@@ -29,17 +29,29 @@
#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 TBBATMAP_HEAD_SIZE 28
+
+#define MACX_PREFIX_LEN 7 /* file:// */
+
+#define PLATFORM_MACX 0x5863614d /* big endian */
+#define PLATFORM_W2RU 0x75723257
+#define PLATFORM_W2KU 0x756B3257
+
+#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 +150,15 @@ 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;
@@ -157,6 +178,225 @@ static int vpc_probe(const uint8_t *buf, int buf_size, const char *filename)
return 0;
}
+static int vpc_decode_maxc_loc(BlockDriverState *bs, char data_length)
+{
+ iconv_t cd;
+ char *inbuf, *outbuf, *buf;
+ size_t inbyteleft, outbyteleft, outbyte;
+ int ret;
+
+ if (!bs || !bs->backing_file || !data_length) {
+ return -1;
+ }
+
+ cd = iconv_open("ASCII", "UTF8");
+ if (cd == (iconv_t) -1) {
+ return -1;
+ }
+
+ inbuf = bs->backing_file;
+ outbuf = buf = (char *) g_malloc(data_length + 1);
+ if (!outbuf) {
+ ret = -1;
+ goto fail2;
+ }
+ inbyteleft = outbyteleft = data_length;
+
+ ret = iconv(cd, &inbuf, &inbyteleft, &outbuf, &outbyteleft);
+ if (ret == (size_t) -1 || inbyteleft) {
+ ret = -1;
+ goto fail1;
+ }
+ outbyte = data_length - outbyteleft;
+ if (outbyte > sizeof(bs->backing_file) - 1) {
+ ret = -1;
+ goto fail1;
+ }
+ if (outbyte < MACX_PREFIX_LEN) {
+ ret = -1;
+ goto fail1;
+ }
+ buf[outbyte] = '\0';
+
+ strncpy(bs->backing_file, buf + MACX_PREFIX_LEN,
+ outbyte - MACX_PREFIX_LEN);
+ bs->backing_file[outbyte - MACX_PREFIX_LEN] = '\0';
+
+fail1:
+ g_free(buf);
+ buf = NULL;
+
+fail2:
+ ret = iconv_close(cd);
+ if (ret == -1) {
+ return -1;
+ }
+
+ return 0;
+}
+
+static int vpc_decode_w2u_loc(BlockDriverState *bs, char data_length)
+{
+ iconv_t cd;
+ char *buf, *inbuf, *outbuf;
+ size_t inbyteleft, outbyteleft, outbyte;
+ char *ptr;
+ char len = 0;
+ int ret;
+
+ if (!bs || !bs->backing_file || !data_length) {
+ return -1;
+ }
+
+ cd = iconv_open("ASCII", "UTF-16LE");
+ if (cd == (iconv_t) -1) {
+ return -1;
+ }
+
+ inbuf = bs->backing_file;
+ outbuf = buf = ptr = (char *) g_malloc(data_length + 1);
+ if (!buf) {
+ ret = -1;
+ goto fail2;
+ }
+ inbyteleft = outbyteleft = data_length;
+
+ ret = iconv(cd, &inbuf, &inbyteleft, &outbuf, &outbyteleft);
+ if (ret == (size_t) -1 || inbyteleft) {
+ ret = -1;
+ goto fail1;
+ }
+ outbyte = data_length - outbyteleft;
+ if (outbyte > sizeof(bs->backing_file) - 1) {
+ ret = -1;
+ goto fail1;
+ }
+ buf[outbyte] = '\0';
+
+ while (ptr != outbuf) {
+ if (*ptr == '\\') {
+ *ptr = '/';
+ }
+ ptr++;
+ }
+ ptr = strstr(buf, ":");
+ if (ptr) {
+ *ptr = '.';
+ len = ptr - buf;
+ }
+ strncpy(bs->backing_file, buf + len, outbyte - len);
+ bs->backing_file[outbyte - len] = '\0';
+
+fail1:
+ g_free(buf);
+ buf = NULL;
+
+fail2:
+ ret = iconv_close(cd);
+ if (ret == -1) {
+ return -1;
+ }
+
+ return 0;
+}
+
+static int vpc_decode_parent_loc(uint32_t platform,
+ BlockDriverState *bs,
+ int data_length)
+{
+ int ret;
+
+ switch (platform) {
+ case PLATFORM_MACX:
+ ret = vpc_decode_maxc_loc(bs, data_length);
+ if (ret < 0) {
+ return ret;
+ }
+ break;
+
+ case PLATFORM_W2RU:
+ /* fall through! */
+ case PLATFORM_W2KU:
+ ret = vpc_decode_w2u_loc(bs, data_length);
+ if (ret < 0) {
+ return ret;
+ }
+ break;
+
+ default:
+ return 0;
+ }
+
+ 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++) {
+ /* The PLATFORM_* is big ending, and the dyndisk_header
+ * is always big ending. So whatever this platform in cpu
+ * is, it works. */
+ platform =
+ dyndisk_header->parent_locator[i].platform;
+ data_offset =
+ be64_to_cpu(dyndisk_header->parent_locator[i].data_offset);
+ data_length =
+ be32_to_cpu(dyndisk_header->parent_locator[i].data_length);
+
+ /* Extend the location offset */
+ if (parent_locator_offset < data_offset) {
+ parent_locator_offset = data_offset;
+ }
+
+ if (done) {
+ continue;
+ }
+
+ /* Read location of backing file */
+ if (data_offset > s->max_table_entries * s->block_size) {
+ return -1;
+ }
+ if (data_length > sizeof(bs->backing_file) - 1) {
+ 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';
+
+ /* Decode the parent location */
+ ret = vpc_decode_parent_loc(platform, bs, data_length);
+ if (ret < 0) {
+ return ret;
+ }
+
+ /* Right parent location is got */
+ if (bs->backing_file[0] != '\0') {
+ done = true;
+ }
+ }
+
+ if (!done) {
+ return -1;
+ }
+
+ return parent_locator_offset;
+}
+
static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
Error **errp)
{
@@ -164,11 +404,14 @@ 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 +419,8 @@ 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 +475,9 @@ 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;
}
@@ -278,7 +523,7 @@ static int vpc_open(BlockDriverState *bs, QDict *options, int flags,
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);
+ s->max_table_entries * 4);
if (ret < 0) {
goto fail;
}
@@ -286,6 +531,48 @@ 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 (be32_to_cpu(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);
+ }
+ }
+
+ if (dyndisk_header->parent_name[0] || dyndisk_header->parent_name[1]) {
+ int len;
+
+ /* Read parent location from dyn header table */
+ ret = parent_locator_offset = vpc_read_backing_loc(dyndisk_header,
+ bs, errp);
+ if (ret < 0) {
+ goto fail;
+ }
+
+ /* Fix me : Set parent format to avoid probing to raw in
+ * format probe framework */
+ len = strlen("vpc");
+ if (sizeof(bs->backing_format) - 1 < len) {
+ goto fail;
+ }
+ pstrcpy(bs->backing_format, sizeof(bs->backing_format), "vpc");
+ bs->backing_format[len] = '\0';
+ }
+
+ 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 +627,76 @@ 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.
*
- * The parameter write must be 1 if the offset will be used for a write
- * operation (the block bitmaps is updated then), 0 otherwise.
+ * If error happened, -1 is returned.
+ *
+ * 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.
+ *
+ * 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 +704,7 @@ 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",
// sector_num, pagetable_index, pageentry_index,
// bitmap_offset, block_offset);
@@ -437,7 +765,8 @@ 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 +786,11 @@ 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 +810,7 @@ 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);
@@ -501,37 +834,71 @@ 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;
+ }
+
+ 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;
- 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) {
+ 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--;
+ sector_num++;
+ buf += BDRV_SECTOR_SIZE;
}
+ break;
- nb_sectors -= sectors;
- sector_num += sectors;
- buf += sectors * BDRV_SECTOR_SIZE;
+ default:
+ return -1;
}
+
return 0;
}
@@ -546,41 +913,101 @@ static coroutine_fn int vpc_co_read(BlockDriverState *bs, int64_t sector_num,
return ret;
}
+static inline int64_t write_bitmap(BlockDriverState *bs, int64_t sector_num,
+ int64_t sectors)
+{
+ BDRVVPCState *s = bs->opaque;
+ 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;
+
+ 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;
+ }
+
+ 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;
- int64_t offset;
- int64_t sectors, sectors_per_block;
- int ret;
- VHDFooter *footer = (VHDFooter *) s->footer_buf;
+ 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;
- if (be32_to_cpu(footer->type) == VHD_FIXED) {
+ switch (be32_to_cpu(footer->type)) {
+ case VHD_FIXED:
return bdrv_write(bs->file, sector_num, buf, nb_sectors);
- }
- 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;
- }
+ case VHD_DYNAMIC:
+ diff = false;
+ /* fall-through ! */
+
+ case VHD_DIFF:
+ while (nb_sectors > 0) {
+ sectors = sectors_per_block - (sector_num % sectors_per_block);
+ if (sectors > nb_sectors) {
+ sectors = nb_sectors;
+ }
- if (offset == -1) {
- offset = alloc_block(bs, sector_num);
- if (offset < 0)
+ 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;
+ }
+
+ if (diff) {
+ ret = write_bitmap(bs, sector_num, sectors);
+ if (ret < 0) {
+ return -1;
+ }
+ }
+
+ nb_sectors -= sectors;
+ sector_num += sectors;
+ buf += sectors * BDRV_SECTOR_SIZE;
}
- nb_sectors -= sectors;
- sector_num += sectors;
- buf += sectors * BDRV_SECTOR_SIZE;
+ break;
+
+ default:
+ return -1;
}
return 0;
@@ -906,6 +1333,7 @@ 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,