[Qemu-devel] Re: [PATCH v2 4/7] qed: Add QEMU Enhanced Disk image format

[Kevin Wolf <kwolf@redhat.com>]

Am 08.10.2010 17:48, schrieb Stefan Hajnoczi:
> This patch introduces the qed on-disk layout and implements image
> creation.  Later patches add read/write and other functionality.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
>  Makefile.objs |    1 +
>  block/qed.c   |  530 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  block/qed.h   |  148 ++++++++++++++++
>  3 files changed, 679 insertions(+), 0 deletions(-)
>  create mode 100644 block/qed.c
>  create mode 100644 block/qed.h
> 
> diff --git a/Makefile.objs b/Makefile.objs
> index 816194a..ff15795 100644
> --- a/Makefile.objs
> +++ b/Makefile.objs
> @@ -14,6 +14,7 @@ block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>  
>  block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
>  block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o
> +block-nested-y += qed.o
>  block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
>  block-nested-$(CONFIG_WIN32) += raw-win32.o
>  block-nested-$(CONFIG_POSIX) += raw-posix.o
> diff --git a/block/qed.c b/block/qed.c
> new file mode 100644
> index 0000000..ea03798
> --- /dev/null
> +++ b/block/qed.c
> @@ -0,0 +1,530 @@
> +/*
> + * QEMU Enhanced Disk Format
> + *
> + * Copyright IBM, Corp. 2010
> + *
> + * Authors:
> + *  Stefan Hajnoczi   <stefanha@linux.vnet.ibm.com>
> + *  Anthony Liguori   <aliguori@us.ibm.com>
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + *
> + */
> +
> +#include "qed.h"
> +
> +static int bdrv_qed_probe(const uint8_t *buf, int buf_size,
> +                          const char *filename)
> +{
> +    const QEDHeader *header = (const void *)buf;

Too lazy to type the real type name? ;-)

> +
> +    if (buf_size < sizeof(*header)) {
> +        return 0;
> +    }
> +    if (le32_to_cpu(header->magic) != QED_MAGIC) {
> +        return 0;
> +    }
> +    return 100;
> +}
> +
> +static void qed_header_le_to_cpu(const QEDHeader *le, QEDHeader *cpu)
> +{
> +    cpu->magic = le32_to_cpu(le->magic);
> +    cpu->cluster_size = le32_to_cpu(le->cluster_size);
> +    cpu->table_size = le32_to_cpu(le->table_size);
> +    cpu->header_size = le32_to_cpu(le->header_size);
> +    cpu->features = le64_to_cpu(le->features);
> +    cpu->compat_features = le64_to_cpu(le->compat_features);
> +    cpu->l1_table_offset = le64_to_cpu(le->l1_table_offset);
> +    cpu->image_size = le64_to_cpu(le->image_size);
> +    cpu->backing_filename_offset = le32_to_cpu(le->backing_filename_offset);
> +    cpu->backing_filename_size = le32_to_cpu(le->backing_filename_size);
> +    cpu->backing_fmt_offset = le32_to_cpu(le->backing_fmt_offset);
> +    cpu->backing_fmt_size = le32_to_cpu(le->backing_fmt_size);
> +}
> +
> +static void qed_header_cpu_to_le(const QEDHeader *cpu, QEDHeader *le)
> +{
> +    le->magic = cpu_to_le32(cpu->magic);
> +    le->cluster_size = cpu_to_le32(cpu->cluster_size);
> +    le->table_size = cpu_to_le32(cpu->table_size);
> +    le->header_size = cpu_to_le32(cpu->header_size);
> +    le->features = cpu_to_le64(cpu->features);
> +    le->compat_features = cpu_to_le64(cpu->compat_features);
> +    le->l1_table_offset = cpu_to_le64(cpu->l1_table_offset);
> +    le->image_size = cpu_to_le64(cpu->image_size);
> +    le->backing_filename_offset = cpu_to_le32(cpu->backing_filename_offset);
> +    le->backing_filename_size = cpu_to_le32(cpu->backing_filename_size);
> +    le->backing_fmt_offset = cpu_to_le32(cpu->backing_fmt_offset);
> +    le->backing_fmt_size = cpu_to_le32(cpu->backing_fmt_size);
> +}
> +

/** Returns the maximum virtual disk size in bytes */

> +static uint64_t qed_max_image_size(uint32_t cluster_size, uint32_t table_size)
> +{
> +    uint64_t table_entries;
> +    uint64_t l2_size;
> +
> +    table_entries = (table_size * cluster_size) / sizeof(uint64_t);
> +    l2_size = table_entries * cluster_size;
> +
> +    return l2_size * table_entries;
> +}
> +
> +static bool qed_is_cluster_size_valid(uint32_t cluster_size)
> +{
> +    if (cluster_size < QED_MIN_CLUSTER_SIZE ||
> +        cluster_size > QED_MAX_CLUSTER_SIZE) {
> +        return false;
> +    }
> +    if (cluster_size & (cluster_size - 1)) {
> +        return false; /* not power of 2 */
> +    }
> +    return true;
> +}
> +
> +static bool qed_is_table_size_valid(uint32_t table_size)
> +{
> +    if (table_size < QED_MIN_TABLE_SIZE ||
> +        table_size > QED_MAX_TABLE_SIZE) {
> +        return false;
> +    }
> +    if (table_size & (table_size - 1)) {
> +        return false; /* not power of 2 */
> +    }
> +    return true;
> +}
> +
> +static bool qed_is_image_size_valid(uint64_t image_size, uint32_t cluster_size,
> +                                    uint32_t table_size)
> +{
> +    if (image_size == 0) {
> +        /* Supporting zero size images makes life harder because even the L1
> +         * table is not needed.  Make life simple and forbid zero size images.
> +         */
> +        return false;
> +    }

Should the spec be updated to forbid zero size images?

> +    if (image_size & (cluster_size - 1)) {
> +        return false; /* not multiple of cluster size */
> +    }
> +    if (image_size > qed_max_image_size(cluster_size, table_size)) {
> +        return false; /* image is too large */
> +    }
> +    return true;
> +}
> +
> +/**
> + * Read a string of known length from the image file
> + *
> + * @file:       Image file
> + * @offset:     File offset to start of string, in bytes
> + * @n:          String length in bytes
> + * @buf:        Destination buffer
> + * @buflen:     Destination buffer length in bytes
> + *
> + * The string is NUL-terminated.

Return value?

> + */
> +static int qed_read_string(BlockDriverState *file, uint64_t offset, size_t n,
> +                           char *buf, size_t buflen)
> +{
> +    int ret;
> +    if (n >= buflen) {
> +        return -EINVAL;
> +    }
> +    ret = bdrv_pread(file, offset, buf, n);
> +    if (ret != n) {
> +        return ret;
> +    }
> +    buf[n] = '\0';
> +    return 0;
> +}
> +
> +static int bdrv_qed_open(BlockDriverState *bs, int flags)
> +{
> +    BDRVQEDState *s = bs->opaque;
> +    QEDHeader le_header;
> +    int64_t file_size;
> +    int ret;
> +
> +    s->bs = bs;
> +
> +    ret = bdrv_pread(bs->file, 0, &le_header, sizeof(le_header));
> +    if (ret != sizeof(le_header)) {
> +        return ret;
> +    }

While this is correct because bdrv_pread never returns short reads, I
think "if (ret < 0)" would be easier to read.

> +    qed_header_le_to_cpu(&le_header, &s->header);
> +
> +    if (s->header.magic != QED_MAGIC) {
> +        return -ENOENT;
> +    }
> +    if (s->header.features & ~QED_FEATURE_MASK) {
> +        return -ENOTSUP; /* image uses unsupported feature bits */
> +    }
> +    if (!qed_is_cluster_size_valid(s->header.cluster_size)) {
> +        return -EINVAL;
> +    }
> +
> +    /* Round up file size to the next cluster */
> +    file_size = bdrv_getlength(bs->file);
> +    if (file_size < 0) {
> +        return file_size;
> +    }
> +    s->file_size = qed_start_of_cluster(s, file_size);

Aren't you rounding down despite the comment?

> +
> +    if (!qed_is_table_size_valid(s->header.table_size)) {
> +        return -EINVAL;
> +    }
> +    if (!qed_is_image_size_valid(s->header.image_size,
> +                                 s->header.cluster_size,
> +                                 s->header.table_size)) {
> +        return -EINVAL;
> +    }
> +    if (!qed_check_table_offset(s, s->header.l1_table_offset)) {
> +        return -EINVAL;
> +    }
> +
> +    s->table_nelems = (s->header.cluster_size * s->header.table_size) /
> +                      sizeof(uint64_t);
> +    s->l2_shift = get_bits_from_size(s->header.cluster_size);
> +    s->l2_mask = s->table_nelems - 1;
> +    s->l1_shift = s->l2_shift + get_bits_from_size(s->l2_mask + 1);
> +
> +    if ((s->header.features & QED_F_BACKING_FILE)) {
> +        ret = qed_read_string(bs->file, s->header.backing_filename_offset,
> +                              s->header.backing_filename_size, bs->backing_file,
> +                              sizeof(bs->backing_file));
> +        if (ret < 0) {
> +            return ret;
> +        }
> +
> +        if ((s->header.compat_features & QED_CF_BACKING_FORMAT)) {
> +            ret = qed_read_string(bs->file, s->header.backing_fmt_offset,
> +                                  s->header.backing_fmt_size,
> +                                  bs->backing_format,
> +                                  sizeof(bs->backing_format));
> +            if (ret < 0) {
> +                return ret;
> +            }
> +        }
> +    }
> +    return ret;

I think this should be a return 0 instead of returning a random
non-negative number.

> +}
> +
> +static void bdrv_qed_close(BlockDriverState *bs)
> +{
> +}
> +
> +static void bdrv_qed_flush(BlockDriverState *bs)
> +{
> +    bdrv_flush(bs->file);
> +}
> +
> +static int qed_create(const char *filename, uint32_t cluster_size,
> +                      uint64_t image_size, uint32_t table_size,
> +                      const char *backing_file, const char *backing_fmt)
> +{
> +    QEDHeader header = {
> +        .magic = QED_MAGIC,
> +        .cluster_size = cluster_size,
> +        .table_size = table_size,
> +        .header_size = 1,
> +        .features = 0,
> +        .compat_features = 0,
> +        .l1_table_offset = cluster_size,
> +        .image_size = image_size,
> +    };
> +    QEDHeader le_header;
> +    uint8_t *l1_table = NULL;
> +    size_t l1_size = header.cluster_size * header.table_size;
> +    int ret = 0;
> +    int fd;
> +
> +    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, 0644);

Please use block.c functions (bdrv_create_file etc.) instead of POSIX
functions.

> +    if (fd < 0) {
> +        return -errno;
> +    }
> +
> +    if (backing_file) {
> +        header.features |= QED_F_BACKING_FILE;
> +        header.backing_filename_offset = sizeof(le_header);
> +        header.backing_filename_size = strlen(backing_file);
> +        if (backing_fmt) {
> +            header.compat_features |= QED_CF_BACKING_FORMAT;
> +            header.backing_fmt_offset = header.backing_filename_offset +
> +                                        header.backing_filename_size;
> +            header.backing_fmt_size = strlen(backing_fmt);
> +        }
> +    }
> +
> +    qed_header_cpu_to_le(&header, &le_header);
> +    if (qemu_write_full(fd, &le_header, sizeof(le_header)) != sizeof(le_header)) {
> +        ret = -errno;
> +        goto out;
> +    }
> +    if (qemu_write_full(fd, backing_file, header.backing_filename_size) != header.backing_filename_size) {
> +        ret = -errno;
> +        goto out;
> +    }
> +    if (qemu_write_full(fd, backing_fmt, header.backing_fmt_size) != header.backing_fmt_size) {
> +        ret = -errno;
> +        goto out;
> +    }
> +
> +    l1_table = qemu_mallocz(l1_size);
> +    lseek(fd, header.l1_table_offset, SEEK_SET);
> +    if (qemu_write_full(fd, l1_table, l1_size) != l1_size) {
> +        ret = -errno;
> +        goto out;
> +    }
> +
> +out:
> +    qemu_free(l1_table);
> +    close(fd);
> +    return ret;
> +}
> +
> +static int bdrv_qed_create(const char *filename, QEMUOptionParameter *options)
> +{
> +    uint64_t image_size = 0;
> +    uint32_t cluster_size = QED_DEFAULT_CLUSTER_SIZE;
> +    uint32_t table_size = QED_DEFAULT_TABLE_SIZE;
> +    const char *backing_file = NULL;
> +    const char *backing_fmt = NULL;
> +
> +    while (options && options->name) {
> +        if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
> +            image_size = options->value.n;
> +        } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
> +            backing_file = options->value.s;
> +        } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FMT)) {
> +            backing_fmt = options->value.s;
> +        } else if (!strcmp(options->name, BLOCK_OPT_CLUSTER_SIZE)) {
> +            if (options->value.n) {
> +                cluster_size = options->value.n;
> +            }
> +        } else if (!strcmp(options->name, "table_size")) {
> +            if (options->value.n) {
> +                table_size = options->value.n;
> +            }
> +        }
> +        options++;
> +    }
> +
> +    if (!qed_is_cluster_size_valid(cluster_size)) {
> +        fprintf(stderr, "QED cluster size must be within range [%u, %u] and power of 2\n",
> +                QED_MIN_CLUSTER_SIZE, QED_MAX_CLUSTER_SIZE);
> +        return -EINVAL;
> +    }
> +    if (!qed_is_table_size_valid(table_size)) {
> +        fprintf(stderr, "QED table size must be within range [%u, %u] and power of 2\n",
> +                QED_MIN_TABLE_SIZE, QED_MAX_TABLE_SIZE);
> +        return -EINVAL;
> +    }
> +    if (!qed_is_image_size_valid(image_size, cluster_size, table_size)) {
> +        char buffer[64];
> +
> +        bytes_to_str(buffer, sizeof(buffer),
> +                     qed_max_image_size(cluster_size, table_size));
> +
> +        fprintf(stderr,
> +                "QED image size must be a non-zero multiple of cluster size and less than %s\n",
> +                buffer);
> +        return -EINVAL;
> +    }
> +
> +    return qed_create(filename, cluster_size, image_size, table_size,
> +                      backing_file, backing_fmt);
> +}
> +
> +static int bdrv_qed_is_allocated(BlockDriverState *bs, int64_t sector_num,
> +                                  int nb_sectors, int *pnum)
> +{
> +    return -ENOTSUP;
> +}
> +
> +static int bdrv_qed_make_empty(BlockDriverState *bs)
> +{
> +    return -ENOTSUP;
> +}
> +
> +static BlockDriverAIOCB *bdrv_qed_aio_readv(BlockDriverState *bs,
> +                                            int64_t sector_num,
> +                                            QEMUIOVector *qiov, int nb_sectors,
> +                                            BlockDriverCompletionFunc *cb,
> +                                            void *opaque)
> +{
> +    return NULL;
> +}
> +
> +static BlockDriverAIOCB *bdrv_qed_aio_writev(BlockDriverState *bs,
> +                                             int64_t sector_num,
> +                                             QEMUIOVector *qiov, int nb_sectors,
> +                                             BlockDriverCompletionFunc *cb,
> +                                             void *opaque)
> +{
> +    return NULL;
> +}
> +
> +static BlockDriverAIOCB *bdrv_qed_aio_flush(BlockDriverState *bs,
> +                                            BlockDriverCompletionFunc *cb,
> +                                            void *opaque)
> +{
> +    return bdrv_aio_flush(bs->file, cb, opaque);
> +}
> +
> +static int bdrv_qed_truncate(BlockDriverState *bs, int64_t offset)
> +{
> +    return -ENOTSUP;
> +}
> +
> +static int64_t bdrv_qed_getlength(BlockDriverState *bs)
> +{
> +    BDRVQEDState *s = bs->opaque;
> +    return s->header.image_size;
> +}
> +
> +static int bdrv_qed_get_info(BlockDriverState *bs, BlockDriverInfo *bdi)
> +{
> +    BDRVQEDState *s = bs->opaque;
> +
> +    memset(bdi, 0, sizeof(*bdi));
> +    bdi->cluster_size = s->header.cluster_size;
> +    return 0;
> +}
> +
> +static int bdrv_qed_change_backing_file(BlockDriverState *bs,
> +                                        const char *backing_file,
> +                                        const char *backing_fmt)
> +{
> +    BDRVQEDState *s = bs->opaque;
> +    QEDHeader new_header, le_header;
> +    void *buffer;
> +    size_t buffer_len, backing_file_len, backing_fmt_len;
> +    int ret;
> +
> +    /* Refuse to set backing filename if unknown compat feature bits are
> +     * active.  If the image uses an unknown compat feature then we may not
> +     * know the layout of data following the header structure and cannot safely
> +     * add a new string.
> +     */
> +    if (backing_file && (s->header.compat_features &
> +                         ~QED_COMPAT_FEATURE_MASK)) {
> +        return -ENOTSUP;
> +    }
> +
> +    memcpy(&new_header, &s->header, sizeof(new_header));
> +
> +    new_header.features &= ~QED_F_BACKING_FILE;
> +    new_header.compat_features &= ~QED_CF_BACKING_FORMAT;
> +
> +    /* Adjust feature flags */
> +    if (backing_file) {
> +        new_header.features |= QED_F_BACKING_FILE;
> +        if (backing_fmt) {
> +            new_header.compat_features |= QED_CF_BACKING_FORMAT;
> +        }
> +    }
> +
> +    /* Calculate new header size */
> +    backing_file_len = backing_fmt_len = 0;
> +
> +    if (backing_file) {
> +        backing_file_len = strlen(backing_file);
> +        if (backing_fmt) {
> +            backing_fmt_len = strlen(backing_fmt);
> +        }
> +    }
> +
> +    buffer_len = sizeof(new_header);
> +    new_header.backing_filename_offset = buffer_len;
> +    new_header.backing_filename_size = backing_file_len;
> +    buffer_len += backing_file_len;
> +    new_header.backing_fmt_offset = buffer_len;
> +    new_header.backing_fmt_size = backing_fmt_len;
> +    buffer_len += backing_fmt_len;
> +
> +    /* Make sure we can rewrite header without failing */
> +    if (buffer_len > new_header.header_size * new_header.cluster_size) {
> +        return -ENOSPC;
> +    }
> +
> +    /* Prepare new header */
> +    buffer = qemu_malloc(buffer_len);
> +
> +    qed_header_cpu_to_le(&new_header, &le_header);
> +    memcpy(buffer, &le_header, sizeof(le_header));
> +    buffer_len = sizeof(le_header);
> +
> +    memcpy(buffer + buffer_len, backing_file, backing_file_len);
> +    buffer_len += backing_file_len;
> +
> +    memcpy(buffer + buffer_len, backing_fmt, backing_fmt_len);
> +    buffer_len += backing_fmt_len;
> +
> +    /* Write new header */
> +    ret = bdrv_pwrite_sync(bs->file, 0, buffer, buffer_len);
> +    qemu_free(buffer);
> +    if (ret == 0) {
> +        memcpy(&s->header, &new_header, sizeof(new_header));
> +    }
> +    return ret;
> +}
> +
> +static int bdrv_qed_check(BlockDriverState *bs, BdrvCheckResult *result)
> +{
> +    return -ENOTSUP;
> +}
> +
> +static QEMUOptionParameter qed_create_options[] = {
> +    {
> +        .name = BLOCK_OPT_SIZE,
> +        .type = OPT_SIZE,
> +        .help = "Virtual disk size (in bytes)"
> +    }, {
> +        .name = BLOCK_OPT_BACKING_FILE,
> +        .type = OPT_STRING,
> +        .help = "File name of a base image"
> +    }, {
> +        .name = BLOCK_OPT_BACKING_FMT,
> +        .type = OPT_STRING,
> +        .help = "Image format of the base image"
> +    }, {
> +        .name = BLOCK_OPT_CLUSTER_SIZE,
> +        .type = OPT_SIZE,
> +        .help = "Cluster size (in bytes)"
> +    }, {
> +        .name = "table_size",

What about introducing a constant for this?

> +        .type = OPT_SIZE,
> +        .help = "L1/L2 table size (in clusters)"
> +    },
> +    { /* end of list */ }
> +};
> +
> +static BlockDriver bdrv_qed = {
> +    .format_name = "qed",
> +    .instance_size = sizeof(BDRVQEDState),
> +    .create_options = qed_create_options,
> +
> +    .bdrv_probe = bdrv_qed_probe,
> +    .bdrv_open = bdrv_qed_open,
> +    .bdrv_close = bdrv_qed_close,
> +    .bdrv_create = bdrv_qed_create,
> +    .bdrv_flush = bdrv_qed_flush,
> +    .bdrv_is_allocated = bdrv_qed_is_allocated,
> +    .bdrv_make_empty = bdrv_qed_make_empty,
> +    .bdrv_aio_readv = bdrv_qed_aio_readv,
> +    .bdrv_aio_writev = bdrv_qed_aio_writev,
> +    .bdrv_aio_flush = bdrv_qed_aio_flush,
> +    .bdrv_truncate = bdrv_qed_truncate,
> +    .bdrv_getlength = bdrv_qed_getlength,
> +    .bdrv_get_info = bdrv_qed_get_info,
> +    .bdrv_change_backing_file = bdrv_qed_change_backing_file,
> +    .bdrv_check = bdrv_qed_check,

Please align the = of all definitions vertically.

> +};
> +
> +static void bdrv_qed_init(void)
> +{
> +    bdrv_register(&bdrv_qed);
> +}
> +
> +block_init(bdrv_qed_init);
> diff --git a/block/qed.h b/block/qed.h
> new file mode 100644
> index 0000000..7ce95a7
> --- /dev/null
> +++ b/block/qed.h
> @@ -0,0 +1,148 @@
> +/*
> + * QEMU Enhanced Disk Format
> + *
> + * Copyright IBM, Corp. 2010
> + *
> + * Authors:
> + *  Stefan Hajnoczi   <stefanha@linux.vnet.ibm.com>
> + *  Anthony Liguori   <aliguori@us.ibm.com>
> + *
> + * This work is licensed under the terms of the GNU LGPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + *
> + */
> +
> +#ifndef BLOCK_QED_H
> +#define BLOCK_QED_H
> +
> +#include "block_int.h"
> +
> +/* The layout of a QED file is as follows:
> + *
> + * +--------+----------+----------+----------+-----+
> + * | header | L1 table | cluster0 | cluster1 | ... |
> + * +--------+----------+----------+----------+-----+
> + *
> + * There is a 2-level pagetable for cluster allocation:
> + *
> + *                     +----------+
> + *                     | L1 table |
> + *                     +----------+
> + *                ,------'  |  '------.
> + *           +----------+   |    +----------+
> + *           | L2 table |  ...   | L2 table |
> + *           +----------+        +----------+
> + *       ,------'  |  '------.
> + *  +----------+   |    +----------+
> + *  |   Data   |  ...   |   Data   |
> + *  +----------+        +----------+
> + *
> + * The L1 table is fixed size and always present.  L2 tables are allocated on
> + * demand.  The L1 table size determines the maximum possible image size; it
> + * can be influenced using the cluster_size and table_size values.
> + *
> + * All fields are little-endian on disk.
> + */
> +
> +enum {
> +    QED_MAGIC = 'Q' | 'E' << 8 | 'D' << 16 | '\0' << 24,
> +
> +    /* The image supports a backing file */
> +    QED_F_BACKING_FILE = 0x01,
> +
> +    /* The image has the backing file format */
> +    QED_CF_BACKING_FORMAT = 0x01,
> +
> +    /* Feature bits must be used when the on-disk format changes */
> +    QED_FEATURE_MASK = QED_F_BACKING_FILE,            /* supported feature bits */
> +    QED_COMPAT_FEATURE_MASK = QED_CF_BACKING_FORMAT,  /* supported compat feature bits */
> +
> +    /* Data is stored in groups of sectors called clusters.  Cluster size must
> +     * be large to avoid keeping too much metadata.  I/O requests that have
> +     * sub-cluster size will require read-modify-write.
> +     */
> +    QED_MIN_CLUSTER_SIZE = 4 * 1024, /* in bytes */
> +    QED_MAX_CLUSTER_SIZE = 64 * 1024 * 1024,
> +    QED_DEFAULT_CLUSTER_SIZE = 64 * 1024,
> +
> +    /* Allocated clusters are tracked using a 2-level pagetable.  Table size is
> +     * a multiple of clusters so large maximum image sizes can be supported
> +     * without jacking up the cluster size too much.
> +     */
> +    QED_MIN_TABLE_SIZE = 1,        /* in clusters */
> +    QED_MAX_TABLE_SIZE = 16,
> +    QED_DEFAULT_TABLE_SIZE = 4,
> +};
> +
> +typedef struct {
> +    uint32_t magic;                 /* QED\0 */
> +
> +    uint32_t cluster_size;          /* in bytes */
> +    uint32_t table_size;            /* for L1 and L2 tables, in clusters */
> +    uint32_t header_size;           /* in clusters */
> +
> +    uint64_t features;              /* format feature bits */
> +    uint64_t compat_features;       /* compatible feature bits */
> +    uint64_t l1_table_offset;       /* in bytes */
> +    uint64_t image_size;            /* total logical image size, in bytes */
> +
> +    /* if (features & QED_F_BACKING_FILE) */
> +    uint32_t backing_filename_offset; /* in bytes from start of header */
> +    uint32_t backing_filename_size;   /* in bytes */
> +
> +    /* if (compat_features & QED_CF_BACKING_FORMAT) */
> +    uint32_t backing_fmt_offset;    /* in bytes from start of header */
> +    uint32_t backing_fmt_size;      /* in bytes */
> +} QEDHeader;
> +
> +typedef struct {
> +    BlockDriverState *bs;           /* device */
> +    uint64_t file_size;             /* length of image file, in bytes */
> +
> +    QEDHeader header;               /* always cpu-endian */
> +    uint32_t table_nelems;
> +    uint32_t l1_shift;
> +    uint32_t l2_shift;
> +    uint32_t l2_mask;
> +} BDRVQEDState;
> +
> +/**
> + * Utility functions

Sure that this is the right description for this function? ;-)

> + */
> +static inline uint64_t qed_start_of_cluster(BDRVQEDState *s, uint64_t offset)
> +{
> +    return offset & ~(uint64_t)(s->header.cluster_size - 1);
> +}
> +
> +/**
> + * Test if a cluster offset is valid
> + */
> +static inline bool qed_check_cluster_offset(BDRVQEDState *s, uint64_t offset)
> +{
> +    uint64_t header_size = (uint64_t)s->header.header_size *
> +                           s->header.cluster_size;
> +
> +    if (offset & (s->header.cluster_size - 1)) {
> +        return false;
> +    }
> +    return offset >= header_size && offset < s->file_size;
> +}
> +
> +/**
> + * Test if a table offset is valid
> + */
> +static inline bool qed_check_table_offset(BDRVQEDState *s, uint64_t offset)
> +{
> +    uint64_t end_offset = offset + (s->header.table_size - 1) *
> +                          s->header.cluster_size;
> +
> +    /* Overflow check */
> +    if (end_offset <= offset) {
> +        return false;
> +    }
> +
> +    return qed_check_cluster_offset(s, offset) &&
> +           qed_check_cluster_offset(s, end_offset);
> +}
> +
> +#endif /* BLOCK_QED_H */

Kevin