Re: [Qemu-devel] [PATCH v2 2/5] block: vhdx header for the QEMU support of VHDX images

[Jeff Cody <jcody@redhat.com>]

On Tue, Apr 23, 2013 at 05:10:18PM +0200, Kevin Wolf wrote:
> Am 23.04.2013 um 16:24 hat Jeff Cody geschrieben:
> > This is based on Microsoft's VHDX specification:
> >     "VHDX Format Specification v0.95", published 4/12/2012
> >     https://www.microsoft.com/en-us/download/details.aspx?id=29681
> > 
> > These structures define the various header, metadata, and other
> > block structures defined in the VHDX specification.
> > 
> > Signed-off-by: Jeff Cody <jcody@redhat.com>
> > ---
> >  block/vhdx.h | 327 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 327 insertions(+)
> >  create mode 100644 block/vhdx.h
> > 
> > diff --git a/block/vhdx.h b/block/vhdx.h
> > new file mode 100644
> > index 0000000..f5cf1ed
> > --- /dev/null
> > +++ b/block/vhdx.h
> > @@ -0,0 +1,327 @@
> > +/*
> > + * Block driver for Hyper-V VHDX Images
> > + *
> > + * Copyright (c) 2013 Red Hat, Inc.,
> > + *
> > + * Authors:
> > + *  Jeff Cody <jcody@redhat.com>
> > + *
> > + *  This is based on the "VHDX Format Specification v0.95", published 4/12/2012
> > + *  by Microsoft:
> > + *      https://www.microsoft.com/en-us/download/details.aspx?id=29681
> > + *
> > + * This work is licensed under the terms of the GNU LGPL, version 2 or later.
> > + * See the COPYING.LIB file in the top-level directory.
> > + *
> > + */
> > +
> > +#ifndef BLOCK_VHDX_H
> > +#define BLOCK_VHDX_H
> > +
> > +/* Structures and fields present in the VHDX file */
> > +
> > +/* The header section has the following blocks,
> > + * each block is 64KB:
> > + *
> > + * _____________________________________________________________________________
> > + * | File Id. |   Header 1    | Header 2   | Region Table |  Reserved (768KB)  |
> > + * |----------|---------------|------------|--------------|--------------------|
> > + * |          |               |            |              |                    |
> > + * 0.........64KB...........128KB........192KB..........256KB................1MB
> > + */
> > +
> > +#define VHDX_HEADER_BLOCK_SIZE      (64*1024)
> > +
> > +#define VHDX_FILE_ID_OFFSET         0
> > +#define VHDX_HEADER1_OFFSET         (VHDX_HEADER_BLOCK_SIZE*1)
> > +#define VHDX_HEADER2_OFFSET         (VHDX_HEADER_BLOCK_SIZE*2)
> > +#define VHDX_REGION_TABLE_OFFSET    (VHDX_HEADER_BLOCK_SIZE*3)
> > +
> > +
> > +/*
> > + * A note on the use of MS-GUID fields.  For more details on the GUID,
> > + * please see: https://en.wikipedia.org/wiki/Globally_unique_identifier.
> > + *
> > + * The VHDX specification only states that these are MS GUIDs, and which
> > + * bytes are data1-data4. It makes no mention of what algorithm should be used
> > + * to generate the GUID, nor what standard.  However, looking at the specified
> > + * known GUID fields, it appears the GUIDs are:
> > + *  Standard/DCE GUID type  (noted by 10b in the MSB of byte 0 of .data4)
> > + *  Random algorithm        (noted by 0x4XXX for .data3)
> > + */
> > +
> > +/* ---- HEADER SECTION STRUCTURES ---- */
> > +
> > +/* Important note: these structures are as defined in the VHDX specification,
> > + * including byte order and size.  However, without being packed structures,
> > + * they will not match 1:1 data read from disk.  Rather than use potentially
> > + * non-portable packed structures, data is copied from read buffers into
> > + * the structures below.  However, for reference, please refrain from
> > + * modifying these structures to something that does not represent the spec */
> > +
> > +#define VHDX_FILE_ID_MAGIC 0x656C696678646876  /* 'vhdxfile' */
> > +typedef struct vhdx_file_identifier {
> 
> According to the qemu coding style, this is not a valid name for a
> struct. I think you can choose between VHDXFileIdentifier and
> VhdxFileIdentifier, where existing code tends towards the former.
> 
> More instances of this follow, but I won't comment on each.
>

OK.  I'll go through and change them all; almost all of them will need
to be changed.

> > +    uint64_t    signature;              /* "vhdxfile" in ASCII */
> > +    uint16_t    creator[256];           /* optional; utf-16 string to identify
> > +                                           the vhdx file creator.  Diagnotistic
> > +                                           only */
> > +} vhdx_file_identifier;
> > +
> > +
> > +/* the guid is a 16 byte unique ID - the definition for this used by
> > + * Microsoft is not just 16 bytes though - it is a structure that is defined,
> > + * so we need to follow it here so that endianness does not trip us up */
> > +
> > +typedef struct ms_guid {
> > +    uint32_t    data1;
> > +    uint16_t    data2;
> > +    uint16_t    data3;
> > +    uint8_t     data4[8];
> > +} ms_guid;
> > +
> > +#define guid_eq(a, b) \
> > +    (memcmp(&(a), &(b), sizeof(ms_guid)) == 0)
> > +
> > +#define VHDX_HEADER_SIZE (4*1024)   /* although the vhdx_header struct in disk
> > +                                       is only 582 bytes, for purposes of crc
> > +                                       the header is the first 4KB of the 64KB
> > +                                       block */
> > +
> > +#define VHDX_HDR_MAGIC 0x64616568   /* 'head' */
> > +typedef struct QEMU_PACKED vhdx_header {
> > +    uint32_t    signature;              /* "head" in ASCII */
> > +    uint32_t    checksum;               /* CRC-32C hash of the whole header */
> > +    uint64_t    sequence_number;        /* Seq number of this header.  Each
> > +                                           VHDX file has 2 of these headers,
> > +                                           and only the header with the highest
> > +                                           sequence number is valid */
> > +    ms_guid     file_write_guid;       /* 128 bit unique identifier. Must be
> > +                                           updated to new, unique value before
> > +                                           the first modification is made to
> > +                                           file */
> > +    ms_guid     data_write_guid;        /* 128 bit unique identifier. Must be
> > +                                           updated to new, unique value before
> > +                                           the first modification is made to
> > +                                           visible data.   Visbile data is
> > +                                           defined as:
> > +                                                    - system & user metadata
> > +                                                    - raw block data
> > +                                                    - disk size
> > +                                                    - any change that will
> > +                                                      cause the virtual disk
> > +                                                      sector read to differ
> > +
> > +                                           This does not need to change if
> > +                                           blocks are re-arranged */
> > +    ms_guid     log_guid;               /* 128 bit unique identifier. If zero,
> > +                                           there is no valid log. If non-zero,
> > +                                           log entries with this guid are
> > +                                           valid. */
> > +    uint16_t    log_version;            /* version of the log format. Mustn't be
> > +                                           zero, unless log_guid is also zero */
> > +    uint16_t    version;                /* version of th evhdx file.  Currently,
> > +                                           only supported version is "1" */
> > +    uint32_t    log_length;             /* length of the log.  Must be multiple
> > +                                           of 1MB */
> > +    uint64_t    log_offset;             /* byte offset in the file of the log.
> > +                                           Must also be a multiple of 1MB */
> > +} vhdx_header;
> > +
> > +/* 4KB in packed data size, not to be used except for initial data read */
> > +typedef struct QEMU_PACKED vhdx_header_padded {
> > +    vhdx_header header;
> > +    uint8_t     reserved[502];          /* per the VHDX spec */
> > +    uint8_t     reserved_[3514];        /* for the initial packed struct read */
> > +} vhdx_header_padded;
> > +
> > +/* Header for the region table block */
> > +#define VHDX_RT_MAGIC 0x69676572  /* 'regi ' */
> > +typedef struct QEMU_PACKED vhdx_region_table_header {
> > +    uint32_t    signature;              /* "regi" in ASCII */
> > +    uint32_t    checksum;               /* CRC-32C hash of the 64KB table */
> > +    uint32_t    entry_count;            /* number of valid entries */
> > +    uint32_t    reserved;
> > +} vhdx_region_table_header;
> > +
> > +/* Individual region table entry.  There may be a maximum of 2047 of these
> > + *
> > + *  There are two known region table properties.  Both are required.
> > + *  BAT (block allocation table):  2DC27766F62342009D64115E9BFD4A08
> > + *  Metadata:                      8B7CA20647904B9AB8FE575F050F886E
> > + */
> > +#define VHDX_REGION_ENTRY_REQUIRED  0x01    /* if set, parser must understand
> > +                                               this entry in order to open
> > +                                               file */
> > +typedef struct QEMU_PACKED vhdx_region_table_entry {
> > +    ms_guid     guid;                   /* 128-bit unique identifier */
> > +    uint64_t    file_offset;            /* offset of the object in the file.
> > +                                           Must be multiple of 1MB */
> > +    uint32_t    length;                 /* length, in bytes, of the object */
> > +    uint32_t    data_bits;
> > +} vhdx_region_table_entry;
> > +
> > +
> > +/* ---- LOG ENTRY STRUCTURES ---- */
> > +#define VHDX_LOG_HDR_SIZE 64
> > +#define VHDX_LOGE_MAGIC 0x65676F6C /* 'loge' */
> > +typedef struct QEMU_PACKED vhdx_log_entry_header {
> > +    uint32_t    signature;              /* "loge" in ASCII */
> > +    uint32_t    checksum;               /* CRC-32C hash of the 64KB table */
> > +    uint32_t    entry_length;           /* length in bytes, multiple of 1MB */
> > +    uint32_t    tail;                   /* byte offset of first log entry of a
> > +                                           seq, where this entry is the last
> > +                                           entry */
> > +    uint64_t    sequence_number;        /* incremented with each log entry.
> > +                                           May not be zero. */
> > +    uint32_t    descriptor_count;       /* number of descriptors in this log
> > +                                           entry, must be >= 0 */
> > +    uint32_t    reserved;
> > +    ms_guid     log_guid;               /* value of the log_guid from
> > +                                           vhdx_header.  If not found in
> > +                                           vhdx_header, it is invalid */
> > +    uint64_t    flushed_file_offset;    /* see spec for full details - this
> > +                                           sould be vhdx file size in bytes */
> > +    uint64_t    last_file_offset;       /* size in bytes that all allocated
> > +                                           file structures fit into */
> > +} vhdx_log_entry_header;
> > +
> > +#define VHDX_LOG_DESC_SIZE 32
> > +
> > +#define VHDX_ZERO_MAGIC 0x6F72657A /* 'zero' */
> > +#define VHDX_DATA_MAGIC 0x63736564 /* 'desc' */
> > +typedef struct QEMU_PACKED vhdx_log_descriptor {
> > +    uint32_t    signature;              /* "zero" or "desc" in ASCII */
> > +    union  {
> > +    uint32_t    reserved;               /* zero desc */
> > +    uint32_t    trailing_bytes;         /* data desc: bytes 4092-4096 of the
> > +                                           data sector */
> > +    };
> 
> Indentation inside the union is off.
> 
> > +    union {
> > +    uint64_t    zero_length;            /* zero desc: length of the section to
> > +                                           zero */
> > +    uint64_t    leading_bytes;          /* data desc: bytes 0-7 of the data
> > +                                           sector */
> > +    };
> 
> Here as well.
>

OK, thanks.

> > +    uint64_t    file_offset;            /* file offset to write zeros - multiple
> > +                                           of 4kB */
> > +    uint64_t    sequence_number;        /* must match same field in
> > +                                           vhdx_log_entry_header */
> > +} vhdx_log_descriptor;
> > +
> > +#define VHDX_DATAS_MAGIC 0x61746164 /* 'data' */
> > +typedef struct QEMU_PACKED vhdx_log_data_sector {
> > +    uint32_t    data_signature;         /* "data" in ASCII */
> > +    uint32_t    sequence_high;          /* 4 MSB of 8 byte sequence_number */
> > +    uint8_t     data[4084];             /* raw data, bytes 8-4091 (inclusive).
> > +                                           see the data descriptor field for the
> > +                                           other mising bytes */
> > +    uint32_t    sequence_low;           /* 4 LSB of 8 byte sequence_number */
> > +} vhdx_log_data_sector;
> > +
> > +
> > +
> > +/* block states - different state values depending on whether it is a
> > + * payload block, or a sector block. */
> > +
> > +#define PAYLOAD_BLOCK_NOT_PRESENT       0
> > +#define PAYLOAD_BLOCK_UNDEFINED         1
> > +#define PAYLOAD_BLOCK_ZERO              2
> > +#define PAYLOAD_BLOCK_UNMAPPED          5
> > +#define PAYLOAD_BLOCK_FULL_PRESENT      6
> > +#define PAYLOAD_BLOCK_PARTIALLY_PRESENT 7
> 
> Looks like an enum?
>

The spec lays them out as #define's, so this is a case where I figured
it helped with a reader comparing against the spec.  But I can make
this (and the corresponding ones below) into enums, if you are
worried about namespace pollution.


> > +#define SB_BLOCK_NOT_PRESENT    0
> > +#define SB_BLOCK_PRESENT        6
> > +
> > +/* per the spec */
> > +#define VHDX_MAX_SECTORS_PER_BLOCK  (1<<23)
> > +
> > +/* upper 44 bits are the file offset in 1MB units lower 3 bits are the state
> > +   other bits are reserved */
> > +#define VHDX_BAT_STATE_BIT_MASK 0x07
> > +#define VHDX_BAT_FILE_OFF_BITS (64-44)
> > +typedef uint64_t vhdx_bat_entry;
> > +
> > +/* ---- METADATA REGION STRUCTURES ---- */
> > +
> > +#define VHDX_METADATA_ENTRY_SIZE 32
> > +#define VHDX_METADATA_MAX_ENTRIES 2047  /* not including the header */
> > +#define VHDX_METADATA_TABLE_MAX_SIZE \
> > +    (VHDX_METADATA_ENTRY_SIZE * (VHDX_METADATA_MAX_ENTRIES+1))
> > +#define VHDX_METADATA_MAGIC 0x617461646174656D /* 'metadata' */
> > +typedef struct QEMU_PACKED vhdx_metadata_table_header {
> > +    uint64_t    signature;              /* "metadata" in ASCII */
> > +    uint16_t    reserved;
> > +    uint16_t    entry_count;            /* number table entries. <= 2047 */
> > +    uint32_t    reserved2[5];
> > +} vhdx_metadata_table_header;
> > +
> > +#define VHDX_META_FLAGS_IS_USER         0x01    /* max 1024 entries */
> > +#define VHDX_META_FLAGS_IS_VIRTUAL_DISK 0x02    /* virtual disk metadata if set,
> > +                                                   otherwise file metdata */
> > +#define VHDX_META_FLAGS_IS_REQUIRED     0x04    /* parse must understand this
> > +                                                   entry to open the file */
> > +typedef struct QEMU_PACKED vhdx_metadata_table_entry {
> > +    ms_guid     item_id;                /* 128-bit identifier for metadata */
> > +    uint32_t    offset;                 /* byte offset of the metadata.  At
> > +                                           least 64kB.  Relative to start of
> > +                                           metadata region */
> > +                                        /* note: if length = 0, so is offset */
> > +    uint32_t    length;                 /* length of metadata. <= 1MB. */
> > +    uint32_t    data_bits;      /* least-significant 3 bits are flags, the
> > +                                   rest are reserved (see above) */
> > +    uint32_t    reserved2;
> > +} vhdx_metadata_table_entry;
> > +
> > +#define VHDX_PARAMS_LEAVE_BLOCKS_ALLOCED 0x01   /* Do not change any blocks to
> > +                                                   be BLOCK_NOT_PRESENT.
> > +                                                   If set indicates a fixed
> > +                                                   size VHDX file */
> > +#define VHDX_PARAMS_HAS_PARENT           0x02    /* has parent / backing file */
> > +typedef struct QEMU_PACKED vhdx_file_parameters {
> > +    uint32_t    block_size;             /* size of each payload block, always
> > +                                           power of 2, <= 256MB and >= 1MB. */
> > +    uint32_t data_bits;     /* least-significant 2 bits are flags, the rest
> > +                               are reserved (see above) */
> > +} vhdx_file_parameters;
> > +
> > +typedef struct QEMU_PACKED vhdx_virtual_disk_size {
> > +    uint64_t    virtual_disk_size;      /* Size of the virtual disk, in bytes.
> > +                                           Must be multiple of the sector size,
> > +                                           max of 64TB */
> > +} vhdx_virtual_disk_size;
> > +
> > +typedef struct QEMU_PACKED vhdx_page83_data {
> > +    uint8_t     page_83_data[16];       /* unique id for scsi devices that
> > +                                           support page 0x83 */
> > +} vhdx_page83_data;
> > +
> > +typedef struct QEMU_PACKED vhdx_virtual_disk_logical_sector_size {
> > +    uint32_t    logical_sector_size;    /* virtual disk sector size (in bytes).
> > +                                           Can only be 512 or 4096 bytes */
> > +} vhdx_virtual_disk_logical_sector_size;
> > +
> > +typedef struct QEMU_PACKED vhdx_virtual_disk_physical_sector_size {
> > +    uint32_t    physical_sector_size;   /* physical sector size (in bytes).
> > +                                           Can only be 512 or 4096 bytes */
> > +} vhdx_virtual_disk_physical_sector_size;
> 
> What's the point with all the single-field structs?
> 

Another case of laying out as it was in the spec, for readability.  I
tried to keep the header as faithful to the spec as possible; if you
have a strong preference, I can drop these and just use the single
field types inside vhdx.c.

> > +typedef struct QEMU_PACKED vhdx_parent_locator_header {
> > +    uint8_t     locator_type[16];       /* type of the parent virtual disk. */
> > +    uint16_t    reserved;
> > +    uint16_t    key_value_count;        /* number of key/value pairs for this
> > +                                           locator */
> > +} vhdx_parent_locator_header;
> > +
> > +/* key and value strings are UNICODE strings, UTF-16 LE encoding, no NULs */
> > +typedef struct QEMU_PACKED vhdx_parent_locator_entry {
> > +    uint32_t    key_offset;             /* offset in metadata for key, > 0 */
> > +    uint32_t    value_offset;           /* offset in metadata for value, >0 */
> > +    uint16_t    key_length;             /* length of entry key, > 0 */
> > +    uint16_t    value_length;           /* length of entry value, > 0 */
> > +} vhdx_parent_locator_entry;
> > +
> > +
> > +/* ----- END VHDX SPECIFICATION STRUCTURES ---- */
> > +
> > +#endif
> 
> Kevin