[RESEND] Support vhd type VHD_DIFFERENCING

Now qemu only supports vhd type VHD_FIXED and VHD_DYNAMIC, so qemu
can't read snapshot volume of vhd, and can't support other storage
features of vhd file.

This patch add read parent information in function "vpc_open", read
bitmap in "vpc_read", and change bitmap in "vpc_write".

Signed-off-by: Xiaodong Gong <gongxiaodong1@huawei.com>
---
Changes since v4:
- Parse the batmap only when the version of VHD > 1.2.
- Add support to parent location of W2RU.

Changes since v3:
- Remove the PARENT_MAX_LOC.

Changes since v2:
- Change MACX to PLATFAORM_MACX.
- Return with EINVAL to parent location is W2RU and W2KU.
- Change -1 == ret to a natrual order of ret == -1.
- Get rid of the get_sector_offset_diff, get_sector_offset
  supports VHD_DIFF.
- Return code of get_sector_offset is set to, -1 for error,
  -2 for not allocate, -3 for in parent.
- Fix un init ret of vpc_write, when nb_sector == 0.
- Change if (diff == ture) to if (diff) and so on.
- Add PARENT_MAX_LOC to more understand.
- Restore the boundary check to write on dynamic type in
  get_sector_offset.

Changes since v1:
- Add Boundary check to any input. 
- Clean the code no used after in vpc_open.
- Change bdrv_co_readv() to bdrv_preadv in vpc_read.
- Added some code to make it easy to understand.
---
 block/vpc.c | 428 ++++++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 357 insertions(+), 71 deletions(-)

On 2014/10/11 0:17, Xiaodong Gong wrote:

> Now qemu only supports vhd type VHD_FIXED and VHD_DYNAMIC, so qemu
> can't read snapshot volume of vhd, and can't support other storage
> features of vhd file.
> 
> This patch add read parent information in function "vpc_open", read
> bitmap in "vpc_read", and change bitmap in "vpc_write".
> 
> Signed-off-by: Xiaodong Gong <gongxiaodong1@huawei.com>
> ---
> Changes since v4:
> - Parse the batmap only when the version of VHD > 1.2.
> - Add support to parent location of W2RU.
> 
> Changes since v3:
> - Remove the PARENT_MAX_LOC.
> 
> Changes since v2:
> - Change MACX to PLATFAORM_MACX.
> - Return with EINVAL to parent location is W2RU and W2KU.
> - Change -1 == ret to a natrual order of ret == -1.
> - Get rid of the get_sector_offset_diff, get_sector_offset
>   supports VHD_DIFF.
> - Return code of get_sector_offset is set to, -1 for error,
>   -2 for not allocate, -3 for in parent.
> - Fix un init ret of vpc_write, when nb_sector == 0.
> - Change if (diff == ture) to if (diff) and so on.
> - Add PARENT_MAX_LOC to more understand.
> - Restore the boundary check to write on dynamic type in
>   get_sector_offset.
> 
> Changes since v1:
> - Add Boundary check to any input. 
> - Clean the code no used after in vpc_open.
> - Change bdrv_co_readv() to bdrv_preadv in vpc_read.
> - Added some code to make it easy to understand.
> ---
>  block/vpc.c | 428 ++++++++++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 357 insertions(+), 71 deletions(-)
> 
> diff --git a/block/vpc.c b/block/vpc.c
> index 4947369..1210542 100644
> --- a/block/vpc.c
> +++ b/block/vpc.c
> @@ -29,17 +29,27 @@
>  #if defined(CONFIG_UUID)
>  #include <uuid/uuid.h>
>  #endif
> +#include <iconv.h>
>  
>  /**************************************************************/
>  
>  #define HEADER_SIZE 512
> +#define DYNAMIC_HEADER_SIZE 1024
> +#define PARENT_LOCATOR_NUM 8
> +#define MACX_PREFIX_LEN 7 /* file:// */
> +#define TBBATMAP_HEAD_SIZE 28
> +
> +#define PLATFORM_MACX 0x5863614d /* big endian */
> +#define PLATFORM_W2RU 0x75723257
> +
> +#define VHD_VERSION(major, minor)  (((major) << 16) | ((minor) & 0x0000FFFF))
>  
>  //#define CACHE
>  
>  enum vhd_type {
>      VHD_FIXED           = 2,
>      VHD_DYNAMIC         = 3,
> -    VHD_DIFFERENCING    = 4,
> +    VHD_DIFF            = 4,
>  };
>  
>  // Seconds since Jan 1, 2000 0:00:00 (UTC)
> @@ -138,6 +148,15 @@ typedef struct BDRVVPCState {
>      Error *migration_blocker;
>  } BDRVVPCState;
>  
> +typedef struct vhd_tdbatmap_header {
> +    char magic[8]; /* always "tdbatmap" */
> +
> +    uint64_t batmap_offset;
> +    uint32_t batmap_size;
> +    uint32_t batmap_version;
> +    uint32_t checksum;
> +} QEMU_PACKED VHDTdBatmapHeader;
> +
>  static uint32_t vpc_checksum(uint8_t* buf, size_t size)
>  {
>      uint32_t res = 0;
> @@ -153,10 +172,107 @@ static uint32_t vpc_checksum(uint8_t* buf, size_t size)
>  static int vpc_probe(const uint8_t *buf, int buf_size, const char *filename)
>  {
>      if (buf_size >= 8 && !strncmp((char *)buf, "conectix", 8))
> -	return 100;
> +        return 100;


Adjusting coding style can be made into a separate patch in vpc.c file.
Such as using '{}' at if conditional statement.

>      return 0;
>  }
>  
> +static int vpc_read_backing_loc(VHDDynDiskHeader *dyndisk_header,
> +                                BlockDriverState *bs,
> +                                Error **errp)
> +{
> +    BDRVVPCState *s = bs->opaque;
> +    int64_t data_offset = 0;
> +    int data_length = 0;
> +    uint32_t platform;
> +    bool done = false;
> +    int parent_locator_offset = 0;
> +    int i;
> +    int ret = 0;
> +
> +    for (i = 0; i < PARENT_LOCATOR_NUM; i++) {
> +        data_offset =
> +            be64_to_cpu(dyndisk_header->parent_locator[i].data_offset);
> +        data_length =
> +            be32_to_cpu(dyndisk_header->parent_locator[i].data_length);
> +        platform = dyndisk_header->parent_locator[i].platform;
> +
> +        /* Extend the location offset */
> +        if (parent_locator_offset < data_offset) {
> +            parent_locator_offset = data_offset;
> +        }
> +
> +        if (done) {
> +            continue;
> +        }
> +
> +        /* Skip "file://" in MacX platform */
> +        if (platform == PLATFORM_MACX) {
> +            data_offset += MACX_PREFIX_LEN;
> +            data_length -= MACX_PREFIX_LEN;
> +        }
> +
> +        /* Read location of backing file */
> +        if (platform == PLATFORM_MACX || platform == PLATFORM_W2RU) {
> +            if (data_offset > s->max_table_entries * s->block_size) {
> +                return -1;
> +            }
> +            if (data_length > BDRV_SECTOR_SIZE) {
> +                return -1;
> +            }
> +            ret = bdrv_pread(bs->file, data_offset, bs->backing_file,
> +                data_length);
> +            if (ret < 0) {
> +                return ret;
> +            }
> +            bs->backing_file[data_length] = '\0';
> +        }
> +
> +        /* Convert location to ACSII string */
> +        if (platform == PLATFORM_MACX) {
> +            done = true;
> +
> +        } else if (platform == PLATFORM_W2RU) {
> +            /* Must be UTF16-LE to ASCII */
> +            char *out, *optr;
> +            int j;
> +
> +            optr = out = (char *) malloc(data_length + 1);
> +            if (out == NULL) {
> +                ret = -1;
> +                return ret;
> +            }
> +            memset(out, 0, data_length + 1);


Please use g_malloc0() instead of malloc and memset(,0,).

> +
> +            for (j = 0; j < data_length + 1; j++) {
> +                out[j] = bs->backing_file[2*j];
> +            }
> +            out[data_length + 1] = '\0';
> +
> +            while (*optr != '\0') {
> +                if (*optr == '\\') {
> +                    *optr = '/';
> +                }
> +                optr++;
> +            }
> +
> +            strncpy(bs->backing_file, out, data_length + 1);
> +
> +            out = NULL;
> +            free(out);


A bug, right?

> +
> +            done = true;
> +        }
> +    }
> +
> +    if (bs->backing_file[0] == '\0') {
> +        error_setg(errp, "block-vpc: differencing is not support in w2ku");
> +        ret = -EINVAL;
> +        return ret;
> +    }
> +
> +    return parent_locator_offset;
> +}
> +
>  static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>                      Error **errp)
>  {
> @@ -164,11 +280,14 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>      int i;
>      VHDFooter *footer;
>      VHDDynDiskHeader *dyndisk_header;
> -    uint8_t buf[HEADER_SIZE];
> +    uint8_t buf[DYNAMIC_HEADER_SIZE];
> +    uint8_t tdbatmap_header_buf[TBBATMAP_HEAD_SIZE];
>      uint32_t checksum;
>      uint64_t computed_size;
> -    int disk_type = VHD_DYNAMIC;
> +    uint32_t disk_type;
>      int ret;
> +    VHDTdBatmapHeader *tdbatmap_header;
> +    int parent_locator_offset = 0;
>  
>      ret = bdrv_pread(bs->file, 0, s->footer_buf, HEADER_SIZE);
>      if (ret < 0) {
> @@ -176,6 +295,8 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>      }
>  
>      footer = (VHDFooter *) s->footer_buf;
> +    disk_type = be32_to_cpu(footer->type);
> +
>      if (strncmp(footer->creator, "conectix", 8)) {
>          int64_t offset = bdrv_getlength(bs->file);
>          if (offset < 0) {
> @@ -230,9 +351,9 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>          goto fail;
>      }
>  
> -    if (disk_type == VHD_DYNAMIC) {
> +    if (disk_type == VHD_DYNAMIC || disk_type == VHD_DIFF) {
>          ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset), buf,
> -                         HEADER_SIZE);
> +                         DYNAMIC_HEADER_SIZE);
>          if (ret < 0) {
>              goto fail;
>          }
> @@ -286,6 +407,37 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>          s->free_data_block_offset =
>              (s->bat_offset + (s->max_table_entries * 4) + 511) & ~511;
>  
> +        /* Read tdbatmap header by offset */
> +        if (footer->version >= VHD_VERSION(1, 2)) {
> +            ret = bdrv_pread(bs->file, s->free_data_block_offset,
> +                tdbatmap_header_buf, TBBATMAP_HEAD_SIZE);
> +            if (ret < 0) {
> +                goto fail;
> +            }
> +
> +            tdbatmap_header = (VHDTdBatmapHeader *) tdbatmap_header_buf;
> +            if (!strncmp(tdbatmap_header->magic, "tdbatmap", 8)) {
> +                s->free_data_block_offset =
> +                    be32_to_cpu(tdbatmap_header->batmap_size) * 512
> +                    + be64_to_cpu(tdbatmap_header->batmap_offset);
> +            }
> +        }
> +
> +        /* Read backing file location from dyn header table */
> +        if (dyndisk_header->parent_name[0] || dyndisk_header->parent_name[1]) {
> +            ret = parent_locator_offset = vpc_read_backing_loc(dyndisk_header,
> +                bs, errp);
> +            if (ret < 0) {
> +                goto fail;
> +            }
> +        }
> +
> +        if (s->free_data_block_offset < parent_locator_offset
> +            + BDRV_SECTOR_SIZE) {
> +            s->free_data_block_offset = parent_locator_offset
> +                + BDRV_SECTOR_SIZE;
> +        }
> +
>          for (i = 0; i < s->max_table_entries; i++) {
>              be32_to_cpus(&s->pagetable[i]);
>              if (s->pagetable[i] != 0xFFFFFFFF) {
> @@ -340,35 +492,76 @@ static int vpc_reopen_prepare(BDRVReopenState *state,
>  }
>  
>  /*
> - * Returns the absolute byte offset of the given sector in the image file.
> - * If the sector is not allocated, -1 is returned instead.
> + * Returns the absolute byte offset of the given sector in the differencing
> + * image file.
> + *
> + * If error happened, -1 is returned.


In your patch, if error happened, not only -1 is returned, you have to describe
the reason clearer IMO.

> + *
> + * When write all type or read dynamic, if the sector is not allocated, -2
> + * is returned instead. If the sector is allocated in current file, the block
> + * offset is returned.
>   *
> - * The parameter write must be 1 if the offset will be used for a write
> - * operation (the block bitmaps is updated then), 0 otherwise.
> + * When read diff. If the sector is not allocated, -2 is returned instead.
> + * If the sector is allocated in the backing file, -3 is returned. If the
> + * sector is allocated in current file, the block offset is returned.
>   */
>  static inline int64_t get_sector_offset(BlockDriverState *bs,
> -    int64_t sector_num, int write)
> +    int64_t sector_num, bool write, bool diff)
>  {
>      BDRVVPCState *s = bs->opaque;
> -    uint64_t offset = sector_num * 512;
> -    uint64_t bitmap_offset, block_offset;
> +    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
> +    uint64_t bitmap_offset;
>      uint32_t pagetable_index, pageentry_index;
> +    int64_t block_offset = LONG_MIN;
> +    int ret;
>  
>      pagetable_index = offset / s->block_size;
> -    pageentry_index = (offset % s->block_size) / 512;
> +    pageentry_index = (offset % s->block_size) >> BDRV_SECTOR_BITS;
>  
> -    if (pagetable_index >= s->max_table_entries || s->pagetable[pagetable_index] == 0xffffffff)
> -        return -1; // not allocated
> +    if (pagetable_index >= s->max_table_entries) {
> +        return -2;
> +    }
> +    if (s->pagetable[pagetable_index] == 0xffffffff) {
> +        if (!write && diff) {
> +            return -3; /* parent allocated */
> +        } else {
> +            return -2; /* not allocated */
> +        }
> +    }
>  
> -    bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
> -    block_offset = bitmap_offset + s->bitmap_size + (512 * pageentry_index);
> +    bitmap_offset = (uint64_t) s->pagetable[pagetable_index]
> +        << BDRV_SECTOR_BITS;
> +
> +    if (!diff || write) {
> +        block_offset = bitmap_offset + s->bitmap_size
> +            + (pageentry_index << BDRV_SECTOR_BITS);
> +    } else {
> +        uint32_t bitmap_index, bitmapentry_index;
> +        uint8_t bitmap[s->bitmap_size];
>  
> +        if (bitmap_offset > s->max_table_entries * s->block_size) {
> +            return -1;
> +        }
> +        ret = bdrv_pread(bs->file, bitmap_offset, bitmap, s->bitmap_size);
> +        if (ret < 0) {
> +            return -1;
> +        }
> +
> +        bitmap_index = pageentry_index / 8;
> +        bitmapentry_index = 7 - pageentry_index % 8;
> +        if (bitmap[bitmap_index] & 0x1 << bitmapentry_index) {
> +            block_offset = bitmap_offset + s->bitmap_size
> +                + (pageentry_index << BDRV_SECTOR_BITS);
> +        } else {
> +            return -3;
> +        }
> +    }
>      // We must ensure that we don't write to any sectors which are marked as
>      // unused in the bitmap. We get away with setting all bits in the block
>      // bitmap each time we write to a new block. This might cause Virtual PC to
>      // miss sparse read optimization, but it's not a problem in terms of
>      // correctness.
> -    if (write && (s->last_bitmap_offset != bitmap_offset)) {
> +    if (!diff && write && (s->last_bitmap_offset != bitmap_offset)) {
>          uint8_t bitmap[s->bitmap_size];
>  
>          s->last_bitmap_offset = bitmap_offset;
> @@ -376,7 +569,7 @@ static inline int64_t get_sector_offset(BlockDriverState *bs,
>          bdrv_pwrite_sync(bs->file, bitmap_offset, bitmap, s->bitmap_size);
>      }
>  
> -//    printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ", bloff: %" PRIx64 "\n",
> +//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ", bloff: %" PRIx64 "\n",


WARNING: line over 80 characters
#369: FILE: block/vpc.c:572:
+//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ", bloff: %" PRIx64 "\n",

ERROR: do not use C99 // comments
#369: FILE: block/vpc.c:572:
+//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ", bloff: %" PRIx64 "\n",

total: 1 errors, 1 warnings, 571 lines checked

>  //	sector_num, pagetable_index, pageentry_index,
>  //	bitmap_offset, block_offset);
>  
> @@ -437,7 +630,8 @@ static int rewrite_footer(BlockDriverState* bs)
>   *
>   * Returns the sectors' offset in the image file on success and < 0 on error
>   */
> -static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
> +static int64_t alloc_block(BlockDriverState *bs, int64_t sector_num,
> +    bool diff)
>  {
>      BDRVVPCState *s = bs->opaque;
>      int64_t bat_offset;
> @@ -457,7 +651,11 @@ static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
>      s->pagetable[index] = s->free_data_block_offset / 512;
>  
>      // Initialize the block's bitmap
> -    memset(bitmap, 0xff, s->bitmap_size);
> +    if (diff) {
> +        memset(bitmap, 0x0, s->bitmap_size);
> +    } else {
> +        memset(bitmap, 0xff, s->bitmap_size);
> +    }
>      ret = bdrv_pwrite_sync(bs->file, s->free_data_block_offset, bitmap,
>          s->bitmap_size);
>      if (ret < 0) {
> @@ -477,7 +675,7 @@ static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
>      if (ret < 0)
>          goto fail;
>  
> -    return get_sector_offset(bs, sector_num, 0);
> +    return get_sector_offset(bs, sector_num, false, diff);
>  
>  fail:
>      s->free_data_block_offset -= (s->block_size + s->bitmap_size);
> @@ -501,36 +699,66 @@ static int vpc_read(BlockDriverState *bs, int64_t sector_num,
>                      uint8_t *buf, int nb_sectors)
>  {
>      BDRVVPCState *s = bs->opaque;
> -    int ret;
> -    int64_t offset;
> -    int64_t sectors, sectors_per_block;
>      VHDFooter *footer = (VHDFooter *) s->footer_buf;
> +    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> +    int64_t offset, sectors;
> +    int ret;
>  
> -    if (be32_to_cpu(footer->type) == VHD_FIXED) {
> +    switch (be32_to_cpu(footer->type)) {
> +    case VHD_FIXED:
>          return bdrv_read(bs->file, sector_num, buf, nb_sectors);
> -    }
> -    while (nb_sectors > 0) {
> -        offset = get_sector_offset(bs, sector_num, 0);
> -
> -        sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> -        sectors = sectors_per_block - (sector_num % sectors_per_block);
> -        if (sectors > nb_sectors) {
> -            sectors = nb_sectors;
> -        }
> +    case VHD_DYNAMIC:
> +        while (nb_sectors > 0) {
> +            sectors = sectors_per_block - (sector_num % sectors_per_block);
> +            if (sectors > nb_sectors) {
> +                sectors = nb_sectors;
> +            }
>  
> -        if (offset == -1) {
> -            memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
> -        } else {
> -            ret = bdrv_pread(bs->file, offset, buf,
> -                sectors * BDRV_SECTOR_SIZE);
> -            if (ret != sectors * BDRV_SECTOR_SIZE) {
> +            offset = get_sector_offset(bs, sector_num, false, false);
> +            if (offset == -1) {
>                  return -1;
> +            } else if (offset == -2) {
> +                memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
> +            } else {
> +                ret = bdrv_pread(bs->file, offset, buf,
> +                    sectors * BDRV_SECTOR_SIZE);
> +                if (ret != sectors * BDRV_SECTOR_SIZE) {
> +                    return -1;
> +                }
>              }
> +
> +            nb_sectors -= sectors;
> +            sector_num += sectors;
> +            buf += sectors * BDRV_SECTOR_SIZE;
>          }
> +        break;
> +    case VHD_DIFF:
> +        while (nb_sectors > 0) {
> +            offset = get_sector_offset(bs, sector_num, false, true);
> +            if (offset == -1) {
> +                return -1;
> +            } else if (offset == -2) {
> +                memset(buf, 0, BDRV_SECTOR_SIZE);
> +            } else if (offset == -3) {
> +                ret = bdrv_pread(bs->backing_hd, sector_num << BDRV_SECTOR_BITS
> +                    , buf, BDRV_SECTOR_SIZE);
> +                if (ret < 0) {
> +                    return -1;
> +                }
> +            } else {
> +                ret = bdrv_pread(bs->file, offset, buf, BDRV_SECTOR_SIZE);
> +                if (ret != BDRV_SECTOR_SIZE) {
> +                    return -1;
> +                }
> +            }
>  
> -        nb_sectors -= sectors;
> -        sector_num += sectors;
> -        buf += sectors * BDRV_SECTOR_SIZE;
> +            nb_sectors--;
> +            sector_num++;
> +            buf += BDRV_SECTOR_SIZE;
> +        }
> +        break;
> +    default:
> +        return -1;
>      }
>      return 0;
>  }
> @@ -546,44 +774,101 @@ static coroutine_fn int vpc_co_read(BlockDriverState *bs, int64_t sector_num,
>      return ret;
>  }
>  
> -static int vpc_write(BlockDriverState *bs, int64_t sector_num,
> -    const uint8_t *buf, int nb_sectors)
> +static inline int64_t write_bitmap(BlockDriverState *bs, int64_t sector_num,
> +    int64_t sectors)
>  {
>      BDRVVPCState *s = bs->opaque;
> -    int64_t offset;
> -    int64_t sectors, sectors_per_block;
> +    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
> +    uint64_t bitmap_offset;
> +    uint32_t pagetable_index, pageentry_index;
> +    uint8_t bitmap[s->bitmap_size];
> +    uint32_t bitmap_index, bitmapbit_index;
> +    int i;
>      int ret;
> -    VHDFooter *footer =  (VHDFooter *) s->footer_buf;
>  
> -    if (be32_to_cpu(footer->type) == VHD_FIXED) {
> -        return bdrv_write(bs->file, sector_num, buf, nb_sectors);
> +    pagetable_index = offset / s->block_size;
> +    pageentry_index = (offset % s->block_size) / 512;
> +    bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
> +
> +    if (bitmap_offset > s->max_table_entries * s->block_size) {
> +        return -1;
> +    }
> +    ret = bdrv_pread(bs->file, bitmap_offset, bitmap, s->bitmap_size);
> +    if (ret < 0) {
> +        return -1;
>      }
> -    while (nb_sectors > 0) {
> -        offset = get_sector_offset(bs, sector_num, 1);
>  
> -        sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> -        sectors = sectors_per_block - (sector_num % sectors_per_block);
> -        if (sectors > nb_sectors) {
> -            sectors = nb_sectors;
> +    for (i = 0; i < sectors; i++) {
> +        bitmap_index = pageentry_index / 8;
> +        bitmapbit_index = 7 - pageentry_index % 8;
> +        bitmap[bitmap_index] |= (0x1 << bitmapbit_index);
> +        pageentry_index++;
> +    }
> +    ret = bdrv_pwrite(bs->file, bitmap_offset, bitmap, s->bitmap_size);
> +    if (ret < 0) {
> +        return -1;
> +    }
> +
> +    return 0;
> +}
> +
> +static int vpc_write(BlockDriverState *bs, int64_t sector_num,
> +    const uint8_t *buf, int nb_sectors)
> +{
> +    BDRVVPCState *s = bs->opaque;
> +    VHDFooter *footer = (VHDFooter *) s->footer_buf;
> +    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
> +    int64_t offset, sectors;
> +    bool diff = true;
> +    int ret = 0;
> +
> +    switch (be32_to_cpu(footer->type)) {
> +    case VHD_FIXED:
> +        return bdrv_write(bs->file, sector_num, buf, nb_sectors);
> +    case VHD_DYNAMIC:
> +    case VHD_DIFF:
> +        if (be32_to_cpu(footer->type) == VHD_DYNAMIC) {
> +            diff = false;
>          }
>  
> -        if (offset == -1) {
> -            offset = alloc_block(bs, sector_num);
> -            if (offset < 0)
> +        while (nb_sectors > 0) {
> +            sectors = sectors_per_block - (sector_num % sectors_per_block);
> +            if (sectors > nb_sectors) {
> +                sectors = nb_sectors;
> +            }
> +
> +            offset = get_sector_offset(bs, sector_num, true, diff);
> +            if (offset == -1) {
>                  return -1;
> -        }
> +            } else if (offset == -2) {
> +                offset = alloc_block(bs, sector_num, diff);
> +                if (offset < 0) {
> +                    return -1;
> +                }
> +            }
>  
> -        ret = bdrv_pwrite(bs->file, offset, buf, sectors * BDRV_SECTOR_SIZE);
> -        if (ret != sectors * BDRV_SECTOR_SIZE) {
> -            return -1;
> -        }
> +            ret = bdrv_pwrite(bs->file, offset, buf,
> +                sectors * BDRV_SECTOR_SIZE);
> +            if (ret != sectors * BDRV_SECTOR_SIZE) {
> +                return -1;
> +            }
>  
> -        nb_sectors -= sectors;
> -        sector_num += sectors;
> -        buf += sectors * BDRV_SECTOR_SIZE;
> -    }
> +            if (diff) {
> +                ret = write_bitmap(bs, sector_num, sectors);
> +                if (ret < 0) {
> +                    return -1;
> +                }
> +            }
>  
> -    return 0;
> +            nb_sectors -= sectors;
> +            sector_num += sectors;
> +            buf += sectors * BDRV_SECTOR_SIZE;
> +        }
> +        break;
> +    default:
> +        return -1;
> +    }
> +    return ret;
>  }
>  
>  static coroutine_fn int vpc_co_write(BlockDriverState *bs, int64_t sector_num,
> @@ -911,6 +1196,7 @@ static BlockDriver bdrv_vpc = {
>      .bdrv_close             = vpc_close,
>      .bdrv_reopen_prepare    = vpc_reopen_prepare,
>      .bdrv_create            = vpc_create,
> +    .supports_backing       = true,
>  
>      .bdrv_read              = vpc_co_read,
>      .bdrv_write             = vpc_co_write,

>Adjusting coding style can be made into a separate patch in vpc.c file.
>Such as using '{}' at if conditional statement.
I'll revert this.

>Please use g_malloc0() instead of malloc and memset(,0,).
This api is great, I'll use it.

>A bug, right?
yes, really a bug, thx a lot.


>in your patch, if error happened, not only -1 is returned, you have to describe
>the reason clearer IMO.
-1 is error, -2 is sector not allocated, -3 is allocated in backing
file.described
before get_sector_offset. and what's the meaning of IMO ? :)

On 10/11/14, Gonglei <arei.gonglei@huawei.com> wrote:
> On 2014/10/11 0:17, Xiaodong Gong wrote:
>
>> Now qemu only supports vhd type VHD_FIXED and VHD_DYNAMIC, so qemu
>> can't read snapshot volume of vhd, and can't support other storage
>> features of vhd file.
>>
>> This patch add read parent information in function "vpc_open", read
>> bitmap in "vpc_read", and change bitmap in "vpc_write".
>>
>> Signed-off-by: Xiaodong Gong <gongxiaodong1@huawei.com>
>> ---
>> Changes since v4:
>> - Parse the batmap only when the version of VHD > 1.2.
>> - Add support to parent location of W2RU.
>>
>> Changes since v3:
>> - Remove the PARENT_MAX_LOC.
>>
>> Changes since v2:
>> - Change MACX to PLATFAORM_MACX.
>> - Return with EINVAL to parent location is W2RU and W2KU.
>> - Change -1 == ret to a natrual order of ret == -1.
>> - Get rid of the get_sector_offset_diff, get_sector_offset
>>   supports VHD_DIFF.
>> - Return code of get_sector_offset is set to, -1 for error,
>>   -2 for not allocate, -3 for in parent.
>> - Fix un init ret of vpc_write, when nb_sector == 0.
>> - Change if (diff == ture) to if (diff) and so on.
>> - Add PARENT_MAX_LOC to more understand.
>> - Restore the boundary check to write on dynamic type in
>>   get_sector_offset.
>>
>> Changes since v1:
>> - Add Boundary check to any input.
>> - Clean the code no used after in vpc_open.
>> - Change bdrv_co_readv() to bdrv_preadv in vpc_read.
>> - Added some code to make it easy to understand.
>> ---
>>  block/vpc.c | 428
>> ++++++++++++++++++++++++++++++++++++++++++++++++++----------
>>  1 file changed, 357 insertions(+), 71 deletions(-)
>>
>> diff --git a/block/vpc.c b/block/vpc.c
>> index 4947369..1210542 100644
>> --- a/block/vpc.c
>> +++ b/block/vpc.c
>> @@ -29,17 +29,27 @@
>>  #if defined(CONFIG_UUID)
>>  #include <uuid/uuid.h>
>>  #endif
>> +#include <iconv.h>
>>
>>  /**************************************************************/
>>
>>  #define HEADER_SIZE 512
>> +#define DYNAMIC_HEADER_SIZE 1024
>> +#define PARENT_LOCATOR_NUM 8
>> +#define MACX_PREFIX_LEN 7 /* file:// */
>> +#define TBBATMAP_HEAD_SIZE 28
>> +
>> +#define PLATFORM_MACX 0x5863614d /* big endian */
>> +#define PLATFORM_W2RU 0x75723257
>> +
>> +#define VHD_VERSION(major, minor)  (((major) << 16) | ((minor) &
>> 0x0000FFFF))
>>
>>  //#define CACHE
>>
>>  enum vhd_type {
>>      VHD_FIXED           = 2,
>>      VHD_DYNAMIC         = 3,
>> -    VHD_DIFFERENCING    = 4,
>> +    VHD_DIFF            = 4,
>>  };
>>
>>  // Seconds since Jan 1, 2000 0:00:00 (UTC)
>> @@ -138,6 +148,15 @@ typedef struct BDRVVPCState {
>>      Error *migration_blocker;
>>  } BDRVVPCState;
>>
>> +typedef struct vhd_tdbatmap_header {
>> +    char magic[8]; /* always "tdbatmap" */
>> +
>> +    uint64_t batmap_offset;
>> +    uint32_t batmap_size;
>> +    uint32_t batmap_version;
>> +    uint32_t checksum;
>> +} QEMU_PACKED VHDTdBatmapHeader;
>> +
>>  static uint32_t vpc_checksum(uint8_t* buf, size_t size)
>>  {
>>      uint32_t res = 0;
>> @@ -153,10 +172,107 @@ static uint32_t vpc_checksum(uint8_t* buf, size_t
>> size)
>>  static int vpc_probe(const uint8_t *buf, int buf_size, const char
>> *filename)
>>  {
>>      if (buf_size >= 8 && !strncmp((char *)buf, "conectix", 8))
>> -	return 100;
>> +        return 100;
>
>
> Adjusting coding style can be made into a separate patch in vpc.c file.
> Such as using '{}' at if conditional statement.
>
>>      return 0;
>>  }
>>
>> +static int vpc_read_backing_loc(VHDDynDiskHeader *dyndisk_header,
>> +                                BlockDriverState *bs,
>> +                                Error **errp)
>> +{
>> +    BDRVVPCState *s = bs->opaque;
>> +    int64_t data_offset = 0;
>> +    int data_length = 0;
>> +    uint32_t platform;
>> +    bool done = false;
>> +    int parent_locator_offset = 0;
>> +    int i;
>> +    int ret = 0;
>> +
>> +    for (i = 0; i < PARENT_LOCATOR_NUM; i++) {
>> +        data_offset =
>> +            be64_to_cpu(dyndisk_header->parent_locator[i].data_offset);
>> +        data_length =
>> +            be32_to_cpu(dyndisk_header->parent_locator[i].data_length);
>> +        platform = dyndisk_header->parent_locator[i].platform;
>> +
>> +        /* Extend the location offset */
>> +        if (parent_locator_offset < data_offset) {
>> +            parent_locator_offset = data_offset;
>> +        }
>> +
>> +        if (done) {
>> +            continue;
>> +        }
>> +
>> +        /* Skip "file://" in MacX platform */
>> +        if (platform == PLATFORM_MACX) {
>> +            data_offset += MACX_PREFIX_LEN;
>> +            data_length -= MACX_PREFIX_LEN;
>> +        }
>> +
>> +        /* Read location of backing file */
>> +        if (platform == PLATFORM_MACX || platform == PLATFORM_W2RU) {
>> +            if (data_offset > s->max_table_entries * s->block_size) {
>> +                return -1;
>> +            }
>> +            if (data_length > BDRV_SECTOR_SIZE) {
>> +                return -1;
>> +            }
>> +            ret = bdrv_pread(bs->file, data_offset, bs->backing_file,
>> +                data_length);
>> +            if (ret < 0) {
>> +                return ret;
>> +            }
>> +            bs->backing_file[data_length] = '\0';
>> +        }
>> +
>> +        /* Convert location to ACSII string */
>> +        if (platform == PLATFORM_MACX) {
>> +            done = true;
>> +
>> +        } else if (platform == PLATFORM_W2RU) {
>> +            /* Must be UTF16-LE to ASCII */
>> +            char *out, *optr;
>> +            int j;
>> +
>> +            optr = out = (char *) malloc(data_length + 1);
>> +            if (out == NULL) {
>> +                ret = -1;
>> +                return ret;
>> +            }
>> +            memset(out, 0, data_length + 1);
>
>
> Please use g_malloc0() instead of malloc and memset(,0,).
>
>> +
>> +            for (j = 0; j < data_length + 1; j++) {
>> +                out[j] = bs->backing_file[2*j];
>> +            }
>> +            out[data_length + 1] = '\0';
>> +
>> +            while (*optr != '\0') {
>> +                if (*optr == '\\') {
>> +                    *optr = '/';
>> +                }
>> +                optr++;
>> +            }
>> +
>> +            strncpy(bs->backing_file, out, data_length + 1);
>> +
>> +            out = NULL;
>> +            free(out);
>
>
> A bug, right?
>
>> +
>> +            done = true;
>> +        }
>> +    }
>> +
>> +    if (bs->backing_file[0] == '\0') {
>> +        error_setg(errp, "block-vpc: differencing is not support in
>> w2ku");
>> +        ret = -EINVAL;
>> +        return ret;
>> +    }
>> +
>> +    return parent_locator_offset;
>> +}
>> +
>>  static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
>>                      Error **errp)
>>  {
>> @@ -164,11 +280,14 @@ static int vpc_open(BlockDriverState *bs, QDict
>> *options, int flags,
>>      int i;
>>      VHDFooter *footer;
>>      VHDDynDiskHeader *dyndisk_header;
>> -    uint8_t buf[HEADER_SIZE];
>> +    uint8_t buf[DYNAMIC_HEADER_SIZE];
>> +    uint8_t tdbatmap_header_buf[TBBATMAP_HEAD_SIZE];
>>      uint32_t checksum;
>>      uint64_t computed_size;
>> -    int disk_type = VHD_DYNAMIC;
>> +    uint32_t disk_type;
>>      int ret;
>> +    VHDTdBatmapHeader *tdbatmap_header;
>> +    int parent_locator_offset = 0;
>>
>>      ret = bdrv_pread(bs->file, 0, s->footer_buf, HEADER_SIZE);
>>      if (ret < 0) {
>> @@ -176,6 +295,8 @@ static int vpc_open(BlockDriverState *bs, QDict
>> *options, int flags,
>>      }
>>
>>      footer = (VHDFooter *) s->footer_buf;
>> +    disk_type = be32_to_cpu(footer->type);
>> +
>>      if (strncmp(footer->creator, "conectix", 8)) {
>>          int64_t offset = bdrv_getlength(bs->file);
>>          if (offset < 0) {
>> @@ -230,9 +351,9 @@ static int vpc_open(BlockDriverState *bs, QDict
>> *options, int flags,
>>          goto fail;
>>      }
>>
>> -    if (disk_type == VHD_DYNAMIC) {
>> +    if (disk_type == VHD_DYNAMIC || disk_type == VHD_DIFF) {
>>          ret = bdrv_pread(bs->file, be64_to_cpu(footer->data_offset),
>> buf,
>> -                         HEADER_SIZE);
>> +                         DYNAMIC_HEADER_SIZE);
>>          if (ret < 0) {
>>              goto fail;
>>          }
>> @@ -286,6 +407,37 @@ static int vpc_open(BlockDriverState *bs, QDict
>> *options, int flags,
>>          s->free_data_block_offset =
>>              (s->bat_offset + (s->max_table_entries * 4) + 511) & ~511;
>>
>> +        /* Read tdbatmap header by offset */
>> +        if (footer->version >= VHD_VERSION(1, 2)) {
>> +            ret = bdrv_pread(bs->file, s->free_data_block_offset,
>> +                tdbatmap_header_buf, TBBATMAP_HEAD_SIZE);
>> +            if (ret < 0) {
>> +                goto fail;
>> +            }
>> +
>> +            tdbatmap_header = (VHDTdBatmapHeader *) tdbatmap_header_buf;
>> +            if (!strncmp(tdbatmap_header->magic, "tdbatmap", 8)) {
>> +                s->free_data_block_offset =
>> +                    be32_to_cpu(tdbatmap_header->batmap_size) * 512
>> +                    + be64_to_cpu(tdbatmap_header->batmap_offset);
>> +            }
>> +        }
>> +
>> +        /* Read backing file location from dyn header table */
>> +        if (dyndisk_header->parent_name[0] ||
>> dyndisk_header->parent_name[1]) {
>> +            ret = parent_locator_offset =
>> vpc_read_backing_loc(dyndisk_header,
>> +                bs, errp);
>> +            if (ret < 0) {
>> +                goto fail;
>> +            }
>> +        }
>> +
>> +        if (s->free_data_block_offset < parent_locator_offset
>> +            + BDRV_SECTOR_SIZE) {
>> +            s->free_data_block_offset = parent_locator_offset
>> +                + BDRV_SECTOR_SIZE;
>> +        }
>> +
>>          for (i = 0; i < s->max_table_entries; i++) {
>>              be32_to_cpus(&s->pagetable[i]);
>>              if (s->pagetable[i] != 0xFFFFFFFF) {
>> @@ -340,35 +492,76 @@ static int vpc_reopen_prepare(BDRVReopenState
>> *state,
>>  }
>>
>>  /*
>> - * Returns the absolute byte offset of the given sector in the image
>> file.
>> - * If the sector is not allocated, -1 is returned instead.
>> + * Returns the absolute byte offset of the given sector in the
>> differencing
>> + * image file.
>> + *
>> + * If error happened, -1 is returned.
>
>
> In your patch, if error happened, not only -1 is returned, you have to
> describe
> the reason clearer IMO.
>
>> + *
>> + * When write all type or read dynamic, if the sector is not allocated,
>> -2
>> + * is returned instead. If the sector is allocated in current file, the
>> block
>> + * offset is returned.
>>   *
>> - * The parameter write must be 1 if the offset will be used for a write
>> - * operation (the block bitmaps is updated then), 0 otherwise.
>> + * When read diff. If the sector is not allocated, -2 is returned
>> instead.
>> + * If the sector is allocated in the backing file, -3 is returned. If
>> the
>> + * sector is allocated in current file, the block offset is returned.
>>   */
>>  static inline int64_t get_sector_offset(BlockDriverState *bs,
>> -    int64_t sector_num, int write)
>> +    int64_t sector_num, bool write, bool diff)
>>  {
>>      BDRVVPCState *s = bs->opaque;
>> -    uint64_t offset = sector_num * 512;
>> -    uint64_t bitmap_offset, block_offset;
>> +    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
>> +    uint64_t bitmap_offset;
>>      uint32_t pagetable_index, pageentry_index;
>> +    int64_t block_offset = LONG_MIN;
>> +    int ret;
>>
>>      pagetable_index = offset / s->block_size;
>> -    pageentry_index = (offset % s->block_size) / 512;
>> +    pageentry_index = (offset % s->block_size) >> BDRV_SECTOR_BITS;
>>
>> -    if (pagetable_index >= s->max_table_entries ||
>> s->pagetable[pagetable_index] == 0xffffffff)
>> -        return -1; // not allocated
>> +    if (pagetable_index >= s->max_table_entries) {
>> +        return -2;
>> +    }
>> +    if (s->pagetable[pagetable_index] == 0xffffffff) {
>> +        if (!write && diff) {
>> +            return -3; /* parent allocated */
>> +        } else {
>> +            return -2; /* not allocated */
>> +        }
>> +    }
>>
>> -    bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
>> -    block_offset = bitmap_offset + s->bitmap_size + (512 *
>> pageentry_index);
>> +    bitmap_offset = (uint64_t) s->pagetable[pagetable_index]
>> +        << BDRV_SECTOR_BITS;
>> +
>> +    if (!diff || write) {
>> +        block_offset = bitmap_offset + s->bitmap_size
>> +            + (pageentry_index << BDRV_SECTOR_BITS);
>> +    } else {
>> +        uint32_t bitmap_index, bitmapentry_index;
>> +        uint8_t bitmap[s->bitmap_size];
>>
>> +        if (bitmap_offset > s->max_table_entries * s->block_size) {
>> +            return -1;
>> +        }
>> +        ret = bdrv_pread(bs->file, bitmap_offset, bitmap,
>> s->bitmap_size);
>> +        if (ret < 0) {
>> +            return -1;
>> +        }
>> +
>> +        bitmap_index = pageentry_index / 8;
>> +        bitmapentry_index = 7 - pageentry_index % 8;
>> +        if (bitmap[bitmap_index] & 0x1 << bitmapentry_index) {
>> +            block_offset = bitmap_offset + s->bitmap_size
>> +                + (pageentry_index << BDRV_SECTOR_BITS);
>> +        } else {
>> +            return -3;
>> +        }
>> +    }
>>      // We must ensure that we don't write to any sectors which are marked
>> as
>>      // unused in the bitmap. We get away with setting all bits in the
>> block
>>      // bitmap each time we write to a new block. This might cause Virtual
>> PC to
>>      // miss sparse read optimization, but it's not a problem in terms of
>>      // correctness.
>> -    if (write && (s->last_bitmap_offset != bitmap_offset)) {
>> +    if (!diff && write && (s->last_bitmap_offset != bitmap_offset)) {
>>          uint8_t bitmap[s->bitmap_size];
>>
>>          s->last_bitmap_offset = bitmap_offset;
>> @@ -376,7 +569,7 @@ static inline int64_t
>> get_sector_offset(BlockDriverState *bs,
>>          bdrv_pwrite_sync(bs->file, bitmap_offset, bitmap,
>> s->bitmap_size);
>>      }
>>
>> -//    printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %"
>> PRIx64 ", bloff: %" PRIx64 "\n",
>> +//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64
>> ", bloff: %" PRIx64 "\n",
>
>
> WARNING: line over 80 characters
> #369: FILE: block/vpc.c:572:
> +//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ",
> bloff: %" PRIx64 "\n",
>
> ERROR: do not use C99 // comments
> #369: FILE: block/vpc.c:572:
> +//  printf("sector: %" PRIx64 ", index: %x, offset: %x, bioff: %" PRIx64 ",
> bloff: %" PRIx64 "\n",
>
> total: 1 errors, 1 warnings, 571 lines checked
>
>>  //	sector_num, pagetable_index, pageentry_index,
>>  //	bitmap_offset, block_offset);
>>
>> @@ -437,7 +630,8 @@ static int rewrite_footer(BlockDriverState* bs)
>>   *
>>   * Returns the sectors' offset in the image file on success and < 0 on
>> error
>>   */
>> -static int64_t alloc_block(BlockDriverState* bs, int64_t sector_num)
>> +static int64_t alloc_block(BlockDriverState *bs, int64_t sector_num,
>> +    bool diff)
>>  {
>>      BDRVVPCState *s = bs->opaque;
>>      int64_t bat_offset;
>> @@ -457,7 +651,11 @@ static int64_t alloc_block(BlockDriverState* bs,
>> int64_t sector_num)
>>      s->pagetable[index] = s->free_data_block_offset / 512;
>>
>>      // Initialize the block's bitmap
>> -    memset(bitmap, 0xff, s->bitmap_size);
>> +    if (diff) {
>> +        memset(bitmap, 0x0, s->bitmap_size);
>> +    } else {
>> +        memset(bitmap, 0xff, s->bitmap_size);
>> +    }
>>      ret = bdrv_pwrite_sync(bs->file, s->free_data_block_offset, bitmap,
>>          s->bitmap_size);
>>      if (ret < 0) {
>> @@ -477,7 +675,7 @@ static int64_t alloc_block(BlockDriverState* bs,
>> int64_t sector_num)
>>      if (ret < 0)
>>          goto fail;
>>
>> -    return get_sector_offset(bs, sector_num, 0);
>> +    return get_sector_offset(bs, sector_num, false, diff);
>>
>>  fail:
>>      s->free_data_block_offset -= (s->block_size + s->bitmap_size);
>> @@ -501,36 +699,66 @@ static int vpc_read(BlockDriverState *bs, int64_t
>> sector_num,
>>                      uint8_t *buf, int nb_sectors)
>>  {
>>      BDRVVPCState *s = bs->opaque;
>> -    int ret;
>> -    int64_t offset;
>> -    int64_t sectors, sectors_per_block;
>>      VHDFooter *footer = (VHDFooter *) s->footer_buf;
>> +    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
>> +    int64_t offset, sectors;
>> +    int ret;
>>
>> -    if (be32_to_cpu(footer->type) == VHD_FIXED) {
>> +    switch (be32_to_cpu(footer->type)) {
>> +    case VHD_FIXED:
>>          return bdrv_read(bs->file, sector_num, buf, nb_sectors);
>> -    }
>> -    while (nb_sectors > 0) {
>> -        offset = get_sector_offset(bs, sector_num, 0);
>> -
>> -        sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
>> -        sectors = sectors_per_block - (sector_num % sectors_per_block);
>> -        if (sectors > nb_sectors) {
>> -            sectors = nb_sectors;
>> -        }
>> +    case VHD_DYNAMIC:
>> +        while (nb_sectors > 0) {
>> +            sectors = sectors_per_block - (sector_num %
>> sectors_per_block);
>> +            if (sectors > nb_sectors) {
>> +                sectors = nb_sectors;
>> +            }
>>
>> -        if (offset == -1) {
>> -            memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
>> -        } else {
>> -            ret = bdrv_pread(bs->file, offset, buf,
>> -                sectors * BDRV_SECTOR_SIZE);
>> -            if (ret != sectors * BDRV_SECTOR_SIZE) {
>> +            offset = get_sector_offset(bs, sector_num, false, false);
>> +            if (offset == -1) {
>>                  return -1;
>> +            } else if (offset == -2) {
>> +                memset(buf, 0, sectors * BDRV_SECTOR_SIZE);
>> +            } else {
>> +                ret = bdrv_pread(bs->file, offset, buf,
>> +                    sectors * BDRV_SECTOR_SIZE);
>> +                if (ret != sectors * BDRV_SECTOR_SIZE) {
>> +                    return -1;
>> +                }
>>              }
>> +
>> +            nb_sectors -= sectors;
>> +            sector_num += sectors;
>> +            buf += sectors * BDRV_SECTOR_SIZE;
>>          }
>> +        break;
>> +    case VHD_DIFF:
>> +        while (nb_sectors > 0) {
>> +            offset = get_sector_offset(bs, sector_num, false, true);
>> +            if (offset == -1) {
>> +                return -1;
>> +            } else if (offset == -2) {
>> +                memset(buf, 0, BDRV_SECTOR_SIZE);
>> +            } else if (offset == -3) {
>> +                ret = bdrv_pread(bs->backing_hd, sector_num <<
>> BDRV_SECTOR_BITS
>> +                    , buf, BDRV_SECTOR_SIZE);
>> +                if (ret < 0) {
>> +                    return -1;
>> +                }
>> +            } else {
>> +                ret = bdrv_pread(bs->file, offset, buf,
>> BDRV_SECTOR_SIZE);
>> +                if (ret != BDRV_SECTOR_SIZE) {
>> +                    return -1;
>> +                }
>> +            }
>>
>> -        nb_sectors -= sectors;
>> -        sector_num += sectors;
>> -        buf += sectors * BDRV_SECTOR_SIZE;
>> +            nb_sectors--;
>> +            sector_num++;
>> +            buf += BDRV_SECTOR_SIZE;
>> +        }
>> +        break;
>> +    default:
>> +        return -1;
>>      }
>>      return 0;
>>  }
>> @@ -546,44 +774,101 @@ static coroutine_fn int
>> vpc_co_read(BlockDriverState *bs, int64_t sector_num,
>>      return ret;
>>  }
>>
>> -static int vpc_write(BlockDriverState *bs, int64_t sector_num,
>> -    const uint8_t *buf, int nb_sectors)
>> +static inline int64_t write_bitmap(BlockDriverState *bs, int64_t
>> sector_num,
>> +    int64_t sectors)
>>  {
>>      BDRVVPCState *s = bs->opaque;
>> -    int64_t offset;
>> -    int64_t sectors, sectors_per_block;
>> +    uint64_t offset = sector_num << BDRV_SECTOR_BITS;
>> +    uint64_t bitmap_offset;
>> +    uint32_t pagetable_index, pageentry_index;
>> +    uint8_t bitmap[s->bitmap_size];
>> +    uint32_t bitmap_index, bitmapbit_index;
>> +    int i;
>>      int ret;
>> -    VHDFooter *footer =  (VHDFooter *) s->footer_buf;
>>
>> -    if (be32_to_cpu(footer->type) == VHD_FIXED) {
>> -        return bdrv_write(bs->file, sector_num, buf, nb_sectors);
>> +    pagetable_index = offset / s->block_size;
>> +    pageentry_index = (offset % s->block_size) / 512;
>> +    bitmap_offset = 512 * (uint64_t) s->pagetable[pagetable_index];
>> +
>> +    if (bitmap_offset > s->max_table_entries * s->block_size) {
>> +        return -1;
>> +    }
>> +    ret = bdrv_pread(bs->file, bitmap_offset, bitmap, s->bitmap_size);
>> +    if (ret < 0) {
>> +        return -1;
>>      }
>> -    while (nb_sectors > 0) {
>> -        offset = get_sector_offset(bs, sector_num, 1);
>>
>> -        sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
>> -        sectors = sectors_per_block - (sector_num % sectors_per_block);
>> -        if (sectors > nb_sectors) {
>> -            sectors = nb_sectors;
>> +    for (i = 0; i < sectors; i++) {
>> +        bitmap_index = pageentry_index / 8;
>> +        bitmapbit_index = 7 - pageentry_index % 8;
>> +        bitmap[bitmap_index] |= (0x1 << bitmapbit_index);
>> +        pageentry_index++;
>> +    }
>> +    ret = bdrv_pwrite(bs->file, bitmap_offset, bitmap, s->bitmap_size);
>> +    if (ret < 0) {
>> +        return -1;
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>> +static int vpc_write(BlockDriverState *bs, int64_t sector_num,
>> +    const uint8_t *buf, int nb_sectors)
>> +{
>> +    BDRVVPCState *s = bs->opaque;
>> +    VHDFooter *footer = (VHDFooter *) s->footer_buf;
>> +    int64_t sectors_per_block = s->block_size >> BDRV_SECTOR_BITS;
>> +    int64_t offset, sectors;
>> +    bool diff = true;
>> +    int ret = 0;
>> +
>> +    switch (be32_to_cpu(footer->type)) {
>> +    case VHD_FIXED:
>> +        return bdrv_write(bs->file, sector_num, buf, nb_sectors);
>> +    case VHD_DYNAMIC:
>> +    case VHD_DIFF:
>> +        if (be32_to_cpu(footer->type) == VHD_DYNAMIC) {
>> +            diff = false;
>>          }
>>
>> -        if (offset == -1) {
>> -            offset = alloc_block(bs, sector_num);
>> -            if (offset < 0)
>> +        while (nb_sectors > 0) {
>> +            sectors = sectors_per_block - (sector_num %
>> sectors_per_block);
>> +            if (sectors > nb_sectors) {
>> +                sectors = nb_sectors;
>> +            }
>> +
>> +            offset = get_sector_offset(bs, sector_num, true, diff);
>> +            if (offset == -1) {
>>                  return -1;
>> -        }
>> +            } else if (offset == -2) {
>> +                offset = alloc_block(bs, sector_num, diff);
>> +                if (offset < 0) {
>> +                    return -1;
>> +                }
>> +            }
>>
>> -        ret = bdrv_pwrite(bs->file, offset, buf, sectors *
>> BDRV_SECTOR_SIZE);
>> -        if (ret != sectors * BDRV_SECTOR_SIZE) {
>> -            return -1;
>> -        }
>> +            ret = bdrv_pwrite(bs->file, offset, buf,
>> +                sectors * BDRV_SECTOR_SIZE);
>> +            if (ret != sectors * BDRV_SECTOR_SIZE) {
>> +                return -1;
>> +            }
>>
>> -        nb_sectors -= sectors;
>> -        sector_num += sectors;
>> -        buf += sectors * BDRV_SECTOR_SIZE;
>> -    }
>> +            if (diff) {
>> +                ret = write_bitmap(bs, sector_num, sectors);
>> +                if (ret < 0) {
>> +                    return -1;
>> +                }
>> +            }
>>
>> -    return 0;
>> +            nb_sectors -= sectors;
>> +            sector_num += sectors;
>> +            buf += sectors * BDRV_SECTOR_SIZE;
>> +        }
>> +        break;
>> +    default:
>> +        return -1;
>> +    }
>> +    return ret;
>>  }
>>
>>  static coroutine_fn int vpc_co_write(BlockDriverState *bs, int64_t
>> sector_num,
>> @@ -911,6 +1196,7 @@ static BlockDriver bdrv_vpc = {
>>      .bdrv_close             = vpc_close,
>>      .bdrv_reopen_prepare    = vpc_reopen_prepare,
>>      .bdrv_create            = vpc_create,
>> +    .supports_backing       = true,
>>
>>      .bdrv_read              = vpc_co_read,
>>      .bdrv_write             = vpc_co_write,
>
>
>
>