static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
                    Error **errp)
{
    BDRVVPCState *s = bs->opaque;
    int i;
    VHDFooter *footer;
    VHDDynDiskHeader *dyndisk_header;
    uint8_t buf[HEADER_SIZE];
    uint32_t checksum;
    int disk_type = VHD_DYNAMIC;
    int ret;

    ret = bdrv_pread(bs->file, 0, s->footer_buf, HEADER_SIZE);
    if (ret < 0) {
        goto fail;
    }

    footer = (VHDFooter *) s->footer_buf;
    if (strncmp(footer->creator, "conectix", 8)) {
        int64_t offset = bdrv_getlength(bs->file);
        if (offset < 0) {
            ret = offset;
            goto fail;
        } else if (offset < HEADER_SIZE) {
            ret = -EINVAL;
            goto fail;
        }

        /* If a fixed disk, the footer is found only at the end of the file */
        ret = bdrv_pread(bs->file, offset-HEADER_SIZE, s->footer_buf,
                         HEADER_SIZE);
        if (ret < 0) {
            goto fail;
        }
        if (strncmp(footer->creator, "conectix", 8)) {
            error_setg(errp, "invalid VPC image");
            ret = -EINVAL;
            goto fail;
        }
        disk_type = VHD_FIXED;
    }

    checksum = be32_to_cpu(footer->checksum);
    footer->checksum = 0;
    if (vpc_checksum(s->footer_buf, HEADER_SIZE) != checksum)
        fprintf(stderr, "block-vpc: The header checksum of '%s' is "
            "incorrect.\n", bs->filename);

    /* Write 'checksum' back to footer, or else will leave it with zero. */
    footer->checksum = be32_to_cpu(checksum);

    // The visible size of a image in Virtual PC depends on the geometry
    // rather than on the size stored in the footer (the size in the footer
    // is too large usually)
    bs->total_sectors = (int64_t)
        be16_to_cpu(footer->cyls) * footer->heads * footer->secs_per_cyl;

    /* images created with disk2vhd report a far higher virtual size
     * than expected with the cyls * heads * sectors_per_cyl formula.
     * use the footer->size instead if the image was created with
     * disk2vhd.
     */
    if (!strncmp(footer->creator_app, "d2v", 4)) {
        bs->total_sectors = be64_to_cpu(footer->size) / BDRV_SECTOR_SIZE;
    }

    /* Allow a maximum disk size of approximately 2 TB */
    if (bs->total_sectors >= 65535LL * 255 * 255) {
        ret = -EFBIG;
        goto fail;
    }

    if (disk_type == VHD_DYNAMIC) {
        ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), buf,
                         HEADER_SIZE);
        if (ret < 0) {
            goto fail;
        }

        dyndisk_header = (VHDDynDiskHeader *) buf;

        if (strncmp(dyndisk_header->magic, "cxsparse", 8)) {
            ret = -EINVAL;
            goto fail;
        }

        s->block_size = be32_to_cpu(dyndisk_header->block_size);
        s->bitmap_size = ((s->block_size / (8 * 512)) + 511) & ~511;

        s->max_table_entries = be32_to_cpu(dyndisk_header->max_table_entries);
        s->pagetable = g_malloc(s->max_table_entries * 4);

        s->bat_offset = be64_to_cpu(dyndisk_header->table_offset);

        ret = bdrv_pread(bs->file, s->bat_offset, s->pagetable,
                         s->max_table_entries * 4);
        if (ret < 0) {
            goto fail;
        }

        s->free_data_block_offset =
            (s->bat_offset + (s->max_table_entries * 4) + 511) & ~511;

        for (i = 0; i < s->max_table_entries; i++) {
            be32_to_cpus(&s->pagetable[i]);
            if (s->pagetable[i] != 0xFFFFFFFF) {
                int64_t next = (512 * (int64_t) s->pagetable[i]) +
                    s->bitmap_size + s->block_size;

                if (next > s->free_data_block_offset) {
                    s->free_data_block_offset = next;
                }
            }
        }

        if (s->free_data_block_offset > bdrv_getlength(bs->file)) {
            error_setg(errp, "block-vpc: free_data_block_offset points after "
                             "the end of file. The image has been truncated.");
            ret = -EINVAL;
            goto fail;
        }

        s->last_bitmap_offset = (int64_t) -1;

#ifdef CACHE
        s->pageentry_u8 = g_malloc(512);
        s->pageentry_u32 = s->pageentry_u8;
        s->pageentry_u16 = s->pageentry_u8;
        s->last_pagetable = -1;
#endif
    }

    qemu_co_mutex_init(&s->lock);

    /* Disable migration when VHD images are used */
    error_set(&s->migration_blocker,
              QERR_BLOCK_FORMAT_FEATURE_NOT_SUPPORTED,
              "vpc", bs->device_name, "live migration");
    migrate_add_blocker(s->migration_blocker);

    return 0;

fail:
    g_free(s->pagetable);
#ifdef CACHE
    g_free(s->pageentry_u8);
#endif
    return ret;
}