ext.magic);

        be32_to_cpus(&ext.len);

        offset += sizeof(ext);

#ifdef DEBUG_EXT

        printf("ext.magic = 0x%x\n", ext.magic);

#endif

        switch (ext.magic) {

        case QCOW_EXT_MAGIC_END:

            return 0;

        case QCOW_EXT_MAGIC_BACKING_FORMAT:

            if (ext.len >= sizeof(bs->backing_format)) {

                fprintf(stderr, "ERROR: ext_backing_format: len=%u too large"

                        " (>=%zu)\n",

                        ext.len, sizeof(bs->backing_format));

                return 2;

            }

            if (bdrv_pread(s->hd, offset , bs->backing_format,

                           ext.len) != ext.len)

                return 3;

            bs->backing_format[ext.len] = '\0';

#ifdef DEBUG_EXT

            printf("Qcow2: Got format extension %s\n", bs->backing_format);

#endif

            offset += ((ext.len + 7) & ~7);

            break;

        default:

            /* unknown magic -- just skip it */

            offset += ((ext.len + 7) & ~7);

            break;

        }

    }

    return 0;

}

static int qcow_open(BlockDriverState *bs, const char *filename, int flags)

{

    BDRVQcowState *s = bs->opaque;

    int len, i, shift, ret;

    QCowHeader header;

    uint64_t ext_end;

    /* Performance is terrible right now with cache=writethrough due mainly

     * to reference count updates.  If the user does not explicitly specify

     * a caching type, force to writeback caching.

     */

    if ((flags & BDRV_O_CACHE_DEF)) {

        flags |= BDRV_O_CACHE_WB;

        flags &= ~BDRV_O_CACHE_DEF;

    }

    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 < MIN_CLUSTER_BITS ||

        header.cluster_bits > MAX_CLUSTER_BITS)

        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 (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));

    s->cluster_cache = qemu_malloc(s->cluster_size);

    /* one more sector for decompressed data alignment */

    s->cluster_data = qemu_malloc(QCOW_MAX_CRYPT_CLUSTERS * s->cluster_size

                                  + 512);

    s->cluster_cache_offset = -1;

    if (refcount_init(bs) < 0)

        goto fail;

    /* read qcow2 extensions */

    if (header.backing_file_offset)

        ext_end = header.backing_file_offset;

    else

        ext_end = s->cluster_size;

    if (qcow_read_extensions(bs, sizeof(header), ext_end))

        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;

    uint8_t data[12];

    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);

    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 */

    cpu_to_be32w((uint32_t*)data, new_l1_size);

    cpu_to_be64w((uint64_t*)(data + 4), new_l1_table_offset);

    if (bdrv_pwrite(s->hd, offsetof(QCowHeader, l1_size), data,

                sizeof(data)) != sizeof(data))

        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;

}

/*

 * seek_l2_table

 *

 * seek l2_offset in the l2_cache table

 * if not found, return NULL,

 * if found,

 *   increments the l2 cache hit count of the entry,

 *   if counter overflow, divide by two all counters

 *   return the pointer to the l2 cache entry

 *

 */

static uint64_t *seek_l2_table(BDRVQcowState *s, uint64_t l2_offset)

{

    int i, j;

    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;

                }

            }

            return s->l2_cache + (i << s->l2_bits);

        }

    }

    return NULL;

}

/*

 * l2_load

 *

 * Loads a L2 table into memory. If the table is in the cache, the cache

 * is used; otherwise the L2 table is loaded from the image file.

 *

 * Returns a pointer to the L2 table on success, or NULL if the read from

 * the image file failed.

 */

static uint64_t *l2_load(BlockDriverState *bs, uint64_t l2_offset)

{

    BDRVQcowState *s = bs->opaque;

    int min_index;

    uint64_t *l2_table;

    /* seek if the table for the given offset is in the cache */

    l2_table = seek_l2_table(s, l2_offset);

    if (l2_table != NULL)

        return l2_table;

    /* 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 NULL;

    s->l2_cache_offsets[min_index] = l2_offset;

    s->l2_cache_counts[min_index] = 1;

    return l2_table;

}

/*

 * l2_allocate

 *

 * Allocate a new l2 entry in the file. If l1_index points to an already

 * used entry in the L2 table (i.e. we are doing a copy on write for the L2

 * table) copy the contents of the old L2 table into the newly allocated one.

 * Otherwise the new table is initialized with zeros.

 *

 */

static uint64_t *l2_allocate(BlockDriverState *bs, int l1_index)

{

    BDRVQcowState *s = bs->opaque;

    int min_index;

    uint64_t old_l2_offset, tmp;

    uint64_t *l2_table, l2_offset;

    old_l2_offset = s->l1_table[l1_index];

    /* 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 NULL;

    /* allocate a new entry in the l2 cache */

    min_index = l2_cache_new_entry(bs);

    l2_table = s->l2_cache + (min_index << s->l2_bits);

    if (old_l2_offset == 0) {

        /* if there was no old l2 table, clear the new table */

        memset(l2_table, 0, s->l2_size * sizeof(uint64_t));

    } else {

        /* if there was an old l2 table, read it from the disk */

        if (bdrv_pread(s->hd, old_l2_offset,

                       l2_table, s->l2_size * sizeof(uint64_t)) !=

            s->l2_size * sizeof(uint64_t))

            return NULL;

    }

    /* write the l2 table to the file */

    if (bdrv_pwrite(s->hd, l2_offset,

                    l2_table, s->l2_size * sizeof(uint64_t)) !=

        s->l2_size * sizeof(uint64_t))

        return NULL;

    /* update the l2 cache entry */

    s->l2_cache_offsets[min_index] = l2_offset;

    s->l2_cache_counts[min_index] = 1;

    return l2_table;

}

static int size_to_clusters(BDRVQcowState *s, int64_t size)

{

    return (size + (s->cluster_size - 1)) >> s->cluster_bits;

}

static int count_contiguous_clusters(uint64_t nb_clusters, int cluster_size,

        uint64_t *l2_table, uint64_t start, uint64_t mask)

{

    int i;

    uint64_t offset = be64_to_cpu(l2_table[0]) & ~mask;

    if (!offset)

        return 0;

    for (i = start; i < start + nb_clusters; i++)

        if (offset + i * cluster_size != (be64_to_cpu(l2_table[i]) & ~mask))

            break;

        return (i - start);

}

static int count_contiguous_free_clusters(uint64_t nb_clusters, uint64_t *l2_table)

{

    int i = 0;

    while(nb_clusters-- && l2_table[i] == 0)

        i++;

    return i;

}

/*

 * get_cluster_offset

 *

 * For a given offset of the disk image, return cluster offset in

 * qcow2 file.

 *

 * on entry, *num is the number of contiguous clusters we'd like to

 * access following offset.

 *

 * on exit, *num is the number of contiguous clusters we can read.

 *

 * Return 1, if the offset is found

 * Return 0, otherwise.

 *

 */

static uint64_t get_cluster_offset(BlockDriverState *bs,

                                   uint64_t offset, int *num)

{

    BDRVQcowState *s = bs->opaque;

    int l1_index, l2_index;

    uint64_t l2_offset, *l2_table, cluster_offset;

    int l1_bits, c;

    int index_in_cluster, nb_available, nb_needed, nb_clusters;

    index_in_cluster = (offset >> 9) & (s->cluster_sectors - 1);

    nb_needed = *num + index_in_cluster;

    l1_bits = s->l2_bits + s->cluster_bits;

    /* compute how many bytes there are between the offset and

     * the end of the l1 entry

     */

    nb_available = (1 << l1_bits) - (offset & ((1 << l1_bits) - 1));

    /* compute the number of available sectors */

    nb_available = (nb_available >> 9) + index_in_cluster;

    if (nb_needed > nb_available) {

        nb_needed = nb_available;

    }

    cluster_offset = 0;

    /* seek the the l2 offset in the l1 table */

    l1_index = offset >> l1_bits;

    if (l1_index >= s->l1_size)

        goto out;

    l2_offset = s->l1_table[l1_index];

    /* seek the l2 table of the given l2 offset */

    if (!l2_offset)

        goto out;

    /* load the l2 table in memory */

    l2_offset &= ~QCOW_OFLAG_COPIED;

    l2_table = l2_load(bs, l2_offset);

    if (l2_table == NULL)

        return 0;

    /* find the cluster offset for the given disk offset */

    l2_index = (offset >> s->cluster_bits) & (s->l2_size - 1);

    cluster_offset = be64_to_cpu(l2_table[l2_index]);

    nb_clusters = size_to_clusters(s, nb_needed << 9);

    if (!cluster_offset) {

        /* how many empty clusters ? */

        c = count_contiguous_free_clusters(nb_clusters, &l2_table[l2_index]);

    } else {

        /* how many allocated clusters ? */

        c = count_contiguous_clusters(nb_clusters, s->cluster_size,

                &l2_table[l2_index], 0, QCOW_OFLAG_COPIED);

    }

   nb_available = (c * s->cluster_sectors);

out:

    if (nb_available > nb_needed)

        nb_available = nb_needed;

    *num = nb_available - index_in_cluster;

    return cluster_offset & ~QCOW_OFLAG_COPIED;

}

/*

 * free_any_clusters

 *

 * free clusters according to its type: compressed or not

 *

 */

static void free_any_clusters(BlockDriverState *bs,

                              uint64_t cluster_offset, int nb_clusters)

{

    BDRVQcowState *s = bs->opaque;

    /* 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);

        return;

    }

    free_clusters(bs, cluster_offset, nb_clusters << s->cluster_bits);

    return;

}

/*

 * get_cluster_table

 *

 * for a given disk offset, load (and allocate if needed)

 * the l2 table.

 *

 * the l2 table offset in the qcow2 file and the cluster index

 * in the l2 table are given to the caller.

 *

 */

static int get_cluster_table(BlockDriverState *bs, uint64_t offset,

                             uint64_t **new_l2_table,

                             uint64_t *new_l2_offset,

                             int *new_l2_index)

{

    BDRVQcowState *s = bs->opaque;

    int l1_index, l2_index, ret;

    uint64_t l2_offset, *l2_table;

    /* seek the the l2 offset in the l1 table */

    l1_index = offset >> (s->l2_bits + s->cluster_bits);

    if (l1_index >= s->l1_size) {

        ret = grow_l1_table(bs, l1_index + 1);

        if (ret < 0)

            return 0;

    }

    l2_offset = s->l1_table[l1_index];

    /* seek the l2 table of the given l2 offset */

    if (l2_offset & QCOW_OFLAG_COPIED) {

        /* load the l2 table in memory */

        l2_offset &= ~QCOW_OFLAG_COPIED;

        l2_table = l2_load(bs, l2_offset);

        if (l2_table == NULL)

            return 0;

    } else {

        if (l2_offset)

            free_clusters(bs, l2_offset, s->l2_size * sizeof(uint64_t));

        l2_table = l2_allocate(bs, l1_index);

        if (l2_table == NULL)

            return 0;

        l2_offset = s->l1_table[l1_index] & ~QCOW_OFLAG_COPIED;

    }

    /* find the cluster offset for the given disk offset */

    l2_index = (offset >> s->cluster_bits) & (s->l2_size - 1);

    *new_l2_table = l2_table;

    *new_l2_offset = l2_offset;

    *new_l2_index = l2_index;

    return 1;

}