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diff --git a/docs/specs/parallels.txt b/docs/specs/parallels.txt new file mode 100644 index 0000000000..b4fe2295fb --- /dev/null +++ b/docs/specs/parallels.txt @@ -0,0 +1,228 @@ += License = + +Copyright (c) 2015 Denis Lunev +Copyright (c) 2015 Vladimir Sementsov-Ogievskiy + +This work is licensed under the terms of the GNU GPL, version 2 or later. +See the COPYING file in the top-level directory. + += Parallels Expandable Image File Format = + +A Parallels expandable image file consists of three consecutive parts: + * header + * BAT + * data area + +All numbers in a Parallels expandable image are stored in little-endian byte +order. + + +== Definitions == + + Sector A 512-byte data chunk. + + Cluster A data chunk of the size specified in the image header. + Currently, the default size is 1MiB (2048 sectors). In previous + versions, cluster sizes of 63 sectors, 256 and 252 kilobytes were + used. + + BAT Block Allocation Table, an entity that contains information for + guest-to-host I/O data address translation. + + +== Header == + +The header is placed at the start of an image and contains the following +fields: + +Bytes: + 0 - 15: magic + Must contain "WithoutFreeSpace" or "WithouFreSpacExt". + + 16 - 19: version + Must be 2. + + 20 - 23: heads + Disk geometry parameter for guest. + + 24 - 27: cylinders + Disk geometry parameter for guest. + + 28 - 31: tracks + Cluster size, in sectors. + + 32 - 35: nb_bat_entries + Disk size, in clusters (BAT size). + + 36 - 43: nb_sectors + Disk size, in sectors. + + For "WithoutFreeSpace" images: + Only the lowest 4 bytes are used. The highest 4 bytes must be + cleared in this case. + + For "WithouFreSpacExt" images, there are no such + restrictions. + + 44 - 47: in_use + Set to 0x746F6E59 when the image is opened by software in R/W + mode; set to 0x312e3276 when the image is closed. + + A zero in this field means that the image was opened by an old + version of the software that doesn't support Format Extension + (see below). + + Other values are not allowed. + + 48 - 51: data_off + An offset, in sectors, from the start of the file to the start of + the data area. + + For "WithoutFreeSpace" images: + - If data_off is zero, the offset is calculated as the end of BAT + table plus some padding to ensure sector size alignment. + - If data_off is non-zero, the offset should be aligned to sector + size. However it is recommended to align it to cluster size for + newly created images. + + For "WithouFreSpacExt" images: + data_off must be non-zero and aligned to cluster size. + + 52 - 55: flags + Miscellaneous flags. + + Bit 0: Empty Image bit. If set, the image should be + considered clear. + + Bits 2-31: Unused. + + 56 - 63: ext_off + Format Extension offset, an offset, in sectors, from the start of + the file to the start of the Format Extension Cluster. + + ext_off must meet the same requirements as cluster offsets + defined by BAT entries (see below). + + +== BAT == + +BAT is placed immediately after the image header. In the file, BAT is a +contiguous array of 32-bit unsigned little-endian integers with +(bat_entries * 4) bytes size. + +Each BAT entry contains an offset from the start of the file to the +corresponding cluster. The offset set in clusters for "WithouFreSpacExt" images +and in sectors for "WithoutFreeSpace" images. + +If a BAT entry is zero, the corresponding cluster is not allocated and should +be considered as filled with zeroes. + +Cluster offsets specified by BAT entries must meet the following requirements: + - the value must not be lower than data offset (provided by header.data_off + or calculated as specified above), + - the value must be lower than the desired file size, + - the value must be unique among all BAT entries, + - the result of (cluster offset - data offset) must be aligned to cluster + size. + + +== Data Area == + +The data area is an area from the data offset (provided by header.data_off or +calculated as specified above) to the end of the file. It represents a +contiguous array of clusters. Most of them are allocated by the BAT, some may +be allocated by the ext_off field in the header while other may be allocated by +extensions. All clusters allocated by ext_off and extensions should meet the +same requirements as clusters specified by BAT entries. + + +== Format Extension == + +The Format Extension is an area 1 cluster in size that provides additional +format features. This cluster is addressed by the ext_off field in the header. +The format of the Format Extension area is the following: + + 0 - 7: magic + Must be 0xAB234CEF23DCEA87 + + 8 - 23: m_CheckSum + The MD5 checksum of the entire Header Extension cluster except + the first 24 bytes. + + The above are followed by feature sections or "extensions". The last + extension must be "End of features" (see below). + +Each feature section has the following format: + + 0 - 7: magic + The identifier of the feature: + 0x0000000000000000 - End of features + 0x20385FAE252CB34A - Dirty bitmap + + 8 - 15: flags + External flags for extension: + + Bit 0: NECESSARY + If the software cannot load the extension (due to an + unknown magic number or error), the file should not be + changed. If this flag is unset and there is an error on + loading the extension, said extension should be dropped. + + Bit 1: TRANSIT + If there is an unknown extension with this flag set, + said extension should be left as is. + + If neither NECESSARY nor TRANSIT are set, the extension should be + dropped. + + 16 - 19: data_size + The size of the following feature data, in bytes. + + 20 - 23: unused32 + Align header to 8 bytes boundary. + + variable: data (data_size bytes) + + The above is followed by padding to the next 8 bytes boundary, then the + next extension starts. + + The last extension must be "End of features" with all the fields set to 0. + + +=== Dirty bitmaps feature === + +This feature provides a way of storing dirty bitmaps in the image. The fields +of its data area are: + + 0 - 7: size + The bitmap size, should be equal to disk size in sectors. + + 8 - 23: id + An identifier for backup consistency checking. + + 24 - 27: granularity + Bitmap granularity, in sectors. I.e., the number of sectors + corresponding to one bit of the bitmap. Granularity must be + a power of 2. + + 28 - 31: l1_size + The number of entries in the L1 table of the bitmap. + + variable: l1 (64 * l1_size bytes) + L1 offset table (in bytes) + +A dirty bitmap is stored using a one-level structure for the mapping to host +clusters - an L1 table. + +Given an offset in bytes into the bitmap data, the offset in bytes into the +image file can be obtained as follows: + + offset = l1_table[offset / cluster_size] + (offset % cluster_size) + +If an L1 table entry is 0, the corresponding cluster of the bitmap is assumed +to be zero. + +If an L1 table entry is 1, the corresponding cluster of the bitmap is assumed +to have all bits set. + +If an L1 table entry is not 0 or 1, it allocates a cluster from the data area. |