/* $Id: QED.cpp $ */
/** @file
 * QED - QED Disk image.
 *
 * The QED backend implements support for the qemu enhanced disk format (short QED)
 * The specification for the format is available under http://wiki.qemu.org/Features/QED/Specification
 *
 * Missing things to implement:
 *    - compaction
 *    - resizing which requires block relocation (very rare case)
 */

/*
 * Copyright (C) 2011-2024 Oracle and/or its affiliates.
 *
 * This file is part of VirtualBox base platform packages, as
 * available from https://www.virtualbox.org.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation, in version 3 of the
 * License.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, see <https://www.gnu.org/licenses>.
 *
 * SPDX-License-Identifier: GPL-3.0-only
 */


/*********************************************************************************************************************************
*   Header Files                                                                                                                 *
*********************************************************************************************************************************/
#define LOG_GROUP LOG_GROUP_VD_QED
#include <VBox/vd-plugin.h>
#include <VBox/err.h>

#include <VBox/log.h>
#include <iprt/asm-mem.h>
#include <iprt/asm.h>
#include <iprt/assert.h>
#include <iprt/string.h>
#include <iprt/alloc.h>
#include <iprt/path.h>
#include <iprt/list.h>

#include "VDBackends.h"
#include "VDBackendsInline.h"


/*********************************************************************************************************************************
*   Structures in a QED image, little endian                                                                                     *
*********************************************************************************************************************************/

#pragma pack(1)
typedef struct QedHeader
{
    /** Magic value. */
    uint32_t    u32Magic;
    /** Cluster size in bytes. */
    uint32_t    u32ClusterSize;
    /** Size of L1 and L2 tables in clusters. */
    uint32_t    u32TableSize;
    /** size of this header structure in clusters. */
    uint32_t    u32HeaderSize;
    /** Features used for the image. */
    uint64_t    u64FeatureFlags;
    /** Compatibility features used for the image. */
    uint64_t    u64CompatFeatureFlags;
    /** Self resetting feature bits. */
    uint64_t    u64AutoresetFeatureFlags;
    /** Offset of the L1 table in bytes. */
    uint64_t    u64OffL1Table;
    /** Logical image size as seen by the guest. */
    uint64_t    u64Size;
    /** Offset of the backing filename in bytes. */
    uint32_t    u32OffBackingFilename;
    /** Size of the backing filename. */
    uint32_t    u32BackingFilenameSize;
} QedHeader;
#pragma pack()
/** Pointer to a on disk QED header. */
typedef QedHeader *PQedHeader;

/** QED magic value. */
#define QED_MAGIC                            UINT32_C(0x00444551) /* QED\0 */
/** Cluster size minimum. */
#define QED_CLUSTER_SIZE_MIN                 RT_BIT(12)
/** Cluster size maximum. */
#define QED_CLUSTER_SIZE_MAX                 RT_BIT(26)
/** L1 and L2 Table size minimum. */
#define QED_TABLE_SIZE_MIN                   1
/** L1 and L2 Table size maximum. */
#define QED_TABLE_SIZE_MAX                   16

/** QED default cluster size when creating an image. */
#define QED_CLUSTER_SIZE_DEFAULT             (64 * _1K)
/** The default table size in clusters. */
#define QED_TABLE_SIZE_DEFAULT               4

/** Feature flags.
 * @{
 */
/** Image uses a backing file to provide data for unallocated clusters. */
#define QED_FEATURE_BACKING_FILE             RT_BIT_64(0)
/** Image needs checking before use. */
#define QED_FEATURE_NEED_CHECK               RT_BIT_64(1)
/** Don't probe for format of the backing file, treat as raw image. */
#define QED_FEATURE_BACKING_FILE_NO_PROBE    RT_BIT_64(2)
/** Mask of valid features. */
#define QED_FEATURE_MASK (QED_FEATURE_BACKING_FILE | QED_FEATURE_NEED_CHECK | QED_FEATURE_BACKING_FILE_NO_PROBE)
/** @} */

/** Compatibility feature flags.
 * @{
 */
/** Mask of valid compatibility features. */
#define QED_COMPAT_FEATURE_MASK (0)
/** @} */

/** Autoreset feature flags.
 * @{
 */
/** Mask of valid autoreset features. */
#define QED_AUTORESET_FEATURE_MASK (0)
/** @} */


/*********************************************************************************************************************************
*   Constants And Macros, Structures and Typedefs                                                                                *
*********************************************************************************************************************************/

/**
 * QED L2 cache entry.
 */
typedef struct QEDL2CACHEENTRY
{
    /** List node for the search list. */
    RTLISTNODE              NodeSearch;
    /** List node for the LRU list. */
    RTLISTNODE              NodeLru;
    /** Reference counter. */
    uint32_t                cRefs;
    /** The offset of the L2 table, used as search key. */
    uint64_t                offL2Tbl;
    /** Pointer to the cached L2 table. */
    uint64_t               *paL2Tbl;
} QEDL2CACHEENTRY, *PQEDL2CACHEENTRY;

/** Maximum amount of memory the cache is allowed to use. */
#define QED_L2_CACHE_MEMORY_MAX (2*_1M)

/**
 * QED image data structure.
 */
typedef struct QEDIMAGE
{
    /** Image name. */
    const char          *pszFilename;
    /** Storage handle. */
    PVDIOSTORAGE        pStorage;

    /** Pointer to the per-disk VD interface list. */
    PVDINTERFACE        pVDIfsDisk;
    /** Pointer to the per-image VD interface list. */
    PVDINTERFACE        pVDIfsImage;
    /** Error interface. */
    PVDINTERFACEERROR   pIfError;
    /** I/O interface. */
    PVDINTERFACEIOINT   pIfIo;

    /** Open flags passed by VBoxHD layer. */
    unsigned            uOpenFlags;
    /** Image flags defined during creation or determined during open. */
    unsigned            uImageFlags;
    /** Total size of the image. */
    uint64_t            cbSize;
    /** Physical geometry of this image. */
    VDGEOMETRY          PCHSGeometry;
    /** Logical geometry of this image. */
    VDGEOMETRY          LCHSGeometry;

    /** Filename of the backing file if any. */
    char               *pszBackingFilename;
    /** Offset of the filename in the image. */
    uint32_t            offBackingFilename;
    /** Size of the backing filename excluding \0. */
    uint32_t            cbBackingFilename;

    /** Size of the image, multiple of clusters. */
    uint64_t            cbImage;
    /** Cluster size in bytes. */
    uint32_t            cbCluster;
    /** Number of entries in the L1 and L2 table. */
    uint32_t            cTableEntries;
    /** Size of an L1 or L2 table rounded to the next cluster size. */
    uint32_t            cbTable;
    /** Pointer to the L1 table. */
    uint64_t            *paL1Table;
    /** Offset of the L1 table. */
    uint64_t            offL1Table;

    /** Offset mask for a cluster. */
    uint64_t            fOffsetMask;
    /** L1 table mask to get the L1 index. */
    uint64_t            fL1Mask;
    /** Number of bits to shift to get the L1 index. */
    uint32_t            cL1Shift;
    /** L2 table mask to get the L2 index. */
    uint64_t            fL2Mask;
    /** Number of bits to shift to get the L2 index. */
    uint32_t            cL2Shift;

    /** Pointer to the L2 table we are currently allocating
     * (can be only one at a time). */
    PQEDL2CACHEENTRY    pL2TblAlloc;

    /** Memory occupied by the L2 table cache. */
    size_t              cbL2Cache;
    /** The sorted L2 entry list used for searching. */
    RTLISTNODE          ListSearch;
    /** The LRU L2 entry list used for eviction. */
    RTLISTNODE          ListLru;
    /** The static region list. */
    VDREGIONLIST        RegionList;
} QEDIMAGE, *PQEDIMAGE;

/**
 * State of the async cluster allocation.
 */
typedef enum QEDCLUSTERASYNCALLOCSTATE
{
    /** Invalid. */
    QEDCLUSTERASYNCALLOCSTATE_INVALID = 0,
    /** L2 table allocation. */
    QEDCLUSTERASYNCALLOCSTATE_L2_ALLOC,
    /** Link L2 table into L1. */
    QEDCLUSTERASYNCALLOCSTATE_L2_LINK,
    /** Allocate user data cluster. */
    QEDCLUSTERASYNCALLOCSTATE_USER_ALLOC,
    /** Link user data cluster. */
    QEDCLUSTERASYNCALLOCSTATE_USER_LINK,
    /** 32bit blowup. */
    QEDCLUSTERASYNCALLOCSTATE_32BIT_HACK = 0x7fffffff
} QEDCLUSTERASYNCALLOCSTATE, *PQEDCLUSTERASYNCALLOCSTATE;

/**
 * Data needed to track async cluster allocation.
 */
typedef struct QEDCLUSTERASYNCALLOC
{
    /** The state of the cluster allocation. */
    QEDCLUSTERASYNCALLOCSTATE enmAllocState;
    /** Old image size to rollback in case of an error. */
    uint64_t                  cbImageOld;
    /** L1 index to link if any. */
    uint32_t                  idxL1;
    /** L2 index to link, required in any case. */
    uint32_t                  idxL2;
    /** Start offset of the allocated cluster. */
    uint64_t                  offClusterNew;
    /** L2 cache entry if a L2 table is allocated. */
    PQEDL2CACHEENTRY          pL2Entry;
    /** Number of bytes to write. */
    size_t                    cbToWrite;
} QEDCLUSTERASYNCALLOC, *PQEDCLUSTERASYNCALLOC;


/*********************************************************************************************************************************
*   Static Variables                                                                                                             *
*********************************************************************************************************************************/

/** NULL-terminated array of supported file extensions. */
static const VDFILEEXTENSION s_aQedFileExtensions[] =
{
    {"qed", VDTYPE_HDD},
    {NULL,  VDTYPE_INVALID}
};


/*********************************************************************************************************************************
*   Internal Functions                                                                                                           *
*********************************************************************************************************************************/

/**
 * Converts the image header to the host endianess and performs basic checks.
 *
 * @returns Whether the given header is valid or not.
 * @param   pHeader    Pointer to the header to convert.
 */
static bool qedHdrConvertToHostEndianess(PQedHeader pHeader)
{
    pHeader->u32Magic                 = RT_LE2H_U32(pHeader->u32Magic);
    pHeader->u32ClusterSize           = RT_LE2H_U32(pHeader->u32ClusterSize);
    pHeader->u32TableSize             = RT_LE2H_U32(pHeader->u32TableSize);
    pHeader->u32HeaderSize            = RT_LE2H_U32(pHeader->u32HeaderSize);
    pHeader->u64FeatureFlags          = RT_LE2H_U64(pHeader->u64FeatureFlags);
    pHeader->u64CompatFeatureFlags    = RT_LE2H_U64(pHeader->u64CompatFeatureFlags);
    pHeader->u64AutoresetFeatureFlags = RT_LE2H_U64(pHeader->u64AutoresetFeatureFlags);
    pHeader->u64OffL1Table            = RT_LE2H_U64(pHeader->u64OffL1Table);
    pHeader->u64Size                  = RT_LE2H_U64(pHeader->u64Size);
    pHeader->u32OffBackingFilename    = RT_LE2H_U32(pHeader->u32OffBackingFilename);
    pHeader->u32BackingFilenameSize   = RT_LE2H_U32(pHeader->u32BackingFilenameSize);

    if (RT_UNLIKELY(pHeader->u32Magic != QED_MAGIC))
        return false;
    if (RT_UNLIKELY(   pHeader->u32ClusterSize < QED_CLUSTER_SIZE_MIN
                    || pHeader->u32ClusterSize > QED_CLUSTER_SIZE_MAX))
        return false;
    if (RT_UNLIKELY(   pHeader->u32TableSize < QED_TABLE_SIZE_MIN
                    || pHeader->u32TableSize > QED_TABLE_SIZE_MAX))
        return false;
    if (RT_UNLIKELY(pHeader->u64Size % 512 != 0))
        return false;
    if (RT_UNLIKELY(   pHeader->u64FeatureFlags & QED_FEATURE_BACKING_FILE
                    && (   pHeader->u32BackingFilenameSize == 0
                        || pHeader->u32BackingFilenameSize == UINT32_MAX)))
        return false;

    return true;
}

/**
 * Creates a QED header from the given image state.
 *
 * @param   pImage     Image instance data.
 * @param   pHeader    Pointer to the header to convert.
 */
static void qedHdrConvertFromHostEndianess(PQEDIMAGE pImage, PQedHeader pHeader)
{
    pHeader->u32Magic                 = RT_H2LE_U32(QED_MAGIC);
    pHeader->u32ClusterSize           = RT_H2LE_U32(pImage->cbCluster);
    pHeader->u32TableSize             = RT_H2LE_U32(pImage->cbTable / pImage->cbCluster);
    pHeader->u32HeaderSize            = RT_H2LE_U32(1);
    pHeader->u64FeatureFlags          = RT_H2LE_U64(pImage->pszBackingFilename ? QED_FEATURE_BACKING_FILE : UINT64_C(0));
    pHeader->u64CompatFeatureFlags    = RT_H2LE_U64(UINT64_C(0));
    pHeader->u64AutoresetFeatureFlags = RT_H2LE_U64(UINT64_C(0));
    pHeader->u64OffL1Table            = RT_H2LE_U64(pImage->offL1Table);
    pHeader->u64Size                  = RT_H2LE_U64(pImage->cbSize);
    pHeader->u32OffBackingFilename    = RT_H2LE_U32(pImage->offBackingFilename);
    pHeader->u32BackingFilenameSize   = RT_H2LE_U32(pImage->cbBackingFilename);
}

/**
 * Convert table entries from little endian to host endianess.
 *
 * @param   paTbl       Pointer to the table.
 * @param   cEntries    Number of entries in the table.
 */
static void qedTableConvertToHostEndianess(uint64_t *paTbl, uint32_t cEntries)
{
    while (cEntries-- > 0)
    {
        *paTbl = RT_LE2H_U64(*paTbl);
        paTbl++;
    }
}

#if defined(RT_BIG_ENDIAN)
/**
 * Convert table entries from host to little endian format.
 *
 * @param   paTblImg    Pointer to the table which will store the little endian table.
 * @param   paTbl       The source table to convert.
 * @param   cEntries    Number of entries in the table.
 */
static void qedTableConvertFromHostEndianess(uint64_t *paTblImg, uint64_t const *paTbl,
                                             uint32_t cEntries)
{
    while (cEntries-- > 0)
    {
        *paTblImg = RT_H2LE_U64(*paTbl);
        paTbl++;
        paTblImg++;
    }
}
#endif

/**
 * Creates the L2 table cache.
 *
 * @returns VBox status code.
 * @param   pImage    The image instance data.
 */
static int qedL2TblCacheCreate(PQEDIMAGE pImage)
{
    pImage->cbL2Cache = 0;
    RTListInit(&pImage->ListSearch);
    RTListInit(&pImage->ListLru);

    return VINF_SUCCESS;
}

/**
 * Destroys the L2 table cache.
 *
 * @param   pImage    The image instance data.
 */
static void qedL2TblCacheDestroy(PQEDIMAGE pImage)
{
    PQEDL2CACHEENTRY pL2Entry;
    PQEDL2CACHEENTRY pL2Next;
    RTListForEachSafe(&pImage->ListSearch, pL2Entry, pL2Next, QEDL2CACHEENTRY, NodeSearch)
    {
        Assert(!pL2Entry->cRefs);

        RTListNodeRemove(&pL2Entry->NodeSearch);
        RTMemPageFree(pL2Entry->paL2Tbl, pImage->cbTable);
        RTMemFree(pL2Entry);
    }

    pImage->cbL2Cache       = 0;
    RTListInit(&pImage->ListSearch);
    RTListInit(&pImage->ListLru);
}

/**
 * Returns the L2 table matching the given offset or NULL if none could be found.
 *
 * @returns Pointer to the L2 table cache entry or NULL.
 * @param   pImage    The image instance data.
 * @param   offL2Tbl  Offset of the L2 table to search for.
 */
static PQEDL2CACHEENTRY qedL2TblCacheRetain(PQEDIMAGE pImage, uint64_t offL2Tbl)
{
    if (   pImage->pL2TblAlloc
        && pImage->pL2TblAlloc->offL2Tbl == offL2Tbl)
    {
        pImage->pL2TblAlloc->cRefs++;
        return pImage->pL2TblAlloc;
    }

    PQEDL2CACHEENTRY pL2Entry;
    RTListForEach(&pImage->ListSearch, pL2Entry, QEDL2CACHEENTRY, NodeSearch)
    {
        if (pL2Entry->offL2Tbl == offL2Tbl)
            break;
    }

    if (!RTListNodeIsDummy(&pImage->ListSearch, pL2Entry, QEDL2CACHEENTRY, NodeSearch))
    {
        /* Update LRU list. */
        RTListNodeRemove(&pL2Entry->NodeLru);
        RTListPrepend(&pImage->ListLru, &pL2Entry->NodeLru);
        pL2Entry->cRefs++;
        return pL2Entry;
    }
    else
        return NULL;
}

/**
 * Releases a L2 table cache entry.
 *
 * @param   pL2Entry    The L2 cache entry.
 */
static void qedL2TblCacheEntryRelease(PQEDL2CACHEENTRY pL2Entry)
{
    Assert(pL2Entry->cRefs > 0);
    pL2Entry->cRefs--;
}

/**
 * Allocates a new L2 table from the cache evicting old entries if required.
 *
 * @returns Pointer to the L2 cache entry or NULL.
 * @param   pImage    The image instance data.
 */
static PQEDL2CACHEENTRY qedL2TblCacheEntryAlloc(PQEDIMAGE pImage)
{
    PQEDL2CACHEENTRY pL2Entry = NULL;

    if (pImage->cbL2Cache + pImage->cbTable <= QED_L2_CACHE_MEMORY_MAX)
    {
        /* Add a new entry. */
        pL2Entry = (PQEDL2CACHEENTRY)RTMemAllocZ(sizeof(QEDL2CACHEENTRY));
        if (pL2Entry)
        {
            pL2Entry->paL2Tbl = (uint64_t *)RTMemPageAllocZ(pImage->cbTable);
            if (RT_UNLIKELY(!pL2Entry->paL2Tbl))
            {
                RTMemFree(pL2Entry);
                pL2Entry = NULL;
            }
            else
            {
                pL2Entry->cRefs    = 1;
                pImage->cbL2Cache += pImage->cbTable;
            }
        }
    }
    else
    {
        /* Evict the last not in use entry and use it */
        Assert(!RTListIsEmpty(&pImage->ListLru));

        RTListForEachReverse(&pImage->ListLru, pL2Entry, QEDL2CACHEENTRY, NodeLru)
        {
            if (!pL2Entry->cRefs)
                break;
        }

        if (!RTListNodeIsDummy(&pImage->ListSearch, pL2Entry, QEDL2CACHEENTRY, NodeSearch))
        {
            RTListNodeRemove(&pL2Entry->NodeSearch);
            RTListNodeRemove(&pL2Entry->NodeLru);
            pL2Entry->offL2Tbl = 0;
            pL2Entry->cRefs    = 1;
        }
        else
            pL2Entry = NULL;
    }

    return pL2Entry;
}

/**
 * Frees a L2 table cache entry.
 *
 * @param   pImage    The image instance data.
 * @param   pL2Entry  The L2 cache entry to free.
 */
static void qedL2TblCacheEntryFree(PQEDIMAGE pImage, PQEDL2CACHEENTRY pL2Entry)
{
    Assert(!pL2Entry->cRefs);
    RTMemPageFree(pL2Entry->paL2Tbl, pImage->cbTable);
    RTMemFree(pL2Entry);

    pImage->cbL2Cache -= pImage->cbTable;
}

/**
 * Inserts an entry in the L2 table cache.
 *
 * @param   pImage    The image instance data.
 * @param   pL2Entry  The L2 cache entry to insert.
 */
static void qedL2TblCacheEntryInsert(PQEDIMAGE pImage, PQEDL2CACHEENTRY pL2Entry)
{
    Assert(pL2Entry->offL2Tbl > 0);

    /* Insert at the top of the LRU list. */
    RTListPrepend(&pImage->ListLru, &pL2Entry->NodeLru);

    if (RTListIsEmpty(&pImage->ListSearch))
    {
        RTListAppend(&pImage->ListSearch, &pL2Entry->NodeSearch);
    }
    else
    {
        /* Insert into search list. */
        PQEDL2CACHEENTRY pIt;
        pIt = RTListGetFirst(&pImage->ListSearch, QEDL2CACHEENTRY, NodeSearch);
        if (pIt->offL2Tbl > pL2Entry->offL2Tbl)
            RTListPrepend(&pImage->ListSearch, &pL2Entry->NodeSearch);
        else
        {
            bool fInserted = false;

            RTListForEach(&pImage->ListSearch, pIt, QEDL2CACHEENTRY, NodeSearch)
            {
                Assert(pIt->offL2Tbl != pL2Entry->offL2Tbl);
                if (pIt->offL2Tbl < pL2Entry->offL2Tbl)
                {
                    RTListNodeInsertAfter(&pIt->NodeSearch, &pL2Entry->NodeSearch);
                    fInserted = true;
                    break;
                }
            }
            Assert(fInserted);
        }
    }
}

/**
 * Fetches the L2 from the given offset trying the LRU cache first and
 * reading it from the image after a cache miss - version for async I/O.
 *
 * @returns VBox status code.
 * @param   pImage    Image instance data.
 * @param   pIoCtx    The I/O context.
 * @param   offL2Tbl  The offset of the L2 table in the image.
 * @param   ppL2Entry Where to store the L2 table on success.
 */
static int qedL2TblCacheFetchAsync(PQEDIMAGE pImage, PVDIOCTX pIoCtx,
                                   uint64_t offL2Tbl, PQEDL2CACHEENTRY *ppL2Entry)
{
    int rc = VINF_SUCCESS;

    /* Try to fetch the L2 table from the cache first. */
    PQEDL2CACHEENTRY pL2Entry = qedL2TblCacheRetain(pImage, offL2Tbl);
    if (!pL2Entry)
    {
        pL2Entry = qedL2TblCacheEntryAlloc(pImage);

        if (pL2Entry)
        {
            /* Read from the image. */
            PVDMETAXFER pMetaXfer;

            pL2Entry->offL2Tbl = offL2Tbl;
            rc = vdIfIoIntFileReadMeta(pImage->pIfIo, pImage->pStorage,
                                       offL2Tbl, pL2Entry->paL2Tbl,
                                       pImage->cbTable, pIoCtx,
                                       &pMetaXfer, NULL, NULL);
            if (RT_SUCCESS(rc))
            {
                vdIfIoIntMetaXferRelease(pImage->pIfIo, pMetaXfer);
#if defined(RT_BIG_ENDIAN)
                qedTableConvertToHostEndianess(pL2Entry->paL2Tbl, pImage->cTableEntries);
#endif
                qedL2TblCacheEntryInsert(pImage, pL2Entry);
            }
            else
            {
                qedL2TblCacheEntryRelease(pL2Entry);
                qedL2TblCacheEntryFree(pImage, pL2Entry);
            }
        }
        else
            rc = VERR_NO_MEMORY;
    }

    if (RT_SUCCESS(rc))
        *ppL2Entry = pL2Entry;

    return rc;
}

/**
 * Return power of 2 or 0 if num error.
 *
 * @returns The power of 2 or 0 if the given number is not a power of 2.
 * @param   u32    The number.
 */
static uint32_t qedGetPowerOfTwo(uint32_t u32)
{
    if (u32 == 0)
        return 0;
    uint32_t uPower2 = 0;
    while ((u32 & 1) == 0)
    {
        u32 >>= 1;
        uPower2++;
    }
    return u32 == 1 ? uPower2 : 0;
}

/**
 * Sets the L1, L2 and offset bitmasks and L1 and L2 bit shift members.
 *
 * @param   pImage    The image instance data.
 */
static void qedTableMasksInit(PQEDIMAGE pImage)
{
    uint32_t cClusterBits, cTableBits;

    cClusterBits = qedGetPowerOfTwo(pImage->cbCluster);
    cTableBits   = qedGetPowerOfTwo(pImage->cTableEntries);

    Assert(cClusterBits + 2 * cTableBits <= 64);

    pImage->fOffsetMask = ((uint64_t)pImage->cbCluster - 1);
    pImage->fL2Mask     = ((uint64_t)pImage->cTableEntries - 1) << cClusterBits;
    pImage->cL2Shift    = cClusterBits;
    pImage->fL1Mask     = ((uint64_t)pImage->cTableEntries - 1) << (cClusterBits + cTableBits);
    pImage->cL1Shift    = cClusterBits + cTableBits;
}

/**
 * Converts a given logical offset into the
 *
 * @param   pImage         The image instance data.
 * @param   off            The logical offset to convert.
 * @param   pidxL1         Where to store the index in the L1 table on success.
 * @param   pidxL2         Where to store the index in the L2 table on success.
 * @param   poffCluster    Where to store the offset in the cluster on success.
 */
DECLINLINE(void) qedConvertLogicalOffset(PQEDIMAGE pImage, uint64_t off, uint32_t *pidxL1,
                                         uint32_t *pidxL2, uint32_t *poffCluster)
{
    AssertPtr(pidxL1);
    AssertPtr(pidxL2);
    AssertPtr(poffCluster);

    *poffCluster = off & pImage->fOffsetMask;
    *pidxL1      = (off & pImage->fL1Mask) >> pImage->cL1Shift;
    *pidxL2      = (off & pImage->fL2Mask) >> pImage->cL2Shift;
}

/**
 * Converts Cluster size to a byte size.
 *
 * @returns Number of bytes derived from the given number of clusters.
 * @param   pImage    The image instance data.
 * @param   cClusters The clusters to convert.
 */
DECLINLINE(uint64_t) qedCluster2Byte(PQEDIMAGE pImage, uint64_t cClusters)
{
    return cClusters * pImage->cbCluster;
}

/**
 * Converts number of bytes to cluster size rounding to the next cluster.
 *
 * @returns Number of bytes derived from the given number of clusters.
 * @param   pImage    The image instance data.
 * @param   cb        Number of bytes to convert.
 */
DECLINLINE(uint64_t) qedByte2Cluster(PQEDIMAGE pImage, uint64_t cb)
{
    return cb / pImage->cbCluster + (cb % pImage->cbCluster ? 1 : 0);
}

/**
 * Allocates a new cluster in the image.
 *
 * @returns The start offset of the new cluster in the image.
 * @param   pImage    The image instance data.
 * @param   cClusters Number of clusters to allocate.
 */
DECLINLINE(uint64_t) qedClusterAllocate(PQEDIMAGE pImage, uint32_t cClusters)
{
    uint64_t offCluster;

    offCluster = pImage->cbImage;
    pImage->cbImage += cClusters*pImage->cbCluster;

    return offCluster;
}

/**
 * Returns the real image offset for a given cluster or an error if the cluster is not
 * yet allocated.
 *
 * @returns VBox status code.
 *          VERR_VD_BLOCK_FREE if the cluster is not yet allocated.
 * @param   pImage        The image instance data.
 * @param   pIoCtx        The I/O context.
 * @param   idxL1         The L1 index.
 * @param   idxL2         The L2 index.
 * @param   offCluster    Offset inside the cluster.
 * @param   poffImage     Where to store the image offset on success;
 */
static int qedConvertToImageOffset(PQEDIMAGE pImage, PVDIOCTX pIoCtx,
                                   uint32_t idxL1, uint32_t idxL2,
                                   uint32_t offCluster, uint64_t *poffImage)
{
    int rc = VERR_VD_BLOCK_FREE;

    AssertReturn(idxL1 < pImage->cTableEntries, VERR_INVALID_PARAMETER);
    AssertReturn(idxL2 < pImage->cTableEntries, VERR_INVALID_PARAMETER);

    if (pImage->paL1Table[idxL1])
    {
        PQEDL2CACHEENTRY pL2Entry;

        rc = qedL2TblCacheFetchAsync(pImage, pIoCtx, pImage->paL1Table[idxL1],
                                     &pL2Entry);
        if (RT_SUCCESS(rc))
        {
            /* Get real file offset. */
            if (pL2Entry->paL2Tbl[idxL2])
                *poffImage = pL2Entry->paL2Tbl[idxL2] + offCluster;
            else
                rc = VERR_VD_BLOCK_FREE;

            qedL2TblCacheEntryRelease(pL2Entry);
        }
    }

    return rc;
}

/**
 * Write the given table to image converting to the image endianess if required.
 *
 * @returns VBox status code.
 * @param   pImage        The image instance data.
 * @param   pIoCtx        The I/O context.
 * @param   offTbl        The offset the table should be written to.
 * @param   paTbl         The table to write.
 * @param   pfnComplete   Callback called when the write completes.
 * @param   pvUser        Opaque user data to pass in the completion callback.
 */
static int qedTblWrite(PQEDIMAGE pImage, PVDIOCTX pIoCtx, uint64_t offTbl, uint64_t *paTbl,
                       PFNVDXFERCOMPLETED pfnComplete, void *pvUser)
{
    int rc = VINF_SUCCESS;

#if defined(RT_BIG_ENDIAN)
    uint64_t *paTblImg = (uint64_t *)RTMemAllocZ(pImage->cbTable);
    if (paTblImg)
    {
        qedTableConvertFromHostEndianess(paTblImg, paTbl,
                                         pImage->cTableEntries);
        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                    offTbl, paTblImg, pImage->cbTable,
                                    pIoCtx, pfnComplete, pvUser);
        RTMemFree(paTblImg);
    }
    else
        rc = VERR_NO_MEMORY;
#else
    /* Write table directly. */
    rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                offTbl, paTbl, pImage->cbTable, pIoCtx,
                                pfnComplete, pvUser);
#endif

    return rc;
}

/**
 * Internal. Flush image data to disk.
 */
static int qedFlushImage(PQEDIMAGE pImage)
{
    int rc = VINF_SUCCESS;

    if (   pImage->pStorage
        && !(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
    {
        QedHeader Header;

        Assert(!(pImage->cbTable % pImage->cbCluster));
#if defined(RT_BIG_ENDIAN)
        uint64_t *paL1TblImg = (uint64_t *)RTMemAllocZ(pImage->cbTable);
        if (paL1TblImg)
        {
            qedTableConvertFromHostEndianess(paL1TblImg, pImage->paL1Table,
                                             pImage->cTableEntries);
            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
                                        pImage->offL1Table, paL1TblImg,
                                        pImage->cbTable);
            RTMemFree(paL1TblImg);
        }
        else
            rc = VERR_NO_MEMORY;
#else
        /* Write L1 table directly. */
        rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, pImage->offL1Table,
                                    pImage->paL1Table, pImage->cbTable);
#endif
        if (RT_SUCCESS(rc))
        {
            /* Write header. */
            qedHdrConvertFromHostEndianess(pImage, &Header);
            rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage, 0, &Header,
                                        sizeof(Header));
            if (RT_SUCCESS(rc))
                rc = vdIfIoIntFileFlushSync(pImage->pIfIo, pImage->pStorage);
        }
    }

    return rc;
}

/**
 * Checks whether the given cluster offset is valid.
 *
 * @returns Whether the given cluster offset is valid.
 * @param   offCluster    The table offset to check.
 * @param   cbFile        The real file size of the image.
 * @param   cbCluster     The cluster size in bytes.
 */
DECLINLINE(bool) qedIsClusterOffsetValid(uint64_t offCluster, uint64_t cbFile, size_t cbCluster)
{
    return    (offCluster <= cbFile - cbCluster)
           && !(offCluster & (cbCluster - 1));
}

/**
 * Checks whether the given table offset is valid.
 *
 * @returns Whether the given table offset is valid.
 * @param   offTbl    The table offset to check.
 * @param   cbFile    The real file size of the image.
 * @param   cbTable   The table size in bytes.
 * @param   cbCluster The cluster size in bytes.
 */
DECLINLINE(bool) qedIsTblOffsetValid(uint64_t offTbl, uint64_t cbFile, size_t cbTable, size_t cbCluster)
{
    return    (offTbl <= cbFile - cbTable)
           && !(offTbl & (cbCluster - 1));
}

/**
 * Sets the specified range in the cluster bitmap checking whether any of the clusters is already
 * used before.
 *
 * @returns Whether the range was clear and is set now.
 * @param   pvClusterBitmap    The cluster bitmap to use.
 * @param   offClusterStart    The first cluster to check and set.
 * @param   offClusterEnd      The first cluster to not check and set anymore.
 */
static bool qedClusterBitmapCheckAndSet(void *pvClusterBitmap, uint32_t offClusterStart, uint32_t offClusterEnd)
{
    for (uint32_t offCluster = offClusterStart; offCluster < offClusterEnd; offCluster++)
        if (ASMBitTest(pvClusterBitmap, offCluster))
            return false;

    ASMBitSetRange(pvClusterBitmap, offClusterStart, offClusterEnd);
    return true;
}

/**
 * Checks the given image for consistency, usually called when the
 * QED_FEATURE_NEED_CHECK bit is set.
 *
 * @returns VBox status code.
 * @retval VINF_SUCCESS when the image can be accessed.
 * @param   pImage    The image instance data.
 * @param   pHeader   The header to use for checking.
 *
 * @note It is not required that the image state is fully initialized  Only
 *       The I/O interface and storage handle need to be valid.
 * @note The header must be converted to the host CPU endian format already
 *       and should be validated already.
 */
static int qedCheckImage(PQEDIMAGE pImage, PQedHeader pHeader)
{
    uint64_t cbFile;
    uint32_t cbTable;
    uint32_t cTableEntries;
    uint64_t *paL1Tbl = NULL;
    uint64_t *paL2Tbl = NULL;
    void *pvClusterBitmap = NULL;
    uint32_t offClusterStart;
    int rc = VINF_SUCCESS;

    pImage->cbCluster = pHeader->u32ClusterSize;
    cbTable       = pHeader->u32TableSize * pHeader->u32ClusterSize;
    cTableEntries = cbTable / sizeof(uint64_t);

    do
    {
        rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &cbFile);
        if (RT_FAILURE(rc))
        {
            rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                           N_("Qed: Querying the file size of image '%s' failed"),
                           pImage->pszFilename);
            break;
        }

        /* Allocate L1 table. */
        paL1Tbl = (uint64_t *)RTMemAllocZ(cbTable);
        if (!paL1Tbl)
        {
            rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                          N_("Qed: Allocating memory for the L1 table for image '%s' failed"),
                          pImage->pszFilename);
            break;
        }

        paL2Tbl = (uint64_t *)RTMemAllocZ(cbTable);
        if (!paL2Tbl)
        {
            rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                           N_("Qed: Allocating memory for the L2 table for image '%s' failed"),
                           pImage->pszFilename);
            break;
        }

        pvClusterBitmap = RTMemAllocZ(cbFile / pHeader->u32ClusterSize / 8);
        if (!pvClusterBitmap)
        {
            rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                           N_("Qed: Allocating memory for the cluster bitmap for image '%s' failed"),
                           pImage->pszFilename);
            break;
        }

        /* Validate L1 table offset. */
        if (!qedIsTblOffsetValid(pHeader->u64OffL1Table, cbFile, cbTable, pHeader->u32ClusterSize))
        {
            rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                           N_("Qed: L1 table offset of image '%s' is corrupt (%llu)"),
                           pImage->pszFilename, pHeader->u64OffL1Table);
            break;
        }

        /* Read L1 table. */
        rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                   pHeader->u64OffL1Table, paL1Tbl, cbTable);
        if (RT_FAILURE(rc))
        {
            rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                           N_("Qed: Reading the L1 table from image '%s' failed"),
                           pImage->pszFilename);
            break;
        }

        /* Mark the L1 table in cluster bitmap. */
        ASMBitSet(pvClusterBitmap, 0); /* Header is always in cluster 0. */
        offClusterStart = qedByte2Cluster(pImage, pHeader->u64OffL1Table);
        bool fSet = qedClusterBitmapCheckAndSet(pvClusterBitmap, offClusterStart, offClusterStart + pHeader->u32TableSize);
        Assert(fSet);

        /* Scan the L1 and L2 tables for invalid entries. */
        qedTableConvertToHostEndianess(paL1Tbl, cTableEntries);

        for (unsigned iL1 = 0; iL1 < cTableEntries; iL1++)
        {
            if (!paL1Tbl[iL1])
                continue; /* Skip unallocated clusters. */

            if (!qedIsTblOffsetValid(paL1Tbl[iL1], cbFile, cbTable, pHeader->u32ClusterSize))
            {
                rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                               N_("Qed: Entry %d of the L1 table from image '%s' is invalid (%llu)"),
                               iL1, pImage->pszFilename, paL1Tbl[iL1]);
                break;
            }

            /* Now check that the clusters are not allocated already. */
            offClusterStart = qedByte2Cluster(pImage, paL1Tbl[iL1]);
            fSet = qedClusterBitmapCheckAndSet(pvClusterBitmap, offClusterStart, offClusterStart + pHeader->u32TableSize);
            if (!fSet)
            {
                rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                               N_("Qed: Entry %d of the L1 table from image '%s' points to a already used cluster (%llu)"),
                               iL1, pImage->pszFilename, paL1Tbl[iL1]);
                break;
            }

            /* Read the linked L2 table and check it. */
            rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                       paL1Tbl[iL1], paL2Tbl, cbTable);
            if (RT_FAILURE(rc))
            {
                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                               N_("Qed: Reading the L2 table from image '%s' failed"),
                               pImage->pszFilename);
                break;
            }

            /* Check all L2 entries. */
            for (unsigned iL2 = 0; iL2 < cTableEntries; iL2++)
            {
                if (paL2Tbl[iL2])
                    continue; /* Skip unallocated clusters. */

                if (!qedIsClusterOffsetValid(paL2Tbl[iL2], cbFile, pHeader->u32ClusterSize))
                {
                    rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                                   N_("Qed: Entry %d of the L2 table from image '%s' is invalid (%llu)"),
                                   iL2, pImage->pszFilename, paL2Tbl[iL2]);
                    break;
                }

                /* Now check that the clusters are not allocated already. */
                offClusterStart = qedByte2Cluster(pImage, paL2Tbl[iL2]);
                fSet = qedClusterBitmapCheckAndSet(pvClusterBitmap, offClusterStart, offClusterStart + 1);
                if (!fSet)
                {
                    rc = vdIfError(pImage->pIfError, VERR_VD_GEN_INVALID_HEADER, RT_SRC_POS,
                                   N_("Qed: Entry %d of the L2 table from image '%s' points to a already used cluster (%llu)"),
                                   iL2, pImage->pszFilename, paL2Tbl[iL2]);
                    break;
                }
            }
        }
    } while(0);

    if (paL1Tbl)
        RTMemFree(paL1Tbl);
    if (paL2Tbl)
        RTMemFree(paL2Tbl);
    if (pvClusterBitmap)
        RTMemFree(pvClusterBitmap);

    return rc;
}

/**
 * Internal. Free all allocated space for representing an image except pImage,
 * and optionally delete the image from disk.
 */
static int qedFreeImage(PQEDIMAGE pImage, bool fDelete)
{
    int rc = VINF_SUCCESS;

    /* Freeing a never allocated image (e.g. because the open failed) is
     * not signalled as an error. After all nothing bad happens. */
    if (pImage)
    {
        if (pImage->pStorage)
        {
            /* No point updating the file that is deleted anyway. */
            if (!fDelete)
                qedFlushImage(pImage);

            rc = vdIfIoIntFileClose(pImage->pIfIo, pImage->pStorage);
            pImage->pStorage = NULL;
        }

        if (pImage->paL1Table)
            RTMemFree(pImage->paL1Table);

        if (pImage->pszBackingFilename)
        {
            RTStrFree(pImage->pszBackingFilename);
            pImage->pszBackingFilename = NULL;
        }

        qedL2TblCacheDestroy(pImage);

        if (fDelete && pImage->pszFilename)
            vdIfIoIntFileDelete(pImage->pIfIo, pImage->pszFilename);
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/**
 * Internal: Open an image, constructing all necessary data structures.
 */
static int qedOpenImage(PQEDIMAGE pImage, unsigned uOpenFlags)
{
    pImage->uOpenFlags = uOpenFlags;

    pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
    pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
    AssertPtrReturn(pImage->pIfIo, VERR_INVALID_PARAMETER);

    /*
     * Create the L2 cache before opening the image so we can call qedFreeImage()
     * even if opening the image file fails.
     */
    int rc = qedL2TblCacheCreate(pImage);
    if (RT_SUCCESS(rc))
    {
        /* Open the image. */
        rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename,
                               VDOpenFlagsToFileOpenFlags(uOpenFlags,
                                                          false /* fCreate */),
                               &pImage->pStorage);
        if (RT_SUCCESS(rc))
        {
            uint64_t cbFile;
            rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &cbFile);
            if (   RT_SUCCESS(rc)
                && cbFile > sizeof(QedHeader))
            {
                QedHeader Header;

                rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage, 0, &Header, sizeof(Header));
                if (   RT_SUCCESS(rc)
                    && qedHdrConvertToHostEndianess(&Header))
                {
                    if (   !(Header.u64FeatureFlags & ~QED_FEATURE_MASK)
                        && !(Header.u64FeatureFlags & QED_FEATURE_BACKING_FILE_NO_PROBE))
                    {
                        if (Header.u64FeatureFlags & QED_FEATURE_NEED_CHECK)
                        {
                            /* Image needs checking. */
                            if (!(uOpenFlags & VD_OPEN_FLAGS_READONLY))
                                rc = qedCheckImage(pImage, &Header);
                            else
                                rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                               N_("Qed: Image '%s' needs checking but is opened readonly"),
                                               pImage->pszFilename);
                        }

                        if (   RT_SUCCESS(rc)
                            && (Header.u64FeatureFlags & QED_FEATURE_BACKING_FILE))
                        {
                            /* Load backing filename from image. */
                            pImage->pszBackingFilename = RTStrAlloc(Header.u32BackingFilenameSize + 1); /* +1 for \0 terminator. */
                            if (pImage->pszBackingFilename)
                            {
                                RT_BZERO(pImage->pszBackingFilename, Header.u32BackingFilenameSize + 1);
                                pImage->cbBackingFilename  = Header.u32BackingFilenameSize;
                                pImage->offBackingFilename = Header.u32OffBackingFilename;
                                rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                                           Header.u32OffBackingFilename, pImage->pszBackingFilename,
                                                           Header.u32BackingFilenameSize);
                                if (RT_SUCCESS(rc))
                                    rc = RTStrValidateEncoding(pImage->pszBackingFilename);
                            }
                            else
                                rc = VERR_NO_STR_MEMORY;
                        }

                        if (RT_SUCCESS(rc))
                        {
                            pImage->cbImage       = cbFile;
                            pImage->cbCluster     = Header.u32ClusterSize;
                            pImage->cbTable       = Header.u32TableSize * pImage->cbCluster;
                            pImage->cTableEntries = pImage->cbTable / sizeof(uint64_t);
                            pImage->offL1Table    = Header.u64OffL1Table;
                            pImage->cbSize        = Header.u64Size;
                            qedTableMasksInit(pImage);

                            /* Allocate L1 table. */
                            pImage->paL1Table     = (uint64_t *)RTMemAllocZ(pImage->cbTable);
                            if (pImage->paL1Table)
                            {
                                /* Read from the image. */
                                rc = vdIfIoIntFileReadSync(pImage->pIfIo, pImage->pStorage,
                                                           pImage->offL1Table, pImage->paL1Table,
                                                           pImage->cbTable);
                                if (RT_SUCCESS(rc))
                                {
                                    qedTableConvertToHostEndianess(pImage->paL1Table, pImage->cTableEntries);

                                    /* If the consistency check succeeded, clear the flag by flushing the image. */
                                    if (Header.u64FeatureFlags & QED_FEATURE_NEED_CHECK)
                                        rc = qedFlushImage(pImage);
                                }
                                else
                                    rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                                                   N_("Qed: Reading the L1 table for image '%s' failed"),
                                                   pImage->pszFilename);
                            }
                            else
                                rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS,
                                               N_("Qed: Out of memory allocating L1 table for image '%s'"),
                                               pImage->pszFilename);
                        }
                    }
                    else
                        rc = vdIfError(pImage->pIfError, VERR_NOT_SUPPORTED, RT_SRC_POS,
                                       N_("Qed: The image '%s' makes use of unsupported features"),
                                       pImage->pszFilename);
                }
                else if (RT_SUCCESS(rc))
                    rc = VERR_VD_GEN_INVALID_HEADER;
            }
            else if (RT_SUCCESS(rc))
                rc = VERR_VD_GEN_INVALID_HEADER;
        }
        /* else: Do NOT signal an appropriate error here, as the VD layer has the
         *       choice of retrying the open if it failed. */
    }
    else
        rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS,
                       N_("Qed: Creating the L2 table cache for image '%s' failed"),
                       pImage->pszFilename);

    if (RT_SUCCESS(rc))
    {
        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
        pImage->RegionList.fFlags   = 0;
        pImage->RegionList.cRegions = 1;

        pRegion->offRegion            = 0; /* Disk start. */
        pRegion->cbBlock              = 512;
        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
        pRegion->cbData               = 512;
        pRegion->cbMetadata           = 0;
        pRegion->cRegionBlocksOrBytes = pImage->cbSize;
    }
    else
        qedFreeImage(pImage, false);
    return rc;
}

/**
 * Internal: Create a qed image.
 */
static int qedCreateImage(PQEDIMAGE pImage, uint64_t cbSize,
                          unsigned uImageFlags, const char *pszComment,
                          PCVDGEOMETRY pPCHSGeometry,
                          PCVDGEOMETRY pLCHSGeometry, unsigned uOpenFlags,
                          PVDINTERFACEPROGRESS pIfProgress,
                          unsigned uPercentStart, unsigned uPercentSpan)
{
    RT_NOREF1(pszComment);
    int rc;

    if (!(uImageFlags & VD_IMAGE_FLAGS_FIXED))
    {
        rc = qedL2TblCacheCreate(pImage);
        if (RT_SUCCESS(rc))
        {
            pImage->uOpenFlags   = uOpenFlags & ~VD_OPEN_FLAGS_READONLY;
            pImage->uImageFlags  = uImageFlags;
            pImage->PCHSGeometry = *pPCHSGeometry;
            pImage->LCHSGeometry = *pLCHSGeometry;

            pImage->pIfError = VDIfErrorGet(pImage->pVDIfsDisk);
            pImage->pIfIo = VDIfIoIntGet(pImage->pVDIfsImage);
            AssertPtrReturn(pImage->pIfIo, VERR_INVALID_PARAMETER);

            /* Create image file. */
            uint32_t fOpen = VDOpenFlagsToFileOpenFlags(pImage->uOpenFlags, true /* fCreate */);
            rc = vdIfIoIntFileOpen(pImage->pIfIo, pImage->pszFilename, fOpen, &pImage->pStorage);
            if (RT_SUCCESS(rc))
            {
                /* Init image state. */
                pImage->cbSize             = cbSize;
                pImage->cbCluster          = QED_CLUSTER_SIZE_DEFAULT;
                pImage->cbTable            = qedCluster2Byte(pImage, QED_TABLE_SIZE_DEFAULT);
                pImage->cTableEntries      = pImage->cbTable / sizeof(uint64_t);
                pImage->offL1Table         = qedCluster2Byte(pImage, 1); /* Cluster 0 is the header. */
                pImage->cbImage            = (1 * pImage->cbCluster) + pImage->cbTable; /* Header + L1 table size. */
                pImage->cbBackingFilename  = 0;
                pImage->offBackingFilename = 0;
                qedTableMasksInit(pImage);

                /* Init L1 table. */
                pImage->paL1Table = (uint64_t *)RTMemAllocZ(pImage->cbTable);
                if (RT_LIKELY(pImage->paL1Table))
                {
                    vdIfProgress(pIfProgress, uPercentStart + uPercentSpan * 98 / 100);
                    rc = qedFlushImage(pImage);
                }
                else
                    rc = vdIfError(pImage->pIfError, VERR_NO_MEMORY, RT_SRC_POS, N_("Qed: cannot allocate memory for L1 table of image '%s'"),
                                   pImage->pszFilename);
            }
            else
                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("Qed: cannot create image '%s'"), pImage->pszFilename);
        }
        else
            rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("Qed: Failed to create L2 cache for image '%s'"),
                           pImage->pszFilename);
    }
    else
        rc = vdIfError(pImage->pIfError, VERR_VD_INVALID_TYPE, RT_SRC_POS, N_("Qed: cannot create fixed image '%s'"), pImage->pszFilename);

    if (RT_SUCCESS(rc))
    {
        PVDREGIONDESC pRegion = &pImage->RegionList.aRegions[0];
        pImage->RegionList.fFlags   = 0;
        pImage->RegionList.cRegions = 1;

        pRegion->offRegion            = 0; /* Disk start. */
        pRegion->cbBlock              = 512;
        pRegion->enmDataForm          = VDREGIONDATAFORM_RAW;
        pRegion->enmMetadataForm      = VDREGIONMETADATAFORM_NONE;
        pRegion->cbData               = 512;
        pRegion->cbMetadata           = 0;
        pRegion->cRegionBlocksOrBytes = pImage->cbSize;

        vdIfProgress(pIfProgress, uPercentStart + uPercentSpan);
    }
    else
        qedFreeImage(pImage, rc != VERR_ALREADY_EXISTS);

    return rc;
}

/**
 * Rollback anything done during async cluster allocation.
 *
 * @returns VBox status code.
 * @param   pImage           The image instance data.
 * @param   pIoCtx           The I/O context.
 * @param   pClusterAlloc    The cluster allocation to rollback.
 */
static int qedAsyncClusterAllocRollback(PQEDIMAGE pImage, PVDIOCTX pIoCtx, PQEDCLUSTERASYNCALLOC pClusterAlloc)
{
    RT_NOREF1(pIoCtx);
    int rc = VINF_SUCCESS;

    switch (pClusterAlloc->enmAllocState)
    {
        case QEDCLUSTERASYNCALLOCSTATE_L2_ALLOC:
        case QEDCLUSTERASYNCALLOCSTATE_L2_LINK:
        {
            /* Revert the L1 table entry */
            pImage->paL1Table[pClusterAlloc->idxL1] = 0;
            pImage->pL2TblAlloc = NULL;

            /* Assumption right now is that the L1 table is not modified on storage if the link fails. */
            rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage, pClusterAlloc->cbImageOld);
            qedL2TblCacheEntryRelease(pClusterAlloc->pL2Entry); /* Release L2 cache entry. */
            Assert(!pClusterAlloc->pL2Entry->cRefs);
            qedL2TblCacheEntryFree(pImage, pClusterAlloc->pL2Entry); /* Free it, it is not in the cache yet. */
            break;
        }
        case QEDCLUSTERASYNCALLOCSTATE_USER_ALLOC:
        case QEDCLUSTERASYNCALLOCSTATE_USER_LINK:
        {
            /* Assumption right now is that the L2 table is not modified if the link fails. */
            pClusterAlloc->pL2Entry->paL2Tbl[pClusterAlloc->idxL2] = 0;
            rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage, pClusterAlloc->cbImageOld);
            qedL2TblCacheEntryRelease(pClusterAlloc->pL2Entry); /* Release L2 cache entry. */
            break;
        }
        default:
            AssertMsgFailed(("Invalid cluster allocation state %d\n", pClusterAlloc->enmAllocState));
            rc = VERR_INVALID_STATE;
    }

    RTMemFree(pClusterAlloc);
    return rc;
}

/**
 * Updates the state of the async cluster allocation.
 *
 * @returns VBox status code.
 * @param   pBackendData    The opaque backend data.
 * @param   pIoCtx          I/O context associated with this request.
 * @param   pvUser          Opaque user data passed during a read/write request.
 * @param   rcReq           Status code for the completed request.
 */
static DECLCALLBACK(int) qedAsyncClusterAllocUpdate(void *pBackendData, PVDIOCTX pIoCtx, void *pvUser, int rcReq)
{
    int rc = VINF_SUCCESS;
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    PQEDCLUSTERASYNCALLOC pClusterAlloc = (PQEDCLUSTERASYNCALLOC)pvUser;

    if (RT_FAILURE(rcReq))
        return qedAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);

    AssertPtr(pClusterAlloc->pL2Entry);

    switch (pClusterAlloc->enmAllocState)
    {
        case QEDCLUSTERASYNCALLOCSTATE_L2_ALLOC:
        {
            /* Update the link in the in memory L1 table now. */
            pImage->paL1Table[pClusterAlloc->idxL1] = pClusterAlloc->pL2Entry->offL2Tbl;

            /* Update the link in the on disk L1 table now. */
            pClusterAlloc->enmAllocState = QEDCLUSTERASYNCALLOCSTATE_L2_LINK;
            rc = qedTblWrite(pImage, pIoCtx, pImage->offL1Table, pImage->paL1Table,
                             qedAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                /* Rollback. */
                qedAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                break;
            }
        }
        RT_FALL_THRU();
        case QEDCLUSTERASYNCALLOCSTATE_L2_LINK:
        {
            /* L2 link updated in L1 , save L2 entry in cache and allocate new user data cluster. */
            uint64_t offData = qedClusterAllocate(pImage, 1);

            pImage->pL2TblAlloc = NULL;
            qedL2TblCacheEntryInsert(pImage, pClusterAlloc->pL2Entry);

            pClusterAlloc->enmAllocState = QEDCLUSTERASYNCALLOCSTATE_USER_ALLOC;
            pClusterAlloc->cbImageOld    = offData;
            pClusterAlloc->offClusterNew = offData;

            /* Write data. */
            rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                        offData, pIoCtx, pClusterAlloc->cbToWrite,
                                        qedAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                qedAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                RTMemFree(pClusterAlloc);
                break;
            }
        }
        RT_FALL_THRU();
        case QEDCLUSTERASYNCALLOCSTATE_USER_ALLOC:
        {
            pClusterAlloc->enmAllocState = QEDCLUSTERASYNCALLOCSTATE_USER_LINK;
            pClusterAlloc->pL2Entry->paL2Tbl[pClusterAlloc->idxL2] = pClusterAlloc->offClusterNew;

            /* Link L2 table and update it. */
            rc = qedTblWrite(pImage, pIoCtx, pImage->paL1Table[pClusterAlloc->idxL1],
                             pClusterAlloc->pL2Entry->paL2Tbl,
                             qedAsyncClusterAllocUpdate, pClusterAlloc);
            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                break;
            else if (RT_FAILURE(rc))
            {
                qedAsyncClusterAllocRollback(pImage, pIoCtx, pClusterAlloc);
                RTMemFree(pClusterAlloc);
                break;
            }
        }
        RT_FALL_THRU();
        case QEDCLUSTERASYNCALLOCSTATE_USER_LINK:
        {
            /* Everything done without errors, signal completion. */
            qedL2TblCacheEntryRelease(pClusterAlloc->pL2Entry);
            RTMemFree(pClusterAlloc);
            rc = VINF_SUCCESS;
            break;
        }
        default:
            AssertMsgFailed(("Invalid async cluster allocation state %d\n",
                             pClusterAlloc->enmAllocState));
    }

    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnProbe */
static DECLCALLBACK(int) qedProbe(const char *pszFilename, PVDINTERFACE pVDIfsDisk,
                                  PVDINTERFACE pVDIfsImage, VDTYPE enmDesiredType, VDTYPE *penmType)
{
    RT_NOREF(pVDIfsDisk, enmDesiredType);
    LogFlowFunc(("pszFilename=\"%s\" pVDIfsDisk=%#p pVDIfsImage=%#p\n", pszFilename, pVDIfsDisk, pVDIfsImage));
    PVDIOSTORAGE pStorage = NULL;
    int rc = VINF_SUCCESS;

    /* Get I/O interface. */
    PVDINTERFACEIOINT pIfIo = VDIfIoIntGet(pVDIfsImage);
    AssertPtrReturn(pIfIo, VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);


    /*
     * Open the file and read the footer.
     */
    rc = vdIfIoIntFileOpen(pIfIo, pszFilename,
                           VDOpenFlagsToFileOpenFlags(VD_OPEN_FLAGS_READONLY,
                                                      false /* fCreate */),
                           &pStorage);
    if (RT_SUCCESS(rc))
    {
        uint64_t cbFile;

        rc = vdIfIoIntFileGetSize(pIfIo, pStorage, &cbFile);
        if (   RT_SUCCESS(rc)
            && cbFile > sizeof(QedHeader))
        {
            QedHeader Header;

            rc = vdIfIoIntFileReadSync(pIfIo, pStorage, 0, &Header, sizeof(Header));
            if (   RT_SUCCESS(rc)
                && qedHdrConvertToHostEndianess(&Header))
                *penmType = VDTYPE_HDD;
            else
                rc = VERR_VD_GEN_INVALID_HEADER;
        }
        else
            rc = VERR_VD_GEN_INVALID_HEADER;
    }

    if (pStorage)
        vdIfIoIntFileClose(pIfIo, pStorage);

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnOpen */
static DECLCALLBACK(int) qedOpen(const char *pszFilename, unsigned uOpenFlags,
                                 PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
                                 VDTYPE enmType, void **ppBackendData)
{
    RT_NOREF1(enmType); /**< @todo r=klaus make use of the type info. */

    LogFlowFunc(("pszFilename=\"%s\" uOpenFlags=%#x pVDIfsDisk=%#p pVDIfsImage=%#p enmType=%u ppBackendData=%#p\n",
                 pszFilename, uOpenFlags, pVDIfsDisk, pVDIfsImage, enmType, ppBackendData));
    int rc;

    /* Check open flags. All valid flags are supported. */
    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);


    PQEDIMAGE pImage = (PQEDIMAGE)RTMemAllocZ(RT_UOFFSETOF(QEDIMAGE, RegionList.aRegions[1]));
    if (RT_LIKELY(pImage))
    {
        pImage->pszFilename = pszFilename;
        pImage->pStorage = NULL;
        pImage->pVDIfsDisk = pVDIfsDisk;
        pImage->pVDIfsImage = pVDIfsImage;

        rc = qedOpenImage(pImage, uOpenFlags);
        if (RT_SUCCESS(rc))
            *ppBackendData = pImage;
        else
            RTMemFree(pImage);
    }
    else
        rc = VERR_NO_MEMORY;

    LogFlowFunc(("returns %Rrc (pBackendData=%#p)\n", rc, *ppBackendData));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnCreate */
static DECLCALLBACK(int) qedCreate(const char *pszFilename, uint64_t cbSize,
                                   unsigned uImageFlags, const char *pszComment,
                                   PCVDGEOMETRY pPCHSGeometry, PCVDGEOMETRY pLCHSGeometry,
                                   PCRTUUID pUuid, unsigned uOpenFlags,
                                   unsigned uPercentStart, unsigned uPercentSpan,
                                   PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
                                   PVDINTERFACE pVDIfsOperation, VDTYPE enmType,
                                   void **ppBackendData)
{
    RT_NOREF1(pUuid);
    LogFlowFunc(("pszFilename=\"%s\" cbSize=%llu uImageFlags=%#x pszComment=\"%s\" pPCHSGeometry=%#p pLCHSGeometry=%#p Uuid=%RTuuid uOpenFlags=%#x uPercentStart=%u uPercentSpan=%u pVDIfsDisk=%#p pVDIfsImage=%#p pVDIfsOperation=%#p enmType=%d ppBackendData=%#p",
                 pszFilename, cbSize, uImageFlags, pszComment, pPCHSGeometry, pLCHSGeometry, pUuid, uOpenFlags, uPercentStart, uPercentSpan, pVDIfsDisk, pVDIfsImage, pVDIfsOperation, enmType, ppBackendData));
    int rc;

    /* Check the VD container type. */
    if (enmType != VDTYPE_HDD)
        return VERR_VD_INVALID_TYPE;

    /* Check open flags. All valid flags are supported. */
    AssertReturn(!(uOpenFlags & ~VD_OPEN_FLAGS_MASK), VERR_INVALID_PARAMETER);
    AssertPtrReturn(pszFilename, VERR_INVALID_POINTER);
    AssertReturn(*pszFilename != '\0', VERR_INVALID_PARAMETER);
    AssertPtrReturn(pPCHSGeometry, VERR_INVALID_POINTER);
    AssertPtrReturn(pLCHSGeometry, VERR_INVALID_POINTER);

    PQEDIMAGE pImage = (PQEDIMAGE)RTMemAllocZ(RT_UOFFSETOF(QEDIMAGE, RegionList.aRegions[1]));
    if (RT_LIKELY(pImage))
    {
        PVDINTERFACEPROGRESS pIfProgress = VDIfProgressGet(pVDIfsOperation);

        pImage->pszFilename = pszFilename;
        pImage->pStorage = NULL;
        pImage->pVDIfsDisk = pVDIfsDisk;
        pImage->pVDIfsImage = pVDIfsImage;

        rc = qedCreateImage(pImage, cbSize, uImageFlags, pszComment,
                            pPCHSGeometry, pLCHSGeometry, uOpenFlags,
                            pIfProgress, uPercentStart, uPercentSpan);
        if (RT_SUCCESS(rc))
        {
            /* So far the image is opened in read/write mode. Make sure the
             * image is opened in read-only mode if the caller requested that. */
            if (uOpenFlags & VD_OPEN_FLAGS_READONLY)
            {
                qedFreeImage(pImage, false);
                rc = qedOpenImage(pImage, uOpenFlags);
            }

            if (RT_SUCCESS(rc))
                *ppBackendData = pImage;
        }

        if (RT_FAILURE(rc))
            RTMemFree(pImage);
    }
    else
        rc = VERR_NO_MEMORY;

    LogFlowFunc(("returns %Rrc (pBackendData=%#p)\n", rc, *ppBackendData));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnRename */
static DECLCALLBACK(int) qedRename(void *pBackendData, const char *pszFilename)
{
    LogFlowFunc(("pBackendData=%#p pszFilename=%#p\n", pBackendData, pszFilename));
    int rc = VINF_SUCCESS;
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    /* Check arguments. */
    AssertReturn((pImage && pszFilename && *pszFilename), VERR_INVALID_PARAMETER);

    /* Close the image. */
    rc = qedFreeImage(pImage, false);
    if (RT_SUCCESS(rc))
    {
        /* Rename the file. */
        rc = vdIfIoIntFileMove(pImage->pIfIo, pImage->pszFilename, pszFilename, 0);
        if (RT_SUCCESS(rc))
        {
            /* Update pImage with the new information. */
            pImage->pszFilename = pszFilename;

            /* Open the old image with new name. */
            rc = qedOpenImage(pImage, pImage->uOpenFlags);
        }
        else
        {
            /* The move failed, try to reopen the original image. */
            int rc2 = qedOpenImage(pImage, pImage->uOpenFlags);
            if (RT_FAILURE(rc2))
                rc = rc2;
        }
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnClose */
static DECLCALLBACK(int) qedClose(void *pBackendData, bool fDelete)
{
    LogFlowFunc(("pBackendData=%#p fDelete=%d\n", pBackendData, fDelete));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    int rc = qedFreeImage(pImage, fDelete);
    RTMemFree(pImage);

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnRead */
static DECLCALLBACK(int) qedRead(void *pBackendData, uint64_t uOffset, size_t cbToRead,
                                 PVDIOCTX pIoCtx, size_t *pcbActuallyRead)
{
    LogFlowFunc(("pBackendData=%#p uOffset=%llu pIoCtx=%#p cbToRead=%zu pcbActuallyRead=%#p\n",
                 pBackendData, uOffset, pIoCtx, cbToRead, pcbActuallyRead));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    uint32_t offCluster = 0;
    uint32_t idxL1      = 0;
    uint32_t idxL2      = 0;
    uint64_t offFile    = 0;

    AssertPtr(pImage);
    Assert(uOffset % 512 == 0);
    Assert(cbToRead % 512 == 0);
    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
    AssertReturn(cbToRead, VERR_INVALID_PARAMETER);
    AssertReturn(uOffset + cbToRead <= pImage->cbSize, VERR_INVALID_PARAMETER);

    qedConvertLogicalOffset(pImage, uOffset, &idxL1, &idxL2, &offCluster);

    /* Clip read size to remain in the cluster. */
    cbToRead = RT_MIN(cbToRead, pImage->cbCluster - offCluster);

    /* Get offset in image. */
    int rc = qedConvertToImageOffset(pImage, pIoCtx, idxL1, idxL2, offCluster, &offFile);
    if (RT_SUCCESS(rc))
        rc = vdIfIoIntFileReadUser(pImage->pIfIo, pImage->pStorage, offFile,
                                   pIoCtx, cbToRead);

    if (   (   RT_SUCCESS(rc)
            || rc == VERR_VD_BLOCK_FREE
            || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
        && pcbActuallyRead)
        *pcbActuallyRead = cbToRead;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnWrite */
static DECLCALLBACK(int) qedWrite(void *pBackendData, uint64_t uOffset, size_t cbToWrite,
                                  PVDIOCTX pIoCtx, size_t *pcbWriteProcess, size_t *pcbPreRead,
                                  size_t *pcbPostRead, unsigned fWrite)
{
    LogFlowFunc(("pBackendData=%#p uOffset=%llu pIoCtx=%#p cbToWrite=%zu pcbWriteProcess=%#p pcbPreRead=%#p pcbPostRead=%#p\n",
                 pBackendData, uOffset, pIoCtx, cbToWrite, pcbWriteProcess, pcbPreRead, pcbPostRead));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    uint32_t offCluster = 0;
    uint32_t idxL1      = 0;
    uint32_t idxL2      = 0;
    uint64_t offImage   = 0;
    int rc = VINF_SUCCESS;

    AssertPtr(pImage);
    Assert(!(uOffset % 512));
    Assert(!(cbToWrite % 512));
    AssertPtrReturn(pIoCtx, VERR_INVALID_POINTER);
    AssertReturn(cbToWrite, VERR_INVALID_PARAMETER);
    AssertReturn(uOffset + cbToWrite <= pImage->cbSize, VERR_INVALID_PARAMETER);

    if (!(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
    {
        /* Convert offset to L1, L2 index and cluster offset. */
        qedConvertLogicalOffset(pImage, uOffset, &idxL1, &idxL2, &offCluster);

        /* Clip write size to remain in the cluster. */
        cbToWrite = RT_MIN(cbToWrite, pImage->cbCluster - offCluster);
        Assert(!(cbToWrite % 512));

        /* Get offset in image. */
        rc = qedConvertToImageOffset(pImage, pIoCtx, idxL1, idxL2, offCluster, &offImage);
        if (RT_SUCCESS(rc))
            rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                        offImage, pIoCtx, cbToWrite, NULL, NULL);
        else if (rc == VERR_VD_BLOCK_FREE)
        {
            if (   cbToWrite == pImage->cbCluster
                && !(fWrite & VD_WRITE_NO_ALLOC))
            {
                PQEDL2CACHEENTRY pL2Entry = NULL;

                /* Full cluster write to previously unallocated cluster.
                 * Allocate cluster and write data. */
                Assert(!offCluster);

                do
                {
                    /* Check if we have to allocate a new cluster for L2 tables. */
                    if (!pImage->paL1Table[idxL1])
                    {
                        uint64_t offL2Tbl;
                        PQEDCLUSTERASYNCALLOC pL2ClusterAlloc = NULL;

                        /* Allocate new async cluster allocation state. */
                        pL2ClusterAlloc = (PQEDCLUSTERASYNCALLOC)RTMemAllocZ(sizeof(QEDCLUSTERASYNCALLOC));
                        if (RT_UNLIKELY(!pL2ClusterAlloc))
                        {
                            rc = VERR_NO_MEMORY;
                            break;
                        }

                        pL2Entry = qedL2TblCacheEntryAlloc(pImage);
                        if (!pL2Entry)
                        {
                            rc = VERR_NO_MEMORY;
                            RTMemFree(pL2ClusterAlloc);
                            break;
                        }

                        offL2Tbl = qedClusterAllocate(pImage, qedByte2Cluster(pImage, pImage->cbTable));
                        pL2Entry->offL2Tbl = offL2Tbl;
                        memset(pL2Entry->paL2Tbl, 0, pImage->cbTable);

                        pL2ClusterAlloc->enmAllocState = QEDCLUSTERASYNCALLOCSTATE_L2_ALLOC;
                        pL2ClusterAlloc->cbImageOld    = offL2Tbl;
                        pL2ClusterAlloc->offClusterNew = offL2Tbl;
                        pL2ClusterAlloc->idxL1         = idxL1;
                        pL2ClusterAlloc->idxL2         = idxL2;
                        pL2ClusterAlloc->cbToWrite     = cbToWrite;
                        pL2ClusterAlloc->pL2Entry      = pL2Entry;

                        pImage->pL2TblAlloc = pL2Entry;

                        LogFlowFunc(("Allocating new L2 table at cluster offset %llu\n", offL2Tbl));

                        /*
                         * Write the L2 table first and link to the L1 table afterwards.
                         * If something unexpected happens the worst case which can happen
                         * is a leak of some clusters.
                         */
                        rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                                    offL2Tbl, pL2Entry->paL2Tbl, pImage->cbTable, pIoCtx,
                                                    qedAsyncClusterAllocUpdate, pL2ClusterAlloc);
                        if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                            break;
                        else if (RT_FAILURE(rc))
                        {
                            RTMemFree(pL2ClusterAlloc);
                            qedL2TblCacheEntryFree(pImage, pL2Entry);
                            break;
                        }

                        rc = qedAsyncClusterAllocUpdate(pImage, pIoCtx, pL2ClusterAlloc, rc);
                    }
                    else
                    {
                        LogFlowFunc(("Fetching L2 table at cluster offset %llu\n", pImage->paL1Table[idxL1]));

                        rc = qedL2TblCacheFetchAsync(pImage, pIoCtx, pImage->paL1Table[idxL1],
                                                     &pL2Entry);

                        if (RT_SUCCESS(rc))
                        {
                            PQEDCLUSTERASYNCALLOC pDataClusterAlloc = NULL;

                            /* Allocate new async cluster allocation state. */
                            pDataClusterAlloc = (PQEDCLUSTERASYNCALLOC)RTMemAllocZ(sizeof(QEDCLUSTERASYNCALLOC));
                            if (RT_UNLIKELY(!pDataClusterAlloc))
                            {
                                rc = VERR_NO_MEMORY;
                                break;
                            }

                            /* Allocate new cluster for the data. */
                            uint64_t offData = qedClusterAllocate(pImage, 1);

                            pDataClusterAlloc->enmAllocState = QEDCLUSTERASYNCALLOCSTATE_USER_ALLOC;
                            pDataClusterAlloc->cbImageOld    = offData;
                            pDataClusterAlloc->offClusterNew = offData;
                            pDataClusterAlloc->idxL1         = idxL1;
                            pDataClusterAlloc->idxL2         = idxL2;
                            pDataClusterAlloc->cbToWrite     = cbToWrite;
                            pDataClusterAlloc->pL2Entry      = pL2Entry;

                            /* Write data. */
                            rc = vdIfIoIntFileWriteUser(pImage->pIfIo, pImage->pStorage,
                                                        offData, pIoCtx, cbToWrite,
                                                        qedAsyncClusterAllocUpdate, pDataClusterAlloc);
                            if (rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                                break;
                            else if (RT_FAILURE(rc))
                            {
                                RTMemFree(pDataClusterAlloc);
                                break;
                            }

                            rc = qedAsyncClusterAllocUpdate(pImage, pIoCtx, pDataClusterAlloc, rc);
                        }
                    }

                } while (0);

                *pcbPreRead = 0;
                *pcbPostRead = 0;
            }
            else
            {
                /* Trying to do a partial write to an unallocated cluster. Don't do
                 * anything except letting the upper layer know what to do. */
                *pcbPreRead = offCluster;
                *pcbPostRead = pImage->cbCluster - cbToWrite - *pcbPreRead;
            }
        }

        if (pcbWriteProcess)
            *pcbWriteProcess = cbToWrite;
    }
    else
        rc = VERR_VD_IMAGE_READ_ONLY;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnFlush */
static DECLCALLBACK(int) qedFlush(void *pBackendData, PVDIOCTX pIoCtx)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtr(pImage);
    AssertPtrReturn(pIoCtx, VERR_INVALID_PARAMETER);

    if (   pImage->pStorage
        && !(pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY))
    {
        QedHeader Header;

        Assert(!(pImage->cbTable % pImage->cbCluster));
        rc = qedTblWrite(pImage, pIoCtx, pImage->offL1Table, pImage->paL1Table,
                         NULL, NULL);
        if (RT_SUCCESS(rc) || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
        {
            /* Write header. */
            qedHdrConvertFromHostEndianess(pImage, &Header);
            rc = vdIfIoIntFileWriteMeta(pImage->pIfIo, pImage->pStorage,
                                        0, &Header, sizeof(Header),
                                        pIoCtx, NULL, NULL);
            if (RT_SUCCESS(rc) || rc == VERR_VD_ASYNC_IO_IN_PROGRESS)
                rc = vdIfIoIntFileFlush(pImage->pIfIo, pImage->pStorage,
                                        pIoCtx, NULL, NULL);
        }
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetVersion */
static DECLCALLBACK(unsigned) qedGetVersion(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    return 1;
}

/** @copydoc VDIMAGEBACKEND::pfnGetFileSize */
static DECLCALLBACK(uint64_t) qedGetFileSize(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    uint64_t cb = 0;

    AssertPtrReturn(pImage, 0);

    uint64_t cbFile;
    if (pImage->pStorage)
    {
        int rc = vdIfIoIntFileGetSize(pImage->pIfIo, pImage->pStorage, &cbFile);
        if (RT_SUCCESS(rc))
            cb += cbFile;
    }

    LogFlowFunc(("returns %lld\n", cb));
    return cb;
}

/** @copydoc VDIMAGEBACKEND::pfnGetPCHSGeometry */
static DECLCALLBACK(int) qedGetPCHSGeometry(void *pBackendData,
                                            PVDGEOMETRY pPCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p\n", pBackendData, pPCHSGeometry));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->PCHSGeometry.cCylinders)
        *pPCHSGeometry = pImage->PCHSGeometry;
    else
        rc = VERR_VD_GEOMETRY_NOT_SET;

    LogFlowFunc(("returns %Rrc (PCHS=%u/%u/%u)\n", rc, pPCHSGeometry->cCylinders, pPCHSGeometry->cHeads, pPCHSGeometry->cSectors));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetPCHSGeometry */
static DECLCALLBACK(int) qedSetPCHSGeometry(void *pBackendData,
                                            PCVDGEOMETRY pPCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pPCHSGeometry=%#p PCHS=%u/%u/%u\n",
                 pBackendData, pPCHSGeometry, pPCHSGeometry->cCylinders, pPCHSGeometry->cHeads, pPCHSGeometry->cSectors));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        rc = VERR_VD_IMAGE_READ_ONLY;
    else
        pImage->PCHSGeometry = *pPCHSGeometry;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetLCHSGeometry */
static DECLCALLBACK(int) qedGetLCHSGeometry(void *pBackendData, PVDGEOMETRY pLCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pLCHSGeometry=%#p\n", pBackendData, pLCHSGeometry));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->LCHSGeometry.cCylinders)
        *pLCHSGeometry = pImage->LCHSGeometry;
    else
        rc = VERR_VD_GEOMETRY_NOT_SET;

    LogFlowFunc(("returns %Rrc (LCHS=%u/%u/%u)\n", rc, pLCHSGeometry->cCylinders,
                 pLCHSGeometry->cHeads, pLCHSGeometry->cSectors));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetLCHSGeometry */
static DECLCALLBACK(int) qedSetLCHSGeometry(void *pBackendData, PCVDGEOMETRY pLCHSGeometry)
{
    LogFlowFunc(("pBackendData=%#p pLCHSGeometry=%#p LCHS=%u/%u/%u\n", pBackendData,
                 pLCHSGeometry, pLCHSGeometry->cCylinders, pLCHSGeometry->cHeads, pLCHSGeometry->cSectors));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        rc = VERR_VD_IMAGE_READ_ONLY;
    else
        pImage->LCHSGeometry = *pLCHSGeometry;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnQueryRegions */
static DECLCALLBACK(int) qedQueryRegions(void *pBackendData, PCVDREGIONLIST *ppRegionList)
{
    LogFlowFunc(("pBackendData=%#p ppRegionList=%#p\n", pBackendData, ppRegionList));
    PQEDIMAGE pThis = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pThis, VERR_VD_NOT_OPENED);

    *ppRegionList = &pThis->RegionList;
    LogFlowFunc(("returns %Rrc\n", VINF_SUCCESS));
    return VINF_SUCCESS;
}

/** @copydoc VDIMAGEBACKEND::pfnRegionListRelease */
static DECLCALLBACK(void) qedRegionListRelease(void *pBackendData, PCVDREGIONLIST pRegionList)
{
    RT_NOREF1(pRegionList);
    LogFlowFunc(("pBackendData=%#p pRegionList=%#p\n", pBackendData, pRegionList));
    PQEDIMAGE pThis = (PQEDIMAGE)pBackendData;
    AssertPtr(pThis); RT_NOREF(pThis);

    /* Nothing to do here. */
}

/** @copydoc VDIMAGEBACKEND::pfnGetImageFlags */
static DECLCALLBACK(unsigned) qedGetImageFlags(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    LogFlowFunc(("returns %#x\n", pImage->uImageFlags));
    return pImage->uImageFlags;
}

/** @copydoc VDIMAGEBACKEND::pfnGetOpenFlags */
static DECLCALLBACK(unsigned) qedGetOpenFlags(void *pBackendData)
{
    LogFlowFunc(("pBackendData=%#p\n", pBackendData));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pImage, 0);

    LogFlowFunc(("returns %#x\n", pImage->uOpenFlags));
    return pImage->uOpenFlags;
}

/** @copydoc VDIMAGEBACKEND::pfnSetOpenFlags */
static DECLCALLBACK(int) qedSetOpenFlags(void *pBackendData, unsigned uOpenFlags)
{
    LogFlowFunc(("pBackendData=%#p\n uOpenFlags=%#x", pBackendData, uOpenFlags));
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    /* Image must be opened and the new flags must be valid. */
    if (!pImage || (uOpenFlags & ~(  VD_OPEN_FLAGS_READONLY | VD_OPEN_FLAGS_INFO
                                   | VD_OPEN_FLAGS_ASYNC_IO | VD_OPEN_FLAGS_SHAREABLE
                                   | VD_OPEN_FLAGS_SEQUENTIAL | VD_OPEN_FLAGS_SKIP_CONSISTENCY_CHECKS)))
        rc = VERR_INVALID_PARAMETER;
    else
    {
        /* Implement this operation via reopening the image. */
        rc = qedFreeImage(pImage, false);
        if (RT_SUCCESS(rc))
            rc = qedOpenImage(pImage, uOpenFlags);
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnGetComment */
VD_BACKEND_CALLBACK_GET_COMMENT_DEF_NOT_SUPPORTED(qedGetComment);

/** @copydoc VDIMAGEBACKEND::pfnSetComment */
VD_BACKEND_CALLBACK_SET_COMMENT_DEF_NOT_SUPPORTED(qedSetComment, PQEDIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qedGetUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qedSetUuid, PQEDIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetModificationUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qedGetModificationUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetModificationUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qedSetModificationUuid, PQEDIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetParentUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qedGetParentUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetParentUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qedSetParentUuid, PQEDIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnGetParentModificationUuid */
VD_BACKEND_CALLBACK_GET_UUID_DEF_NOT_SUPPORTED(qedGetParentModificationUuid);

/** @copydoc VDIMAGEBACKEND::pfnSetParentModificationUuid */
VD_BACKEND_CALLBACK_SET_UUID_DEF_NOT_SUPPORTED(qedSetParentModificationUuid, PQEDIMAGE);

/** @copydoc VDIMAGEBACKEND::pfnDump */
static DECLCALLBACK(void) qedDump(void *pBackendData)
{
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturnVoid(pImage);
    vdIfErrorMessage(pImage->pIfError, "Header: Geometry PCHS=%u/%u/%u LCHS=%u/%u/%u cbSector=%llu\n",
                     pImage->PCHSGeometry.cCylinders, pImage->PCHSGeometry.cHeads, pImage->PCHSGeometry.cSectors,
                     pImage->LCHSGeometry.cCylinders, pImage->LCHSGeometry.cHeads, pImage->LCHSGeometry.cSectors,
                     pImage->cbSize / 512);
}

/** @copydoc VDIMAGEBACKEND::pfnGetParentFilename */
static DECLCALLBACK(int) qedGetParentFilename(void *pBackendData, char **ppszParentFilename)
{
    int rc = VINF_SUCCESS;
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->pszBackingFilename)
        *ppszParentFilename = RTStrDup(pImage->pszBackingFilename);
    else
        rc = VERR_NOT_SUPPORTED;

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnSetParentFilename */
static DECLCALLBACK(int) qedSetParentFilename(void *pBackendData, const char *pszParentFilename)
{
    int rc = VINF_SUCCESS;
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;

    AssertPtrReturn(pImage, VERR_VD_NOT_OPENED);

    if (pImage->uOpenFlags & VD_OPEN_FLAGS_READONLY)
        rc = VERR_VD_IMAGE_READ_ONLY;
    else if (   pImage->pszBackingFilename
             && (strlen(pszParentFilename) > pImage->cbBackingFilename))
        rc = VERR_NOT_SUPPORTED; /* The new filename is longer than the old one. */
    else
    {
        if (pImage->pszBackingFilename)
            RTStrFree(pImage->pszBackingFilename);
        pImage->pszBackingFilename = RTStrDup(pszParentFilename);
        if (!pImage->pszBackingFilename)
            rc = VERR_NO_STR_MEMORY;
        else
        {
            if (!pImage->offBackingFilename)
            {
                /* Allocate new cluster. */
                uint64_t offData = qedClusterAllocate(pImage, 1);

                Assert((offData & UINT32_MAX) == offData);
                pImage->offBackingFilename = (uint32_t)offData;
                pImage->cbBackingFilename  = (uint32_t)strlen(pszParentFilename);
                rc = vdIfIoIntFileSetSize(pImage->pIfIo, pImage->pStorage,
                                          offData + pImage->cbCluster);
            }

            if (RT_SUCCESS(rc))
                rc = vdIfIoIntFileWriteSync(pImage->pIfIo, pImage->pStorage,
                                            pImage->offBackingFilename,
                                            pImage->pszBackingFilename,
                                            strlen(pImage->pszBackingFilename));
        }
    }

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}

/** @copydoc VDIMAGEBACKEND::pfnResize */
static DECLCALLBACK(int) qedResize(void *pBackendData, uint64_t cbSize,
                                   PCVDGEOMETRY pPCHSGeometry, PCVDGEOMETRY pLCHSGeometry,
                                   unsigned uPercentStart, unsigned uPercentSpan,
                                   PVDINTERFACE pVDIfsDisk, PVDINTERFACE pVDIfsImage,
                                   PVDINTERFACE pVDIfsOperation)
{
    RT_NOREF7(pPCHSGeometry, pLCHSGeometry, uPercentStart, uPercentSpan, pVDIfsDisk, pVDIfsImage, pVDIfsOperation);
    PQEDIMAGE pImage = (PQEDIMAGE)pBackendData;
    int rc = VINF_SUCCESS;

    /* Making the image smaller is not supported at the moment. */
    if (cbSize < pImage->cbSize)
        rc = VERR_NOT_SUPPORTED;
    else if (cbSize > pImage->cbSize)
    {
        /*
         * It is enough to just update the size field in the header to complete
         * growing. With the default cluster and table sizes the image can be expanded
         * to 64TB without overflowing the L1 and L2 tables making block relocation
         * superfluous.
         * @todo: The rare case where block relocation is still required (non default
         *        table and/or cluster size or images with more than 64TB) is not
         *        implemented yet and resizing such an image will fail with an error.
         */
        if (qedByte2Cluster(pImage, pImage->cbTable)*pImage->cTableEntries*pImage->cTableEntries*pImage->cbCluster < cbSize)
            rc = vdIfError(pImage->pIfError, VERR_BUFFER_OVERFLOW, RT_SRC_POS,
                           N_("Qed: Resizing the image '%s' is not supported because it would overflow the L1 and L2 table\n"),
                           pImage->pszFilename);
        else
        {
            uint64_t cbSizeOld = pImage->cbSize;

            pImage->cbSize = cbSize;
            rc = qedFlushImage(pImage);
            if (RT_FAILURE(rc))
            {
                pImage->cbSize = cbSizeOld; /* Restore */

                rc = vdIfError(pImage->pIfError, rc, RT_SRC_POS, N_("Qed: Resizing the image '%s' failed\n"),
                               pImage->pszFilename);
            }
        }
    }
    /* Same size doesn't change the image at all. */

    LogFlowFunc(("returns %Rrc\n", rc));
    return rc;
}


const VDIMAGEBACKEND g_QedBackend =
{
    /* u32Version */
    VD_IMGBACKEND_VERSION,
    /* pszBackendName */
    "QED",
    /* uBackendCaps */
    VD_CAP_FILE | VD_CAP_VFS | VD_CAP_CREATE_DYNAMIC | VD_CAP_DIFF | VD_CAP_ASYNC,
    /* paFileExtensions */
    s_aQedFileExtensions,
    /* paConfigInfo */
    NULL,
    /* pfnProbe */
    qedProbe,
    /* pfnOpen */
    qedOpen,
    /* pfnCreate */
    qedCreate,
    /* pfnRename */
    qedRename,
    /* pfnClose */
    qedClose,
    /* pfnRead */
    qedRead,
    /* pfnWrite */
    qedWrite,
    /* pfnFlush */
    qedFlush,
    /* pfnDiscard */
    NULL,
    /* pfnGetVersion */
    qedGetVersion,
    /* pfnGetFileSize */
    qedGetFileSize,
    /* pfnGetPCHSGeometry */
    qedGetPCHSGeometry,
    /* pfnSetPCHSGeometry */
    qedSetPCHSGeometry,
    /* pfnGetLCHSGeometry */
    qedGetLCHSGeometry,
    /* pfnSetLCHSGeometry */
    qedSetLCHSGeometry,
    /* pfnQueryRegions */
    qedQueryRegions,
    /* pfnRegionListRelease */
    qedRegionListRelease,
    /* pfnGetImageFlags */
    qedGetImageFlags,
    /* pfnGetOpenFlags */
    qedGetOpenFlags,
    /* pfnSetOpenFlags */
    qedSetOpenFlags,
    /* pfnGetComment */
    qedGetComment,
    /* pfnSetComment */
    qedSetComment,
    /* pfnGetUuid */
    qedGetUuid,
    /* pfnSetUuid */
    qedSetUuid,
    /* pfnGetModificationUuid */
    qedGetModificationUuid,
    /* pfnSetModificationUuid */
    qedSetModificationUuid,
    /* pfnGetParentUuid */
    qedGetParentUuid,
    /* pfnSetParentUuid */
    qedSetParentUuid,
    /* pfnGetParentModificationUuid */
    qedGetParentModificationUuid,
    /* pfnSetParentModificationUuid */
    qedSetParentModificationUuid,
    /* pfnDump */
    qedDump,
    /* pfnGetTimestamp */
    NULL,
    /* pfnGetParentTimestamp */
    NULL,
    /* pfnSetParentTimestamp */
    NULL,
    /* pfnGetParentFilename */
    qedGetParentFilename,
    /* pfnSetParentFilename */
    qedSetParentFilename,
    /* pfnComposeLocation */
    genericFileComposeLocation,
    /* pfnComposeName */
    genericFileComposeName,
    /* pfnCompact */
    NULL,
    /* pfnResize */
    qedResize,
    /* pfnRepair */
    NULL,
    /* pfnTraverseMetadata */
    NULL,
    /* u32Version */
    VD_IMGBACKEND_VERSION
};
