block/cow.c - qemu - Git at Google

 /*
  * Block driver for the COW format
  *
  * Copyright (c) 2004 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/block_int.h"
 #include "qemu/module.h"

 /**************************************************************/
 /* COW block driver using file system holes */

 /* user mode linux compatible COW file */
 #define COW_MAGIC 0x4f4f4f4d  /* MOOO */
 #define COW_VERSION 2

 struct cow_header_v2 {
     uint32_t magic;
     uint32_t version;
     char backing_file[1024];
     int32_t mtime;
     uint64_t size;
     uint32_t sectorsize;
 };

 typedef struct BDRVCowState {
     CoMutex lock;
     int64_t cow_sectors_offset;
 } BDRVCowState;

 static int cow_probe(const uint8_t *buf, int buf_size, const char *filename)
 {
     const struct cow_header_v2 *cow_header = (const void *)buf;

     if (buf_size >= sizeof(struct cow_header_v2) &&
         be32_to_cpu(cow_header->magic) == COW_MAGIC &&
         be32_to_cpu(cow_header->version) == COW_VERSION)
         return 100;
     else
         return 0;
 }

 static int cow_open(BlockDriverState *bs, QDict *options, int flags,
                     Error **errp)
 {
     BDRVCowState *s = bs->opaque;
     struct cow_header_v2 cow_header;
     int bitmap_size;
     int64_t size;
     int ret;

     /* see if it is a cow image */
     ret = bdrv_pread(bs->file, 0, &cow_header, sizeof(cow_header));
     if (ret < 0) {
         goto fail;
     }

     if (be32_to_cpu(cow_header.magic) != COW_MAGIC) {
         ret = -EMEDIUMTYPE;
         goto fail;
     }

     if (be32_to_cpu(cow_header.version) != COW_VERSION) {
         char version[64];
         snprintf(version, sizeof(version),
                "COW version %d", cow_header.version);
         qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
             bs->device_name, "cow", version);
         ret = -ENOTSUP;
         goto fail;
     }

     /* cow image found */
     size = be64_to_cpu(cow_header.size);
     bs->total_sectors = size / 512;

     pstrcpy(bs->backing_file, sizeof(bs->backing_file),
             cow_header.backing_file);

     bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
     s->cow_sectors_offset = (bitmap_size + 511) & ~511;
     qemu_co_mutex_init(&s->lock);
     return 0;
  fail:
     return ret;
 }

 static inline void cow_set_bits(uint8_t *bitmap, int start, int64_t nb_sectors)
 {
     int64_t bitnum = start, last = start + nb_sectors;
     while (bitnum < last) {
         if ((bitnum & 7) == 0 && bitnum + 8 <= last) {
             bitmap[bitnum / 8] = 0xFF;
             bitnum += 8;
             continue;
         }
         bitmap[bitnum/8] |= (1 << (bitnum % 8));
         bitnum++;
     }
 }

 #define BITS_PER_BITMAP_SECTOR (512 * 8)

 /* Cannot use bitmap.c on big-endian machines.  */
 static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)
 {
     return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;
 }

 static int cow_find_streak(const uint8_t *bitmap, int value, int start, int nb_sectors)
 {
     int streak_value = value ? 0xFF : 0;
     int last = MIN(start + nb_sectors, BITS_PER_BITMAP_SECTOR);
     int bitnum = start;
     while (bitnum < last) {
         if ((bitnum & 7) == 0 && bitmap[bitnum / 8] == streak_value) {
             bitnum += 8;
             continue;
         }
         if (cow_test_bit(bitnum, bitmap) == value) {
             bitnum++;
             continue;
         }
         break;
     }
     return MIN(bitnum, last) - start;
 }

 /* Return true if first block has been changed (ie. current version is
  * in COW file).  Set the number of continuous blocks for which that
  * is true. */
 static int coroutine_fn cow_co_is_allocated(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, int *num_same)
 {
     int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
     uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
     bool first = true;
     int changed = 0, same = 0;

     do {
         int ret;
         uint8_t bitmap[BDRV_SECTOR_SIZE];

         bitnum &= BITS_PER_BITMAP_SECTOR - 1;
         int sector_bits = MIN(nb_sectors, BITS_PER_BITMAP_SECTOR - bitnum);

         ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
         if (ret < 0) {
             return ret;
         }

         if (first) {
             changed = cow_test_bit(bitnum, bitmap);
             first = false;
         }

         same += cow_find_streak(bitmap, changed, bitnum, nb_sectors);

         bitnum += sector_bits;
         nb_sectors -= sector_bits;
         offset += BDRV_SECTOR_SIZE;
     } while (nb_sectors);

     *num_same = same;
     return changed;
 }

 static int64_t coroutine_fn cow_co_get_block_status(BlockDriverState *bs,
         int64_t sector_num, int nb_sectors, int *num_same)
 {
     BDRVCowState *s = bs->opaque;
     int ret = cow_co_is_allocated(bs, sector_num, nb_sectors, num_same);
     int64_t offset = s->cow_sectors_offset + (sector_num << BDRV_SECTOR_BITS);
     if (ret < 0) {
         return ret;
     }
     return (ret ? BDRV_BLOCK_DATA : 0) | offset | BDRV_BLOCK_OFFSET_VALID;
 }

 static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
         int nb_sectors)
 {
     int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
     uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
     bool first = true;
     int sector_bits;

     for ( ; nb_sectors;
             bitnum += sector_bits,
             nb_sectors -= sector_bits,
             offset += BDRV_SECTOR_SIZE) {
         int ret, set;
         uint8_t bitmap[BDRV_SECTOR_SIZE];

         bitnum &= BITS_PER_BITMAP_SECTOR - 1;
         sector_bits = MIN(nb_sectors, BITS_PER_BITMAP_SECTOR - bitnum);

         ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
         if (ret < 0) {
             return ret;
         }

         /* Skip over any already set bits */
         set = cow_find_streak(bitmap, 1, bitnum, sector_bits);
         bitnum += set;
         sector_bits -= set;
         nb_sectors -= set;
         if (!sector_bits) {
             continue;
         }

         if (first) {
             ret = bdrv_flush(bs->file);
             if (ret < 0) {
                 return ret;
             }
             first = false;
         }

         cow_set_bits(bitmap, bitnum, sector_bits);

         ret = bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap));
         if (ret < 0) {
             return ret;
         }
     }

     return 0;
 }

 static int coroutine_fn cow_read(BlockDriverState *bs, int64_t sector_num,
                                  uint8_t *buf, int nb_sectors)
 {
     BDRVCowState *s = bs->opaque;
     int ret, n;

     while (nb_sectors > 0) {
         ret = cow_co_is_allocated(bs, sector_num, nb_sectors, &n);
         if (ret < 0) {
             return ret;
         }
         if (ret) {
             ret = bdrv_pread(bs->file,
                         s->cow_sectors_offset + sector_num * 512,
                         buf, n * 512);
             if (ret < 0) {
                 return ret;
             }
         } else {
             if (bs->backing_hd) {
                 /* read from the base image */
                 ret = bdrv_read(bs->backing_hd, sector_num, buf, n);
                 if (ret < 0) {
                     return ret;
                 }
             } else {
                 memset(buf, 0, n * 512);
             }
         }
         nb_sectors -= n;
         sector_num += n;
         buf += n * 512;
     }
     return 0;
 }

 static coroutine_fn int cow_co_read(BlockDriverState *bs, int64_t sector_num,
                                     uint8_t *buf, int nb_sectors)
 {
     int ret;
     BDRVCowState *s = bs->opaque;
     qemu_co_mutex_lock(&s->lock);
     ret = cow_read(bs, sector_num, buf, nb_sectors);
     qemu_co_mutex_unlock(&s->lock);
     return ret;
 }

 static int cow_write(BlockDriverState *bs, int64_t sector_num,
                      const uint8_t *buf, int nb_sectors)
 {
     BDRVCowState *s = bs->opaque;
     int ret;

     ret = bdrv_pwrite(bs->file, s->cow_sectors_offset + sector_num * 512,
                       buf, nb_sectors * 512);
     if (ret < 0) {
         return ret;
     }

     return cow_update_bitmap(bs, sector_num, nb_sectors);
 }

 static coroutine_fn int cow_co_write(BlockDriverState *bs, int64_t sector_num,
                                      const uint8_t *buf, int nb_sectors)
 {
     int ret;
     BDRVCowState *s = bs->opaque;
     qemu_co_mutex_lock(&s->lock);
     ret = cow_write(bs, sector_num, buf, nb_sectors);
     qemu_co_mutex_unlock(&s->lock);
     return ret;
 }

 static void cow_close(BlockDriverState *bs)
 {
 }

 static int cow_create(const char *filename, QEMUOptionParameter *options,
                       Error **errp)
 {
     struct cow_header_v2 cow_header;
     struct stat st;
     int64_t image_sectors = 0;
     const char *image_filename = NULL;
     Error *local_err = NULL;
     int ret;
     BlockDriverState *cow_bs;

     /* Read out options */
     while (options && options->name) {
         if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
             image_sectors = options->value.n / 512;
         } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
             image_filename = options->value.s;
         }
         options++;
     }

     ret = bdrv_create_file(filename, options, &local_err);
     if (ret < 0) {
         qerror_report_err(local_err);
         error_free(local_err);
         return ret;
     }

     ret = bdrv_file_open(&cow_bs, filename, NULL, NULL, BDRV_O_RDWR,
                          &local_err);
     if (ret < 0) {
         qerror_report_err(local_err);
         error_free(local_err);
         return ret;
     }

     memset(&cow_header, 0, sizeof(cow_header));
     cow_header.magic = cpu_to_be32(COW_MAGIC);
     cow_header.version = cpu_to_be32(COW_VERSION);
     if (image_filename) {
         /* Note: if no file, we put a dummy mtime */
         cow_header.mtime = cpu_to_be32(0);

         if (stat(image_filename, &st) != 0) {
             goto mtime_fail;
         }
         cow_header.mtime = cpu_to_be32(st.st_mtime);
     mtime_fail:
         pstrcpy(cow_header.backing_file, sizeof(cow_header.backing_file),
                 image_filename);
     }
     cow_header.sectorsize = cpu_to_be32(512);
     cow_header.size = cpu_to_be64(image_sectors * 512);
     ret = bdrv_pwrite(cow_bs, 0, &cow_header, sizeof(cow_header));
     if (ret < 0) {
         goto exit;
     }

     /* resize to include at least all the bitmap */
     ret = bdrv_truncate(cow_bs,
         sizeof(cow_header) + ((image_sectors + 7) >> 3));
     if (ret < 0) {
         goto exit;
     }

 exit:
     bdrv_unref(cow_bs);
     return ret;
 }

 static QEMUOptionParameter cow_create_options[] = {
     {
         .name = BLOCK_OPT_SIZE,
         .type = OPT_SIZE,
         .help = "Virtual disk size"
     },
     {
         .name = BLOCK_OPT_BACKING_FILE,
         .type = OPT_STRING,
         .help = "File name of a base image"
     },
     { NULL }
 };

 static BlockDriver bdrv_cow = {
     .format_name    = "cow",
     .instance_size  = sizeof(BDRVCowState),

     .bdrv_probe     = cow_probe,
     .bdrv_open      = cow_open,
     .bdrv_close     = cow_close,
     .bdrv_create    = cow_create,
     .bdrv_has_zero_init     = bdrv_has_zero_init_1,

     .bdrv_read              = cow_co_read,
     .bdrv_write             = cow_co_write,
     .bdrv_co_get_block_status   = cow_co_get_block_status,

     .create_options = cow_create_options,
 };

 static void bdrv_cow_init(void)
 {
     bdrv_register(&bdrv_cow);
 }

 block_init(bdrv_cow_init);
	/*
	* Block driver for the COW format
	*
	* Copyright (c) 2004 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/block_int.h"
	#include "qemu/module.h"

	/**************************************************************/
	/* COW block driver using file system holes */

	/* user mode linux compatible COW file */
	#define COW_MAGIC 0x4f4f4f4d /* MOOO */
	#define COW_VERSION 2

	struct cow_header_v2 {
	uint32_t magic;
	uint32_t version;
	char backing_file[1024];
	int32_t mtime;
	uint64_t size;
	uint32_t sectorsize;
	};

	typedef struct BDRVCowState {
	CoMutex lock;
	int64_t cow_sectors_offset;
	} BDRVCowState;

	static int cow_probe(const uint8_t buf, int buf_size, const char filename)
	{
	const struct cow_header_v2 cow_header = (const void )buf;

	if (buf_size >= sizeof(struct cow_header_v2) &&
	be32_to_cpu(cow_header->magic) == COW_MAGIC &&
	be32_to_cpu(cow_header->version) == COW_VERSION)
	return 100;
	else
	return 0;
	}

	static int cow_open(BlockDriverState bs, QDict options, int flags,
	Error **errp)
	{
	BDRVCowState *s = bs->opaque;
	struct cow_header_v2 cow_header;
	int bitmap_size;
	int64_t size;
	int ret;

	/* see if it is a cow image */
	ret = bdrv_pread(bs->file, 0, &cow_header, sizeof(cow_header));
	if (ret < 0) {
	goto fail;
	}

	if (be32_to_cpu(cow_header.magic) != COW_MAGIC) {
	ret = -EMEDIUMTYPE;
	goto fail;
	}

	if (be32_to_cpu(cow_header.version) != COW_VERSION) {
	char version[64];
	snprintf(version, sizeof(version),
	"COW version %d", cow_header.version);
	qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,
	bs->device_name, "cow", version);
	ret = -ENOTSUP;
	goto fail;
	}

	/* cow image found */
	size = be64_to_cpu(cow_header.size);
	bs->total_sectors = size / 512;

	pstrcpy(bs->backing_file, sizeof(bs->backing_file),
	cow_header.backing_file);

	bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
	s->cow_sectors_offset = (bitmap_size + 511) & ~511;
	qemu_co_mutex_init(&s->lock);
	return 0;
	fail:
	return ret;
	}

	static inline void cow_set_bits(uint8_t *bitmap, int start, int64_t nb_sectors)
	{
	int64_t bitnum = start, last = start + nb_sectors;
	while (bitnum < last) {
	if ((bitnum & 7) == 0 && bitnum + 8 <= last) {
	bitmap[bitnum / 8] = 0xFF;
	bitnum += 8;
	continue;
	}
	bitmap[bitnum/8] \|= (1 << (bitnum % 8));
	bitnum++;
	}
	}

	#define BITS_PER_BITMAP_SECTOR (512 * 8)

	/* Cannot use bitmap.c on big-endian machines. */
	static int cow_test_bit(int64_t bitnum, const uint8_t *bitmap)
	{
	return (bitmap[bitnum / 8] & (1 << (bitnum & 7))) != 0;
	}

	static int cow_find_streak(const uint8_t *bitmap, int value, int start, int nb_sectors)
	{
	int streak_value = value ? 0xFF : 0;
	int last = MIN(start + nb_sectors, BITS_PER_BITMAP_SECTOR);
	int bitnum = start;
	while (bitnum < last) {
	if ((bitnum & 7) == 0 && bitmap[bitnum / 8] == streak_value) {
	bitnum += 8;
	continue;
	}
	if (cow_test_bit(bitnum, bitmap) == value) {
	bitnum++;
	continue;
	}
	break;
	}
	return MIN(bitnum, last) - start;
	}

	/* Return true if first block has been changed (ie. current version is
	* in COW file). Set the number of continuous blocks for which that
	* is true. */
	static int coroutine_fn cow_co_is_allocated(BlockDriverState *bs,
	int64_t sector_num, int nb_sectors, int *num_same)
	{
	int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
	uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
	bool first = true;
	int changed = 0, same = 0;

	do {
	int ret;
	uint8_t bitmap[BDRV_SECTOR_SIZE];

	bitnum &= BITS_PER_BITMAP_SECTOR - 1;
	int sector_bits = MIN(nb_sectors, BITS_PER_BITMAP_SECTOR - bitnum);

	ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
	if (ret < 0) {
	return ret;
	}

	if (first) {
	changed = cow_test_bit(bitnum, bitmap);
	first = false;
	}

	same += cow_find_streak(bitmap, changed, bitnum, nb_sectors);

	bitnum += sector_bits;
	nb_sectors -= sector_bits;
	offset += BDRV_SECTOR_SIZE;
	} while (nb_sectors);

	*num_same = same;
	return changed;
	}

	static int64_t coroutine_fn cow_co_get_block_status(BlockDriverState *bs,
	int64_t sector_num, int nb_sectors, int *num_same)
	{
	BDRVCowState *s = bs->opaque;
	int ret = cow_co_is_allocated(bs, sector_num, nb_sectors, num_same);
	int64_t offset = s->cow_sectors_offset + (sector_num << BDRV_SECTOR_BITS);
	if (ret < 0) {
	return ret;
	}
	return (ret ? BDRV_BLOCK_DATA : 0) \| offset \| BDRV_BLOCK_OFFSET_VALID;
	}

	static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
	int nb_sectors)
	{
	int64_t bitnum = sector_num + sizeof(struct cow_header_v2) * 8;
	uint64_t offset = (bitnum / 8) & -BDRV_SECTOR_SIZE;
	bool first = true;
	int sector_bits;

	for ( ; nb_sectors;
	bitnum += sector_bits,
	nb_sectors -= sector_bits,
	offset += BDRV_SECTOR_SIZE) {
	int ret, set;
	uint8_t bitmap[BDRV_SECTOR_SIZE];

	bitnum &= BITS_PER_BITMAP_SECTOR - 1;
	sector_bits = MIN(nb_sectors, BITS_PER_BITMAP_SECTOR - bitnum);

	ret = bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap));
	if (ret < 0) {
	return ret;
	}

	/* Skip over any already set bits */
	set = cow_find_streak(bitmap, 1, bitnum, sector_bits);
	bitnum += set;
	sector_bits -= set;
	nb_sectors -= set;
	if (!sector_bits) {
	continue;
	}

	if (first) {
	ret = bdrv_flush(bs->file);
	if (ret < 0) {
	return ret;
	}
	first = false;
	}

	cow_set_bits(bitmap, bitnum, sector_bits);

	ret = bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap));
	if (ret < 0) {
	return ret;
	}
	}

	return 0;
	}

	static int coroutine_fn cow_read(BlockDriverState *bs, int64_t sector_num,
	uint8_t *buf, int nb_sectors)
	{
	BDRVCowState *s = bs->opaque;
	int ret, n;

	while (nb_sectors > 0) {
	ret = cow_co_is_allocated(bs, sector_num, nb_sectors, &n);
	if (ret < 0) {
	return ret;
	}
	if (ret) {
	ret = bdrv_pread(bs->file,
	s->cow_sectors_offset + sector_num * 512,
	buf, n * 512);
	if (ret < 0) {
	return ret;
	}
	} else {
	if (bs->backing_hd) {
	/* read from the base image */
	ret = bdrv_read(bs->backing_hd, sector_num, buf, n);
	if (ret < 0) {
	return ret;
	}
	} else {
	memset(buf, 0, n * 512);
	}
	}
	nb_sectors -= n;
	sector_num += n;
	buf += n * 512;
	}
	return 0;
	}

	static coroutine_fn int cow_co_read(BlockDriverState *bs, int64_t sector_num,
	uint8_t *buf, int nb_sectors)
	{
	int ret;
	BDRVCowState *s = bs->opaque;
	qemu_co_mutex_lock(&s->lock);
	ret = cow_read(bs, sector_num, buf, nb_sectors);
	qemu_co_mutex_unlock(&s->lock);
	return ret;
	}

	static int cow_write(BlockDriverState *bs, int64_t sector_num,
	const uint8_t *buf, int nb_sectors)
	{
	BDRVCowState *s = bs->opaque;
	int ret;

	ret = bdrv_pwrite(bs->file, s->cow_sectors_offset + sector_num * 512,
	buf, nb_sectors * 512);
	if (ret < 0) {
	return ret;
	}

	return cow_update_bitmap(bs, sector_num, nb_sectors);
	}

	static coroutine_fn int cow_co_write(BlockDriverState *bs, int64_t sector_num,
	const uint8_t *buf, int nb_sectors)
	{
	int ret;
	BDRVCowState *s = bs->opaque;
	qemu_co_mutex_lock(&s->lock);
	ret = cow_write(bs, sector_num, buf, nb_sectors);
	qemu_co_mutex_unlock(&s->lock);
	return ret;
	}

	static void cow_close(BlockDriverState *bs)
	{
	}

	static int cow_create(const char filename, QEMUOptionParameter options,
	Error **errp)
	{
	struct cow_header_v2 cow_header;
	struct stat st;
	int64_t image_sectors = 0;
	const char *image_filename = NULL;
	Error *local_err = NULL;
	int ret;
	BlockDriverState *cow_bs;

	/* Read out options */
	while (options && options->name) {
	if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
	image_sectors = options->value.n / 512;
	} else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
	image_filename = options->value.s;
	}
	options++;
	}

	ret = bdrv_create_file(filename, options, &local_err);
	if (ret < 0) {
	qerror_report_err(local_err);
	error_free(local_err);
	return ret;
	}

	ret = bdrv_file_open(&cow_bs, filename, NULL, NULL, BDRV_O_RDWR,
	&local_err);
	if (ret < 0) {
	qerror_report_err(local_err);
	error_free(local_err);
	return ret;
	}

	memset(&cow_header, 0, sizeof(cow_header));
	cow_header.magic = cpu_to_be32(COW_MAGIC);
	cow_header.version = cpu_to_be32(COW_VERSION);
	if (image_filename) {
	/* Note: if no file, we put a dummy mtime */
	cow_header.mtime = cpu_to_be32(0);

	if (stat(image_filename, &st) != 0) {
	goto mtime_fail;
	}
	cow_header.mtime = cpu_to_be32(st.st_mtime);
	mtime_fail:
	pstrcpy(cow_header.backing_file, sizeof(cow_header.backing_file),
	image_filename);
	}
	cow_header.sectorsize = cpu_to_be32(512);
	cow_header.size = cpu_to_be64(image_sectors * 512);
	ret = bdrv_pwrite(cow_bs, 0, &cow_header, sizeof(cow_header));
	if (ret < 0) {
	goto exit;
	}

	/* resize to include at least all the bitmap */
	ret = bdrv_truncate(cow_bs,
	sizeof(cow_header) + ((image_sectors + 7) >> 3));
	if (ret < 0) {
	goto exit;
	}

	exit:
	bdrv_unref(cow_bs);
	return ret;
	}

	static QEMUOptionParameter cow_create_options[] = {
	{
	.name = BLOCK_OPT_SIZE,
	.type = OPT_SIZE,
	.help = "Virtual disk size"
	},
	{
	.name = BLOCK_OPT_BACKING_FILE,
	.type = OPT_STRING,
	.help = "File name of a base image"
	},
	{ NULL }
	};

	static BlockDriver bdrv_cow = {
	.format_name = "cow",
	.instance_size = sizeof(BDRVCowState),

	.bdrv_probe = cow_probe,
	.bdrv_open = cow_open,
	.bdrv_close = cow_close,
	.bdrv_create = cow_create,
	.bdrv_has_zero_init = bdrv_has_zero_init_1,

	.bdrv_read = cow_co_read,
	.bdrv_write = cow_co_write,
	.bdrv_co_get_block_status = cow_co_get_block_status,

	.create_options = cow_create_options,
	};

	static void bdrv_cow_init(void)
	{
	bdrv_register(&bdrv_cow);
	}

	block_init(bdrv_cow_init);