Patchwork [v3,2/3] qcow2: Implement bdrv_amend_options

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Submitter Max Reitz
Date Aug. 30, 2013, 10:27 a.m.
Message ID <1377858440-4004-3-git-send-email-mreitz@redhat.com>
Download mbox | patch
Permalink /patch/271226/
State New
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Comments

Max Reitz - Aug. 30, 2013, 10:27 a.m.
Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
and lazy_refcounts.

Downgrading images from compat=1.1 to compat=0.10 is achieved through
handling all incompatible flags accordingly, clearing all compatible and
autoclear flags and expanding all zero clusters.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cluster.c | 165 ++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c         | 194 +++++++++++++++++++++++++++++++++++++++++++++++++-
 block/qcow2.h         |   3 +
 3 files changed, 361 insertions(+), 1 deletion(-)
Fam Zheng - Sept. 2, 2013, 3:43 a.m.
On Fri, 08/30 12:27, Max Reitz wrote:
> Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
> and lazy_refcounts.
> 
> Downgrading images from compat=1.1 to compat=0.10 is achieved through
> handling all incompatible flags accordingly, clearing all compatible and
> autoclear flags and expanding all zero clusters.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-cluster.c | 165 ++++++++++++++++++++++++++++++++++++++++++
>  block/qcow2.c         | 194 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  block/qcow2.h         |   3 +
>  3 files changed, 361 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> index cca76d4..e0ca104 100644
> --- a/block/qcow2-cluster.c
> +++ b/block/qcow2-cluster.c
> @@ -1476,3 +1476,168 @@ fail:
>  
>      return ret;
>  }
> +
> +/*
> + * Expands all zero clusters in a specific L1 table (or deallocates them, for
> + * non-backed non-pre-allocated zero clusters).
> + */
> +static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
> +                                      int l1_size)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    bool is_active_l1 = (l1_table == s->l1_table);
> +    uint64_t *l2_table;
> +    int ret;
> +    int i, j;
> +
> +    if (!is_active_l1) {
> +        /* inactive L2 tables require a buffer to be stored in when loading
> +         * them from disk */
> +        l2_table = qemu_blockalign(bs, s->cluster_size);
> +    }
> +
> +    for (i = 0; i < l1_size; i++) {
> +        uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
> +        bool l2_dirty = false;
> +
> +        if (!l2_offset) {
> +            /* unallocated */
> +            continue;
> +        }
> +
> +        if (is_active_l1) {
> +            /* get active L2 tables from cache */
> +            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
> +                    (void **)&l2_table);
> +        } else {
> +            /* load inactive L2 tables from disk */
> +            ret = bdrv_read(bs->file, l2_offset / BDRV_SECTOR_SIZE,
> +                    (void *)l2_table, s->cluster_sectors);
> +        }
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +
> +        for (j = 0; j < s->l2_size; j++) {
> +            uint64_t l2_entry = be64_to_cpu(l2_table[j]);
> +            int64_t offset;
> +
> +            if (qcow2_get_cluster_type(l2_entry) != QCOW2_CLUSTER_ZERO) {
> +                continue;
> +            }
> +
> +            offset = l2_entry & L2E_OFFSET_MASK;
> +            if (!offset) {
> +                /* not preallocated */
> +                if (!bs->backing_hd) {
> +                    /* not backed; therefore we can simply deallocate the
> +                     * cluster */
> +                    l2_table[j] = 0;
> +                    l2_dirty = true;
> +                    continue;
> +                }
> +
> +                offset = qcow2_alloc_clusters(bs, s->cluster_size);
> +                if (offset < 0) {
> +                    ret = offset;
> +                    goto fail;
> +                }
> +            }
> +
> +            ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
> +                                    s->cluster_sectors);
> +            if (ret < 0) {
> +                qcow2_free_clusters(bs, offset, s->cluster_size,
> +                        QCOW2_DISCARD_ALWAYS);
> +                goto fail;
> +            }
> +
> +            l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
> +            l2_dirty = true;
> +        }
> +
> +        if (is_active_l1) {
> +            if (l2_dirty) {
> +                qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
> +                qcow2_cache_depends_on_flush(s->l2_table_cache);
> +            }
> +            ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
> +            if (ret < 0) {
> +                l2_table = NULL;
> +                goto fail;
> +            }
> +        } else {
> +            if (l2_dirty) {
> +                ret = bdrv_write(bs->file, l2_offset / BDRV_SECTOR_SIZE,
> +                        (void *)l2_table, s->cluster_sectors);
> +                if (ret < 0) {
> +                    goto fail;
> +                }
> +            }
> +        }
> +    }
> +
> +    ret = 0;
> +
> +fail:
> +    if (l2_table) {
> +        if (!is_active_l1) {
> +            qemu_vfree(l2_table);
> +        } else {
> +            if (ret < 0) {
> +                qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
> +            } else {
> +                ret = qcow2_cache_put(bs, s->l2_table_cache,
> +                        (void **)&l2_table);
> +            }
> +        }
> +    }
> +    return ret;
> +}
> +
> +/*
> + * For backed images, expands all zero clusters on the image. For non-backed
> + * images, deallocates all non-pre-allocated zero clusters (and claims the
> + * allocation for pre-allocated ones). This is important for downgrading to a
> + * qcow2 version which doesn't yet support metadata zero clusters.
> + */
> +int qcow2_expand_zero_clusters(BlockDriverState *bs)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    uint64_t *l1_table = NULL;
> +    int ret;
> +    int i, j;
> +
> +    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size);
> +    if (ret < 0) {
> +        goto fail;
> +    }
> +
> +    for (i = 0; i < s->nb_snapshots; i++) {
> +        int l1_sectors = (s->snapshots[i].l1_size * sizeof(uint64_t) +
> +                BDRV_SECTOR_SIZE - 1) / BDRV_SECTOR_SIZE;
> +
> +        l1_table = g_realloc(l1_table, l1_sectors * BDRV_SECTOR_SIZE);
> +
> +        ret = bdrv_read(bs->file, s->snapshots[i].l1_table_offset /
> +                BDRV_SECTOR_SIZE, (void *)l1_table, l1_sectors);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +
> +        for (j = 0; j < s->snapshots[i].l1_size; j++) {
> +            be64_to_cpus(&l1_table[j]);
> +        }
> +
> +        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size);
> +        if (ret < 0) {
> +            goto fail;
> +        }
> +    }
> +
> +    ret = 0;
> +
> +fail:
> +    g_free(l1_table);
> +    return ret;
> +}
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 78097e5..a8eaf45 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -409,6 +409,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
>          ret = -ENOTSUP;
>          goto fail;
>      }
> +    s->refcount_order = header.refcount_order;
>  
>      if (header.cluster_bits < MIN_CLUSTER_BITS ||
>          header.cluster_bits > MAX_CLUSTER_BITS) {
> @@ -1076,7 +1077,7 @@ int qcow2_update_header(BlockDriverState *bs)
>          .incompatible_features  = cpu_to_be64(s->incompatible_features),
>          .compatible_features    = cpu_to_be64(s->compatible_features),
>          .autoclear_features     = cpu_to_be64(s->autoclear_features),
> -        .refcount_order         = cpu_to_be32(3 + REFCOUNT_SHIFT),
> +        .refcount_order         = cpu_to_be32(s->refcount_order),
>          .header_length          = cpu_to_be32(header_length),
>      };
>  
> @@ -1735,6 +1736,196 @@ static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
>      return ret;
>  }
>  
> +/*
> + * Downgrades an image's version. To achieve this, any incompatible features
> + * have to be removed.
> + */
> +static int qcow2_downgrade(BlockDriverState *bs, int target_version)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int current_version = s->qcow_version;
> +    int ret;
> +
> +    if (target_version == current_version) {
> +        return 0;
> +    } else if (target_version > current_version) {
> +        return -EINVAL;
> +    } else if (target_version != 2) {
> +        return -EINVAL;
> +    }
> +
> +    if (s->refcount_order != 4) {
> +        /* we would have to convert the image to a refcount_order == 4 image
> +         * here; however, since qemu (at the time of writing this) does not
> +         * support anything different than 4 anyway, there is no point in doing
> +         * so right now; however, we should error out (if qemu supports this in
> +         * the future and this code has not been adapted) */
> +        error_report("qcow2_downgrade: Image refcount orders other than 4 are"
> +                     "currently not supported.");
> +        return -ENOTSUP;
> +    }
> +
> +    /* clear incompatible features */
> +    if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
> +        ret = qcow2_mark_clean(bs);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    if (s->incompatible_features) {
> +        return -ENOTSUP;
> +    }
> +
> +    /* since we can ignore compatible features, we can set them to 0 as well */
> +    s->compatible_features = 0;
> +    /* if lazy refcounts have been used, they have already been fixed through
> +     * clearing the dirty flag */
> +
> +    /* clearing autoclear features is trivial */
> +    s->autoclear_features = 0;
> +
> +    ret = qcow2_expand_zero_clusters(bs);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +
> +    s->qcow_version = target_version;
> +    ret = qcow2_update_header(bs);
> +    if (ret < 0) {
> +        s->qcow_version = current_version;
> +        return ret;
> +    }
> +    return 0;
> +}
> +
> +static int qcow2_amend_options(BlockDriverState *bs,
> +                               QEMUOptionParameter *options)
> +{
> +    BDRVQcowState *s = bs->opaque;
> +    int old_version = s->qcow_version, new_version = old_version;
> +    uint64_t new_size = 0;
> +    const char *backing_file = NULL, *backing_format = NULL;
> +    bool lazy_refcounts = s->use_lazy_refcounts;
> +    int ret;
> +    int i;
> +
> +    for (i = 0; options[i].name; i++)
> +    {
> +        if (!strcmp(options[i].name, "compat")) {
> +            if (!options[i].value.s) {
> +                /* preserve default */
> +            } else if (!strcmp(options[i].value.s, "0.10")) {
> +                new_version = 2;
> +            } else if (!strcmp(options[i].value.s, "1.1")) {
> +                new_version = 3;
> +            } else {
> +                fprintf(stderr, "Unknown compatibility level %s.\n",
> +                        options[i].value.s);
> +                return -EINVAL;
> +            }
> +        } else if (!strcmp(options[i].name, "preallocation")) {
> +            if (options[i].assigned) {

For encryption flag and cluster_size, you checked the original value and only
error out on actual change, should check the original preallocation value here
as well?

> +                fprintf(stderr, "Cannot change preallocation mode.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "size")) {
> +            new_size = options[i].value.n;
> +        } else if (!strcmp(options[i].name, "backing_file")) {
> +            backing_file = options[i].value.s;
> +        } else if (!strcmp(options[i].name, "backing_fmt")) {
> +            backing_format = options[i].value.s;
> +        } else if (!strcmp(options[i].name, "encryption")) {
> +            if (options[i].assigned &&
> +                (options[i].value.n != !!s->crypt_method)) {
> +                fprintf(stderr, "Changing the encryption flag is not "
> +                        "supported.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "cluster_size")) {
> +            if (options[i].assigned && (options[i].value.n != s->cluster_size))
> +            {
> +                fprintf(stderr, "Changing the cluster size is not "
> +                        "supported.\n");
> +                return -ENOTSUP;
> +            }
> +        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
> +            if (options[i].assigned) {
> +                lazy_refcounts = options[i].value.n;
> +            }
> +        } else {
> +            /* if this assertion fails, this probably means a new option was
> +             * added without having it covered here */
> +            assert(false);

A unknown option reported as -ENOTSUP with a proper message is good enough,
it's not that critical for an assert.

> +        }
> +    }
> +
> +    if (new_version != old_version) {
> +        if (new_version > old_version) {
> +            /* Upgrade */
> +            s->qcow_version = new_version;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->qcow_version = old_version;
> +                return ret;
> +            }
> +        } else {
> +            ret = qcow2_downgrade(bs, new_version);
> +            if (ret < 0) {
> +                return ret;
> +            }
> +        }
> +    }
> +
> +    if (new_size) {
> +        ret = qcow2_truncate(bs, new_size);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    if (backing_file || backing_format) {
> +        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
> +                                        backing_format ?: bs->backing_format);
> +        if (ret < 0) {
> +            return ret;
> +        }
> +    }
> +
> +    if (s->use_lazy_refcounts != lazy_refcounts) {
> +        if (lazy_refcounts) {
> +            if (s->qcow_version < 3) {
> +                fprintf(stderr, "Lazy refcounts only supported with compatibility "
> +                        "level 1.1 and above (use compat=1.1 or greater)\n");
> +                return -EINVAL;
> +            }
> +            s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
> +                return ret;
> +            }
> +            s->use_lazy_refcounts = true;
> +        } else {
> +            /* make image clean first */
> +            ret = qcow2_mark_clean(bs);
> +            if (ret < 0) {
> +                return ret;
> +            }
> +            /* now disallow lazy refcounts */
> +            s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
> +            ret = qcow2_update_header(bs);
> +            if (ret < 0) {
> +                s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
> +                return ret;
> +            }
> +            s->use_lazy_refcounts = false;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
>  static QEMUOptionParameter qcow2_create_options[] = {
>      {
>          .name = BLOCK_OPT_SIZE,
> @@ -1818,6 +2009,7 @@ static BlockDriver bdrv_qcow2 = {
>  
>      .create_options = qcow2_create_options,
>      .bdrv_check = qcow2_check,
> +    .bdrv_amend_options = qcow2_amend_options,
>  };
>  
>  static void bdrv_qcow2_init(void)
> diff --git a/block/qcow2.h b/block/qcow2.h
> index dba9771..ad3fd21 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -196,6 +196,7 @@ typedef struct BDRVQcowState {
>      int flags;
>      int qcow_version;
>      bool use_lazy_refcounts;
> +    int refcount_order;
>  
>      bool discard_passthrough[QCOW2_DISCARD_MAX];
>  
> @@ -408,6 +409,8 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
>      int nb_sectors);
>  int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
>  
> +int qcow2_expand_zero_clusters(BlockDriverState *bs);
> +
>  /* qcow2-snapshot.c functions */
>  int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
>  int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);
> -- 
> 1.8.3.1
> 
>
Max Reitz - Sept. 2, 2013, 7:43 a.m.
Am 02.09.2013 05:43, schrieb Fam Zheng:
> On Fri, 08/30 12:27, Max Reitz wrote:
>> Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
>> and lazy_refcounts.
>>
>> Downgrading images from compat=1.1 to compat=0.10 is achieved through
>> handling all incompatible flags accordingly, clearing all compatible and
>> autoclear flags and expanding all zero clusters.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   block/qcow2-cluster.c | 165 ++++++++++++++++++++++++++++++++++++++++++
>>   block/qcow2.c         | 194 +++++++++++++++++++++++++++++++++++++++++++++++++-
>>   block/qcow2.h         |   3 +
>>   3 files changed, 361 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
>> index cca76d4..e0ca104 100644
>> --- a/block/qcow2-cluster.c
>> +++ b/block/qcow2-cluster.c
>> @@ -1476,3 +1476,168 @@ fail:
>>   
>>       return ret;
>>   }
>> +
>> +/*
>> + * Expands all zero clusters in a specific L1 table (or deallocates them, for
>> + * non-backed non-pre-allocated zero clusters).
>> + */
>> +static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
>> +                                      int l1_size)
>> +{
>> +    BDRVQcowState *s = bs->opaque;
>> +    bool is_active_l1 = (l1_table == s->l1_table);
>> +    uint64_t *l2_table;
>> +    int ret;
>> +    int i, j;
>> +
>> +    if (!is_active_l1) {
>> +        /* inactive L2 tables require a buffer to be stored in when loading
>> +         * them from disk */
>> +        l2_table = qemu_blockalign(bs, s->cluster_size);
>> +    }
>> +
>> +    for (i = 0; i < l1_size; i++) {
>> +        uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
>> +        bool l2_dirty = false;
>> +
>> +        if (!l2_offset) {
>> +            /* unallocated */
>> +            continue;
>> +        }
>> +
>> +        if (is_active_l1) {
>> +            /* get active L2 tables from cache */
>> +            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
>> +                    (void **)&l2_table);
>> +        } else {
>> +            /* load inactive L2 tables from disk */
>> +            ret = bdrv_read(bs->file, l2_offset / BDRV_SECTOR_SIZE,
>> +                    (void *)l2_table, s->cluster_sectors);
>> +        }
>> +        if (ret < 0) {
>> +            goto fail;
>> +        }
>> +
>> +        for (j = 0; j < s->l2_size; j++) {
>> +            uint64_t l2_entry = be64_to_cpu(l2_table[j]);
>> +            int64_t offset;
>> +
>> +            if (qcow2_get_cluster_type(l2_entry) != QCOW2_CLUSTER_ZERO) {
>> +                continue;
>> +            }
>> +
>> +            offset = l2_entry & L2E_OFFSET_MASK;
>> +            if (!offset) {
>> +                /* not preallocated */
>> +                if (!bs->backing_hd) {
>> +                    /* not backed; therefore we can simply deallocate the
>> +                     * cluster */
>> +                    l2_table[j] = 0;
>> +                    l2_dirty = true;
>> +                    continue;
>> +                }
>> +
>> +                offset = qcow2_alloc_clusters(bs, s->cluster_size);
>> +                if (offset < 0) {
>> +                    ret = offset;
>> +                    goto fail;
>> +                }
>> +            }
>> +
>> +            ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
>> +                                    s->cluster_sectors);
>> +            if (ret < 0) {
>> +                qcow2_free_clusters(bs, offset, s->cluster_size,
>> +                        QCOW2_DISCARD_ALWAYS);
>> +                goto fail;
>> +            }
>> +
>> +            l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
>> +            l2_dirty = true;
>> +        }
>> +
>> +        if (is_active_l1) {
>> +            if (l2_dirty) {
>> +                qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
>> +                qcow2_cache_depends_on_flush(s->l2_table_cache);
>> +            }
>> +            ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
>> +            if (ret < 0) {
>> +                l2_table = NULL;
>> +                goto fail;
>> +            }
>> +        } else {
>> +            if (l2_dirty) {
>> +                ret = bdrv_write(bs->file, l2_offset / BDRV_SECTOR_SIZE,
>> +                        (void *)l2_table, s->cluster_sectors);
>> +                if (ret < 0) {
>> +                    goto fail;
>> +                }
>> +            }
>> +        }
>> +    }
>> +
>> +    ret = 0;
>> +
>> +fail:
>> +    if (l2_table) {
>> +        if (!is_active_l1) {
>> +            qemu_vfree(l2_table);
>> +        } else {
>> +            if (ret < 0) {
>> +                qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
>> +            } else {
>> +                ret = qcow2_cache_put(bs, s->l2_table_cache,
>> +                        (void **)&l2_table);
>> +            }
>> +        }
>> +    }
>> +    return ret;
>> +}
>> +
>> +/*
>> + * For backed images, expands all zero clusters on the image. For non-backed
>> + * images, deallocates all non-pre-allocated zero clusters (and claims the
>> + * allocation for pre-allocated ones). This is important for downgrading to a
>> + * qcow2 version which doesn't yet support metadata zero clusters.
>> + */
>> +int qcow2_expand_zero_clusters(BlockDriverState *bs)
>> +{
>> +    BDRVQcowState *s = bs->opaque;
>> +    uint64_t *l1_table = NULL;
>> +    int ret;
>> +    int i, j;
>> +
>> +    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size);
>> +    if (ret < 0) {
>> +        goto fail;
>> +    }
>> +
>> +    for (i = 0; i < s->nb_snapshots; i++) {
>> +        int l1_sectors = (s->snapshots[i].l1_size * sizeof(uint64_t) +
>> +                BDRV_SECTOR_SIZE - 1) / BDRV_SECTOR_SIZE;
>> +
>> +        l1_table = g_realloc(l1_table, l1_sectors * BDRV_SECTOR_SIZE);
>> +
>> +        ret = bdrv_read(bs->file, s->snapshots[i].l1_table_offset /
>> +                BDRV_SECTOR_SIZE, (void *)l1_table, l1_sectors);
>> +        if (ret < 0) {
>> +            goto fail;
>> +        }
>> +
>> +        for (j = 0; j < s->snapshots[i].l1_size; j++) {
>> +            be64_to_cpus(&l1_table[j]);
>> +        }
>> +
>> +        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size);
>> +        if (ret < 0) {
>> +            goto fail;
>> +        }
>> +    }
>> +
>> +    ret = 0;
>> +
>> +fail:
>> +    g_free(l1_table);
>> +    return ret;
>> +}
>> diff --git a/block/qcow2.c b/block/qcow2.c
>> index 78097e5..a8eaf45 100644
>> --- a/block/qcow2.c
>> +++ b/block/qcow2.c
>> @@ -409,6 +409,7 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
>>           ret = -ENOTSUP;
>>           goto fail;
>>       }
>> +    s->refcount_order = header.refcount_order;
>>   
>>       if (header.cluster_bits < MIN_CLUSTER_BITS ||
>>           header.cluster_bits > MAX_CLUSTER_BITS) {
>> @@ -1076,7 +1077,7 @@ int qcow2_update_header(BlockDriverState *bs)
>>           .incompatible_features  = cpu_to_be64(s->incompatible_features),
>>           .compatible_features    = cpu_to_be64(s->compatible_features),
>>           .autoclear_features     = cpu_to_be64(s->autoclear_features),
>> -        .refcount_order         = cpu_to_be32(3 + REFCOUNT_SHIFT),
>> +        .refcount_order         = cpu_to_be32(s->refcount_order),
>>           .header_length          = cpu_to_be32(header_length),
>>       };
>>   
>> @@ -1735,6 +1736,196 @@ static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
>>       return ret;
>>   }
>>   
>> +/*
>> + * Downgrades an image's version. To achieve this, any incompatible features
>> + * have to be removed.
>> + */
>> +static int qcow2_downgrade(BlockDriverState *bs, int target_version)
>> +{
>> +    BDRVQcowState *s = bs->opaque;
>> +    int current_version = s->qcow_version;
>> +    int ret;
>> +
>> +    if (target_version == current_version) {
>> +        return 0;
>> +    } else if (target_version > current_version) {
>> +        return -EINVAL;
>> +    } else if (target_version != 2) {
>> +        return -EINVAL;
>> +    }
>> +
>> +    if (s->refcount_order != 4) {
>> +        /* we would have to convert the image to a refcount_order == 4 image
>> +         * here; however, since qemu (at the time of writing this) does not
>> +         * support anything different than 4 anyway, there is no point in doing
>> +         * so right now; however, we should error out (if qemu supports this in
>> +         * the future and this code has not been adapted) */
>> +        error_report("qcow2_downgrade: Image refcount orders other than 4 are"
>> +                     "currently not supported.");
>> +        return -ENOTSUP;
>> +    }
>> +
>> +    /* clear incompatible features */
>> +    if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
>> +        ret = qcow2_mark_clean(bs);
>> +        if (ret < 0) {
>> +            return ret;
>> +        }
>> +    }
>> +
>> +    if (s->incompatible_features) {
>> +        return -ENOTSUP;
>> +    }
>> +
>> +    /* since we can ignore compatible features, we can set them to 0 as well */
>> +    s->compatible_features = 0;
>> +    /* if lazy refcounts have been used, they have already been fixed through
>> +     * clearing the dirty flag */
>> +
>> +    /* clearing autoclear features is trivial */
>> +    s->autoclear_features = 0;
>> +
>> +    ret = qcow2_expand_zero_clusters(bs);
>> +    if (ret < 0) {
>> +        return ret;
>> +    }
>> +
>> +    s->qcow_version = target_version;
>> +    ret = qcow2_update_header(bs);
>> +    if (ret < 0) {
>> +        s->qcow_version = current_version;
>> +        return ret;
>> +    }
>> +    return 0;
>> +}
>> +
>> +static int qcow2_amend_options(BlockDriverState *bs,
>> +                               QEMUOptionParameter *options)
>> +{
>> +    BDRVQcowState *s = bs->opaque;
>> +    int old_version = s->qcow_version, new_version = old_version;
>> +    uint64_t new_size = 0;
>> +    const char *backing_file = NULL, *backing_format = NULL;
>> +    bool lazy_refcounts = s->use_lazy_refcounts;
>> +    int ret;
>> +    int i;
>> +
>> +    for (i = 0; options[i].name; i++)
>> +    {
>> +        if (!strcmp(options[i].name, "compat")) {
>> +            if (!options[i].value.s) {
>> +                /* preserve default */
>> +            } else if (!strcmp(options[i].value.s, "0.10")) {
>> +                new_version = 2;
>> +            } else if (!strcmp(options[i].value.s, "1.1")) {
>> +                new_version = 3;
>> +            } else {
>> +                fprintf(stderr, "Unknown compatibility level %s.\n",
>> +                        options[i].value.s);
>> +                return -EINVAL;
>> +            }
>> +        } else if (!strcmp(options[i].name, "preallocation")) {
>> +            if (options[i].assigned) {
> For encryption flag and cluster_size, you checked the original value and only
> error out on actual change, should check the original preallocation value here
> as well?
As fas as I know, there is no way to check the original preallocation value.

>> +                fprintf(stderr, "Cannot change preallocation mode.\n");
>> +                return -ENOTSUP;
>> +            }
>> +        } else if (!strcmp(options[i].name, "size")) {
>> +            new_size = options[i].value.n;
>> +        } else if (!strcmp(options[i].name, "backing_file")) {
>> +            backing_file = options[i].value.s;
>> +        } else if (!strcmp(options[i].name, "backing_fmt")) {
>> +            backing_format = options[i].value.s;
>> +        } else if (!strcmp(options[i].name, "encryption")) {
>> +            if (options[i].assigned &&
>> +                (options[i].value.n != !!s->crypt_method)) {
>> +                fprintf(stderr, "Changing the encryption flag is not "
>> +                        "supported.\n");
>> +                return -ENOTSUP;
>> +            }
>> +        } else if (!strcmp(options[i].name, "cluster_size")) {
>> +            if (options[i].assigned && (options[i].value.n != s->cluster_size))
>> +            {
>> +                fprintf(stderr, "Changing the cluster size is not "
>> +                        "supported.\n");
>> +                return -ENOTSUP;
>> +            }
>> +        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
>> +            if (options[i].assigned) {
>> +                lazy_refcounts = options[i].value.n;
>> +            }
>> +        } else {
>> +            /* if this assertion fails, this probably means a new option was
>> +             * added without having it covered here */
>> +            assert(false);
> A unknown option reported as -ENOTSUP with a proper message is good enough,
> it's not that critical for an assert.
Well, Kevin suggested the assert(false), and since this code would be 
indeed probably reached only because of a programming error, I 
personally think it to be appropriate, too – although I'll gladly change 
it if the majority decides otherwise. ;-)

>> +        }
>> +    }
>> +
>> +    if (new_version != old_version) {
>> +        if (new_version > old_version) {
>> +            /* Upgrade */
>> +            s->qcow_version = new_version;
>> +            ret = qcow2_update_header(bs);
>> +            if (ret < 0) {
>> +                s->qcow_version = old_version;
>> +                return ret;
>> +            }
>> +        } else {
>> +            ret = qcow2_downgrade(bs, new_version);
>> +            if (ret < 0) {
>> +                return ret;
>> +            }
>> +        }
>> +    }
>> +
>> +    if (new_size) {
>> +        ret = qcow2_truncate(bs, new_size);
>> +        if (ret < 0) {
>> +            return ret;
>> +        }
>> +    }
>> +
>> +    if (backing_file || backing_format) {
>> +        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
>> +                                        backing_format ?: bs->backing_format);
>> +        if (ret < 0) {
>> +            return ret;
>> +        }
>> +    }
>> +
>> +    if (s->use_lazy_refcounts != lazy_refcounts) {
>> +        if (lazy_refcounts) {
>> +            if (s->qcow_version < 3) {
>> +                fprintf(stderr, "Lazy refcounts only supported with compatibility "
>> +                        "level 1.1 and above (use compat=1.1 or greater)\n");
>> +                return -EINVAL;
>> +            }
>> +            s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
>> +            ret = qcow2_update_header(bs);
>> +            if (ret < 0) {
>> +                s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
>> +                return ret;
>> +            }
>> +            s->use_lazy_refcounts = true;
>> +        } else {
>> +            /* make image clean first */
>> +            ret = qcow2_mark_clean(bs);
>> +            if (ret < 0) {
>> +                return ret;
>> +            }
>> +            /* now disallow lazy refcounts */
>> +            s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
>> +            ret = qcow2_update_header(bs);
>> +            if (ret < 0) {
>> +                s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
>> +                return ret;
>> +            }
>> +            s->use_lazy_refcounts = false;
>> +        }
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>>   static QEMUOptionParameter qcow2_create_options[] = {
>>       {
>>           .name = BLOCK_OPT_SIZE,
>> @@ -1818,6 +2009,7 @@ static BlockDriver bdrv_qcow2 = {
>>   
>>       .create_options = qcow2_create_options,
>>       .bdrv_check = qcow2_check,
>> +    .bdrv_amend_options = qcow2_amend_options,
>>   };
>>   
>>   static void bdrv_qcow2_init(void)
>> diff --git a/block/qcow2.h b/block/qcow2.h
>> index dba9771..ad3fd21 100644
>> --- a/block/qcow2.h
>> +++ b/block/qcow2.h
>> @@ -196,6 +196,7 @@ typedef struct BDRVQcowState {
>>       int flags;
>>       int qcow_version;
>>       bool use_lazy_refcounts;
>> +    int refcount_order;
>>   
>>       bool discard_passthrough[QCOW2_DISCARD_MAX];
>>   
>> @@ -408,6 +409,8 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
>>       int nb_sectors);
>>   int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
>>   
>> +int qcow2_expand_zero_clusters(BlockDriverState *bs);
>> +
>>   /* qcow2-snapshot.c functions */
>>   int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
>>   int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);
>> -- 
>> 1.8.3.1
>>
>>

Max
Kevin Wolf - Sept. 2, 2013, 7:55 a.m.
Am 02.09.2013 um 05:43 hat Fam Zheng geschrieben:
> On Fri, 08/30 12:27, Max Reitz wrote:
> > Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
> > and lazy_refcounts.
> > 
> > Downgrading images from compat=1.1 to compat=0.10 is achieved through
> > handling all incompatible flags accordingly, clearing all compatible and
> > autoclear flags and expanding all zero clusters.
> > 
> > Signed-off-by: Max Reitz <mreitz@redhat.com>
> > ---
> >  block/qcow2-cluster.c | 165 ++++++++++++++++++++++++++++++++++++++++++
> >  block/qcow2.c         | 194 +++++++++++++++++++++++++++++++++++++++++++++++++-
> >  block/qcow2.h         |   3 +
> >  3 files changed, 361 insertions(+), 1 deletion(-)

> > +static int qcow2_amend_options(BlockDriverState *bs,
> > +                               QEMUOptionParameter *options)
> > +{
> > +    BDRVQcowState *s = bs->opaque;
> > +    int old_version = s->qcow_version, new_version = old_version;
> > +    uint64_t new_size = 0;
> > +    const char *backing_file = NULL, *backing_format = NULL;
> > +    bool lazy_refcounts = s->use_lazy_refcounts;
> > +    int ret;
> > +    int i;
> > +
> > +    for (i = 0; options[i].name; i++)
> > +    {
> > +        if (!strcmp(options[i].name, "compat")) {
> > +            if (!options[i].value.s) {
> > +                /* preserve default */
> > +            } else if (!strcmp(options[i].value.s, "0.10")) {
> > +                new_version = 2;
> > +            } else if (!strcmp(options[i].value.s, "1.1")) {
> > +                new_version = 3;
> > +            } else {
> > +                fprintf(stderr, "Unknown compatibility level %s.\n",
> > +                        options[i].value.s);
> > +                return -EINVAL;
> > +            }
> > +        } else if (!strcmp(options[i].name, "preallocation")) {
> > +            if (options[i].assigned) {
> 
> For encryption flag and cluster_size, you checked the original value and only
> error out on actual change, should check the original preallocation value here
> as well?
> 
> > +                fprintf(stderr, "Cannot change preallocation mode.\n");
> > +                return -ENOTSUP;
> > +            }
> > +        } else if (!strcmp(options[i].name, "size")) {
> > +            new_size = options[i].value.n;
> > +        } else if (!strcmp(options[i].name, "backing_file")) {
> > +            backing_file = options[i].value.s;
> > +        } else if (!strcmp(options[i].name, "backing_fmt")) {
> > +            backing_format = options[i].value.s;
> > +        } else if (!strcmp(options[i].name, "encryption")) {
> > +            if (options[i].assigned &&
> > +                (options[i].value.n != !!s->crypt_method)) {
> > +                fprintf(stderr, "Changing the encryption flag is not "
> > +                        "supported.\n");
> > +                return -ENOTSUP;
> > +            }
> > +        } else if (!strcmp(options[i].name, "cluster_size")) {
> > +            if (options[i].assigned && (options[i].value.n != s->cluster_size))
> > +            {
> > +                fprintf(stderr, "Changing the cluster size is not "
> > +                        "supported.\n");
> > +                return -ENOTSUP;
> > +            }
> > +        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
> > +            if (options[i].assigned) {
> > +                lazy_refcounts = options[i].value.n;
> > +            }
> > +        } else {
> > +            /* if this assertion fails, this probably means a new option was
> > +             * added without having it covered here */
> > +            assert(false);
> 
> A unknown option reported as -ENOTSUP with a proper message is good enough,
> it's not that critical for an assert.

assert() has nothing to do with being critical.

This is not code for handling a user error, but for catching programming
errors. If the else branch is reached, this is a bug, and you want it to
be detected as quickly as possible (and without having to debug a lot in
order to find out where a bogus -ENOTSUP comes from)

Kevin

Patch

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index cca76d4..e0ca104 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1476,3 +1476,168 @@  fail:
 
     return ret;
 }
+
+/*
+ * Expands all zero clusters in a specific L1 table (or deallocates them, for
+ * non-backed non-pre-allocated zero clusters).
+ */
+static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
+                                      int l1_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    bool is_active_l1 = (l1_table == s->l1_table);
+    uint64_t *l2_table;
+    int ret;
+    int i, j;
+
+    if (!is_active_l1) {
+        /* inactive L2 tables require a buffer to be stored in when loading
+         * them from disk */
+        l2_table = qemu_blockalign(bs, s->cluster_size);
+    }
+
+    for (i = 0; i < l1_size; i++) {
+        uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
+        bool l2_dirty = false;
+
+        if (!l2_offset) {
+            /* unallocated */
+            continue;
+        }
+
+        if (is_active_l1) {
+            /* get active L2 tables from cache */
+            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
+                    (void **)&l2_table);
+        } else {
+            /* load inactive L2 tables from disk */
+            ret = bdrv_read(bs->file, l2_offset / BDRV_SECTOR_SIZE,
+                    (void *)l2_table, s->cluster_sectors);
+        }
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (j = 0; j < s->l2_size; j++) {
+            uint64_t l2_entry = be64_to_cpu(l2_table[j]);
+            int64_t offset;
+
+            if (qcow2_get_cluster_type(l2_entry) != QCOW2_CLUSTER_ZERO) {
+                continue;
+            }
+
+            offset = l2_entry & L2E_OFFSET_MASK;
+            if (!offset) {
+                /* not preallocated */
+                if (!bs->backing_hd) {
+                    /* not backed; therefore we can simply deallocate the
+                     * cluster */
+                    l2_table[j] = 0;
+                    l2_dirty = true;
+                    continue;
+                }
+
+                offset = qcow2_alloc_clusters(bs, s->cluster_size);
+                if (offset < 0) {
+                    ret = offset;
+                    goto fail;
+                }
+            }
+
+            ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
+                                    s->cluster_sectors);
+            if (ret < 0) {
+                qcow2_free_clusters(bs, offset, s->cluster_size,
+                        QCOW2_DISCARD_ALWAYS);
+                goto fail;
+            }
+
+            l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
+            l2_dirty = true;
+        }
+
+        if (is_active_l1) {
+            if (l2_dirty) {
+                qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+                qcow2_cache_depends_on_flush(s->l2_table_cache);
+            }
+            ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+            if (ret < 0) {
+                l2_table = NULL;
+                goto fail;
+            }
+        } else {
+            if (l2_dirty) {
+                ret = bdrv_write(bs->file, l2_offset / BDRV_SECTOR_SIZE,
+                        (void *)l2_table, s->cluster_sectors);
+                if (ret < 0) {
+                    goto fail;
+                }
+            }
+        }
+    }
+
+    ret = 0;
+
+fail:
+    if (l2_table) {
+        if (!is_active_l1) {
+            qemu_vfree(l2_table);
+        } else {
+            if (ret < 0) {
+                qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+            } else {
+                ret = qcow2_cache_put(bs, s->l2_table_cache,
+                        (void **)&l2_table);
+            }
+        }
+    }
+    return ret;
+}
+
+/*
+ * For backed images, expands all zero clusters on the image. For non-backed
+ * images, deallocates all non-pre-allocated zero clusters (and claims the
+ * allocation for pre-allocated ones). This is important for downgrading to a
+ * qcow2 version which doesn't yet support metadata zero clusters.
+ */
+int qcow2_expand_zero_clusters(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table = NULL;
+    int ret;
+    int i, j;
+
+    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    for (i = 0; i < s->nb_snapshots; i++) {
+        int l1_sectors = (s->snapshots[i].l1_size * sizeof(uint64_t) +
+                BDRV_SECTOR_SIZE - 1) / BDRV_SECTOR_SIZE;
+
+        l1_table = g_realloc(l1_table, l1_sectors * BDRV_SECTOR_SIZE);
+
+        ret = bdrv_read(bs->file, s->snapshots[i].l1_table_offset /
+                BDRV_SECTOR_SIZE, (void *)l1_table, l1_sectors);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (j = 0; j < s->snapshots[i].l1_size; j++) {
+            be64_to_cpus(&l1_table[j]);
+        }
+
+        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    ret = 0;
+
+fail:
+    g_free(l1_table);
+    return ret;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index 78097e5..a8eaf45 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -409,6 +409,7 @@  static int qcow2_open(BlockDriverState *bs, QDict *options, int flags)
         ret = -ENOTSUP;
         goto fail;
     }
+    s->refcount_order = header.refcount_order;
 
     if (header.cluster_bits < MIN_CLUSTER_BITS ||
         header.cluster_bits > MAX_CLUSTER_BITS) {
@@ -1076,7 +1077,7 @@  int qcow2_update_header(BlockDriverState *bs)
         .incompatible_features  = cpu_to_be64(s->incompatible_features),
         .compatible_features    = cpu_to_be64(s->compatible_features),
         .autoclear_features     = cpu_to_be64(s->autoclear_features),
-        .refcount_order         = cpu_to_be32(3 + REFCOUNT_SHIFT),
+        .refcount_order         = cpu_to_be32(s->refcount_order),
         .header_length          = cpu_to_be32(header_length),
     };
 
@@ -1735,6 +1736,196 @@  static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
     return ret;
 }
 
+/*
+ * Downgrades an image's version. To achieve this, any incompatible features
+ * have to be removed.
+ */
+static int qcow2_downgrade(BlockDriverState *bs, int target_version)
+{
+    BDRVQcowState *s = bs->opaque;
+    int current_version = s->qcow_version;
+    int ret;
+
+    if (target_version == current_version) {
+        return 0;
+    } else if (target_version > current_version) {
+        return -EINVAL;
+    } else if (target_version != 2) {
+        return -EINVAL;
+    }
+
+    if (s->refcount_order != 4) {
+        /* we would have to convert the image to a refcount_order == 4 image
+         * here; however, since qemu (at the time of writing this) does not
+         * support anything different than 4 anyway, there is no point in doing
+         * so right now; however, we should error out (if qemu supports this in
+         * the future and this code has not been adapted) */
+        error_report("qcow2_downgrade: Image refcount orders other than 4 are"
+                     "currently not supported.");
+        return -ENOTSUP;
+    }
+
+    /* clear incompatible features */
+    if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
+        ret = qcow2_mark_clean(bs);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (s->incompatible_features) {
+        return -ENOTSUP;
+    }
+
+    /* since we can ignore compatible features, we can set them to 0 as well */
+    s->compatible_features = 0;
+    /* if lazy refcounts have been used, they have already been fixed through
+     * clearing the dirty flag */
+
+    /* clearing autoclear features is trivial */
+    s->autoclear_features = 0;
+
+    ret = qcow2_expand_zero_clusters(bs);
+    if (ret < 0) {
+        return ret;
+    }
+
+    s->qcow_version = target_version;
+    ret = qcow2_update_header(bs);
+    if (ret < 0) {
+        s->qcow_version = current_version;
+        return ret;
+    }
+    return 0;
+}
+
+static int qcow2_amend_options(BlockDriverState *bs,
+                               QEMUOptionParameter *options)
+{
+    BDRVQcowState *s = bs->opaque;
+    int old_version = s->qcow_version, new_version = old_version;
+    uint64_t new_size = 0;
+    const char *backing_file = NULL, *backing_format = NULL;
+    bool lazy_refcounts = s->use_lazy_refcounts;
+    int ret;
+    int i;
+
+    for (i = 0; options[i].name; i++)
+    {
+        if (!strcmp(options[i].name, "compat")) {
+            if (!options[i].value.s) {
+                /* preserve default */
+            } else if (!strcmp(options[i].value.s, "0.10")) {
+                new_version = 2;
+            } else if (!strcmp(options[i].value.s, "1.1")) {
+                new_version = 3;
+            } else {
+                fprintf(stderr, "Unknown compatibility level %s.\n",
+                        options[i].value.s);
+                return -EINVAL;
+            }
+        } else if (!strcmp(options[i].name, "preallocation")) {
+            if (options[i].assigned) {
+                fprintf(stderr, "Cannot change preallocation mode.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "size")) {
+            new_size = options[i].value.n;
+        } else if (!strcmp(options[i].name, "backing_file")) {
+            backing_file = options[i].value.s;
+        } else if (!strcmp(options[i].name, "backing_fmt")) {
+            backing_format = options[i].value.s;
+        } else if (!strcmp(options[i].name, "encryption")) {
+            if (options[i].assigned &&
+                (options[i].value.n != !!s->crypt_method)) {
+                fprintf(stderr, "Changing the encryption flag is not "
+                        "supported.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "cluster_size")) {
+            if (options[i].assigned && (options[i].value.n != s->cluster_size))
+            {
+                fprintf(stderr, "Changing the cluster size is not "
+                        "supported.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
+            if (options[i].assigned) {
+                lazy_refcounts = options[i].value.n;
+            }
+        } else {
+            /* if this assertion fails, this probably means a new option was
+             * added without having it covered here */
+            assert(false);
+        }
+    }
+
+    if (new_version != old_version) {
+        if (new_version > old_version) {
+            /* Upgrade */
+            s->qcow_version = new_version;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->qcow_version = old_version;
+                return ret;
+            }
+        } else {
+            ret = qcow2_downgrade(bs, new_version);
+            if (ret < 0) {
+                return ret;
+            }
+        }
+    }
+
+    if (new_size) {
+        ret = qcow2_truncate(bs, new_size);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (backing_file || backing_format) {
+        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
+                                        backing_format ?: bs->backing_format);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (s->use_lazy_refcounts != lazy_refcounts) {
+        if (lazy_refcounts) {
+            if (s->qcow_version < 3) {
+                fprintf(stderr, "Lazy refcounts only supported with compatibility "
+                        "level 1.1 and above (use compat=1.1 or greater)\n");
+                return -EINVAL;
+            }
+            s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
+                return ret;
+            }
+            s->use_lazy_refcounts = true;
+        } else {
+            /* make image clean first */
+            ret = qcow2_mark_clean(bs);
+            if (ret < 0) {
+                return ret;
+            }
+            /* now disallow lazy refcounts */
+            s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
+                return ret;
+            }
+            s->use_lazy_refcounts = false;
+        }
+    }
+
+    return 0;
+}
+
 static QEMUOptionParameter qcow2_create_options[] = {
     {
         .name = BLOCK_OPT_SIZE,
@@ -1818,6 +2009,7 @@  static BlockDriver bdrv_qcow2 = {
 
     .create_options = qcow2_create_options,
     .bdrv_check = qcow2_check,
+    .bdrv_amend_options = qcow2_amend_options,
 };
 
 static void bdrv_qcow2_init(void)
diff --git a/block/qcow2.h b/block/qcow2.h
index dba9771..ad3fd21 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -196,6 +196,7 @@  typedef struct BDRVQcowState {
     int flags;
     int qcow_version;
     bool use_lazy_refcounts;
+    int refcount_order;
 
     bool discard_passthrough[QCOW2_DISCARD_MAX];
 
@@ -408,6 +409,8 @@  int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
     int nb_sectors);
 int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
 
+int qcow2_expand_zero_clusters(BlockDriverState *bs);
+
 /* qcow2-snapshot.c functions */
 int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
 int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);