Archipelago » qemu

/*

 * QEMU Enhanced Disk Format

 *

 * Copyright IBM, Corp. 2010

 *

 * Authors:

 *  Stefan Hajnoczi   <stefanha@linux.vnet.ibm.com>

 *  Anthony Liguori   <aliguori@us.ibm.com>

 *

 * 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.

 *

 */

#include "qemu/timer.h"

#include "trace.h"

#include "qed.h"

#include "qapi/qmp/qerror.h"

#include "migration/migration.h"

static void qed_aio_cancel(BlockDriverAIOCB *blockacb)

{

    QEDAIOCB *acb = (QEDAIOCB *)blockacb;

    bool finished = false;

    /* Wait for the request to finish */

    acb->finished = &finished;

    while (!finished) {

        qemu_aio_wait();

    }

}

static const AIOCBInfo qed_aiocb_info = {

    .aiocb_size         = sizeof(QEDAIOCB),

    .cancel             = qed_aio_cancel,

};

static int bdrv_qed_probe(const uint8_t *buf, int buf_size,

                          const char *filename)

{

    const QEDHeader *header = (const QEDHeader *)buf;

    if (buf_size < sizeof(*header)) {

        return 0;

    }

    if (le32_to_cpu(header->magic) != QED_MAGIC) {

        return 0;

    }

    return 100;

}

/**

 * Check whether an image format is raw

 *

 * @fmt:    Backing file format, may be NULL

 */

static bool qed_fmt_is_raw(const char *fmt)

{

    return fmt && strcmp(fmt, "raw") == 0;

}

static void qed_header_le_to_cpu(const QEDHeader *le, QEDHeader *cpu)

{

    cpu->magic = le32_to_cpu(le->magic);

    cpu->cluster_size = le32_to_cpu(le->cluster_size);

    cpu->table_size = le32_to_cpu(le->table_size);

    cpu->header_size = le32_to_cpu(le->header_size);

    cpu->features = le64_to_cpu(le->features);

    cpu->compat_features = le64_to_cpu(le->compat_features);

    cpu->autoclear_features = le64_to_cpu(le->autoclear_features);

    cpu->l1_table_offset = le64_to_cpu(le->l1_table_offset);

    cpu->image_size = le64_to_cpu(le->image_size);

    cpu->backing_filename_offset = le32_to_cpu(le->backing_filename_offset);

    cpu->backing_filename_size = le32_to_cpu(le->backing_filename_size);

}

static void qed_header_cpu_to_le(const QEDHeader *cpu, QEDHeader *le)

{

    le->magic = cpu_to_le32(cpu->magic);

    le->cluster_size = cpu_to_le32(cpu->cluster_size);

    le->table_size = cpu_to_le32(cpu->table_size);

    le->header_size = cpu_to_le32(cpu->header_size);

    le->features = cpu_to_le64(cpu->features);

    le->compat_features = cpu_to_le64(cpu->compat_features);

    le->autoclear_features = cpu_to_le64(cpu->autoclear_features);

    le->l1_table_offset = cpu_to_le64(cpu->l1_table_offset);

    le->image_size = cpu_to_le64(cpu->image_size);

    le->backing_filename_offset = cpu_to_le32(cpu->backing_filename_offset);

    le->backing_filename_size = cpu_to_le32(cpu->backing_filename_size);

}

int qed_write_header_sync(BDRVQEDState *s)

{

    QEDHeader le;

    int ret;

    qed_header_cpu_to_le(&s->header, &le);

    ret = bdrv_pwrite(s->bs->file, 0, &le, sizeof(le));

    if (ret != sizeof(le)) {

        return ret;

    }

    return 0;

}

typedef struct {

    GenericCB gencb;

    BDRVQEDState *s;

    struct iovec iov;

    QEMUIOVector qiov;

    int nsectors;

    uint8_t *buf;

} QEDWriteHeaderCB;

static void qed_write_header_cb(void *opaque, int ret)

{

    QEDWriteHeaderCB *write_header_cb = opaque;

    qemu_vfree(write_header_cb->buf);

    gencb_complete(write_header_cb, ret);

}

static void qed_write_header_read_cb(void *opaque, int ret)

{

    QEDWriteHeaderCB *write_header_cb = opaque;

    BDRVQEDState *s = write_header_cb->s;

    if (ret) {

        qed_write_header_cb(write_header_cb, ret);

        return;

    }

    /* Update header */

    qed_header_cpu_to_le(&s->header, (QEDHeader *)write_header_cb->buf);

    bdrv_aio_writev(s->bs->file, 0, &write_header_cb->qiov,

                    write_header_cb->nsectors, qed_write_header_cb,

                    write_header_cb);

}

/**

 * Update header in-place (does not rewrite backing filename or other strings)

 *

 * This function only updates known header fields in-place and does not affect

 * extra data after the QED header.

 */

static void qed_write_header(BDRVQEDState *s, BlockDriverCompletionFunc cb,

                             void *opaque)

{

    /* We must write full sectors for O_DIRECT but cannot necessarily generate

     * the data following the header if an unrecognized compat feature is

     * active.  Therefore, first read the sectors containing the header, update

     * them, and write back.

     */

    int nsectors = (sizeof(QEDHeader) + BDRV_SECTOR_SIZE - 1) /

                   BDRV_SECTOR_SIZE;

    size_t len = nsectors * BDRV_SECTOR_SIZE;

    QEDWriteHeaderCB *write_header_cb = gencb_alloc(sizeof(*write_header_cb),

                                                    cb, opaque);

    write_header_cb->s = s;

    write_header_cb->nsectors = nsectors;

    write_header_cb->buf = qemu_blockalign(s->bs, len);

    write_header_cb->iov.iov_base = write_header_cb->buf;

    write_header_cb->iov.iov_len = len;

    qemu_iovec_init_external(&write_header_cb->qiov, &write_header_cb->iov, 1);

    bdrv_aio_readv(s->bs->file, 0, &write_header_cb->qiov, nsectors,

                   qed_write_header_read_cb, write_header_cb);

}

static uint64_t qed_max_image_size(uint32_t cluster_size, uint32_t table_size)

{

    uint64_t table_entries;

    uint64_t l2_size;

    table_entries = (table_size * cluster_size) / sizeof(uint64_t);

    l2_size = table_entries * cluster_size;

    return l2_size * table_entries;

}

static bool qed_is_cluster_size_valid(uint32_t cluster_size)

{

    if (cluster_size < QED_MIN_CLUSTER_SIZE ||

        cluster_size > QED_MAX_CLUSTER_SIZE) {

        return false;

    }

    if (cluster_size & (cluster_size - 1)) {

        return false; /* not power of 2 */

    }

    return true;

}

static bool qed_is_table_size_valid(uint32_t table_size)

{

    if (table_size < QED_MIN_TABLE_SIZE ||

        table_size > QED_MAX_TABLE_SIZE) {

        return false;

    }

    if (table_size & (table_size - 1)) {

        return false; /* not power of 2 */

    }

    return true;

}

static bool qed_is_image_size_valid(uint64_t image_size, uint32_t cluster_size,

                                    uint32_t table_size)

{

    if (image_size % BDRV_SECTOR_SIZE != 0) {

        return false; /* not multiple of sector size */

    }

    if (image_size > qed_max_image_size(cluster_size, table_size)) {

        return false; /* image is too large */

    }

    return true;

}

/**

 * Read a string of known length from the image file

 *

 * @file:       Image file

 * @offset:     File offset to start of string, in bytes

 * @n:          String length in bytes

 * @buf:        Destination buffer

 * @buflen:     Destination buffer length in bytes

 * @ret:        0 on success, -errno on failure

 *

 * The string is NUL-terminated.

 */

static int qed_read_string(BlockDriverState *file, uint64_t offset, size_t n,

                           char *buf, size_t buflen)

{

    int ret;

    if (n >= buflen) {

        return -EINVAL;

    }

    ret = bdrv_pread(file, offset, buf, n);

    if (ret < 0) {

        return ret;

    }

    buf[n] = '\0';

    return 0;

}

/**

 * Allocate new clusters

 *

 * @s:          QED state

 * @n:          Number of contiguous clusters to allocate

 * @ret:        Offset of first allocated cluster

 *

 * This function only produces the offset where the new clusters should be

 * written.  It updates BDRVQEDState but does not make any changes to the image

 * file.

 */

static uint64_t qed_alloc_clusters(BDRVQEDState *s, unsigned int n)

{

    uint64_t offset = s->file_size;

    s->file_size += n * s->header.cluster_size;

    return offset;

}

QEDTable *qed_alloc_table(BDRVQEDState *s)

{

    /* Honor O_DIRECT memory alignment requirements */

    return qemu_blockalign(s->bs,

                           s->header.cluster_size * s->header.table_size);

}

/**

 * Allocate a new zeroed L2 table

 */

static CachedL2Table *qed_new_l2_table(BDRVQEDState *s)

{

    CachedL2Table *l2_table = qed_alloc_l2_cache_entry(&s->l2_cache);

    l2_table->table = qed_alloc_table(s);

    l2_table->offset = qed_alloc_clusters(s, s->header.table_size);

    memset(l2_table->table->offsets, 0,

           s->header.cluster_size * s->header.table_size);

    return l2_table;

}

static void qed_aio_next_io(void *opaque, int ret);

static void qed_plug_allocating_write_reqs(BDRVQEDState *s)

{

    assert(!s->allocating_write_reqs_plugged);

    s->allocating_write_reqs_plugged = true;

}

static void qed_unplug_allocating_write_reqs(BDRVQEDState *s)

{

    QEDAIOCB *acb;

    assert(s->allocating_write_reqs_plugged);

    s->allocating_write_reqs_plugged = false;

    acb = QSIMPLEQ_FIRST(&s->allocating_write_reqs);

    if (acb) {

        qed_aio_next_io(acb, 0);

    }

}

static void qed_finish_clear_need_check(void *opaque, int ret)

{

    /* Do nothing */

}

static void qed_flush_after_clear_need_check(void *opaque, int ret)

{

    BDRVQEDState *s = opaque;

    bdrv_aio_flush(s->bs, qed_finish_clear_need_check, s);

    /* No need to wait until flush completes */

    qed_unplug_allocating_write_reqs(s);

}

static void qed_clear_need_check(void *opaque, int ret)

{

    BDRVQEDState *s = opaque;

    if (ret) {

        qed_unplug_allocating_write_reqs(s);

        return;

    }

    s->header.features &= ~QED_F_NEED_CHECK;

    qed_write_header(s, qed_flush_after_clear_need_check, s);

}

static void qed_need_check_timer_cb(void *opaque)

{

    BDRVQEDState *s = opaque;

    /* The timer should only fire when allocating writes have drained */

    assert(!QSIMPLEQ_FIRST(&s->allocating_write_reqs));

    trace_qed_need_check_timer_cb(s);

    qed_plug_allocating_write_reqs(s);

    /* Ensure writes are on disk before clearing flag */

    bdrv_aio_flush(s->bs, qed_clear_need_check, s);

}

static void qed_start_need_check_timer(BDRVQEDState *s)

{

    trace_qed_start_need_check_timer(s);

    /* Use QEMU_CLOCK_VIRTUAL so we don't alter the image file while suspended for

     * migration.

     */

    timer_mod(s->need_check_timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) +

                   get_ticks_per_sec() * QED_NEED_CHECK_TIMEOUT);

}

/* It's okay to call this multiple times or when no timer is started */

static void qed_cancel_need_check_timer(BDRVQEDState *s)

{

    trace_qed_cancel_need_check_timer(s);

    timer_del(s->need_check_timer);

}

static void bdrv_qed_rebind(BlockDriverState *bs)

{

    BDRVQEDState *s = bs->opaque;

    s->bs = bs;

}

static int bdrv_qed_open(BlockDriverState *bs, QDict *options, int flags,

                         Error **errp)

{

    BDRVQEDState *s = bs->opaque;

    QEDHeader le_header;

    int64_t file_size;

    int ret;

    s->bs = bs;

    QSIMPLEQ_INIT(&s->allocating_write_reqs);

    ret = bdrv_pread(bs->file, 0, &le_header, sizeof(le_header));

    if (ret < 0) {

        return ret;

    }

    qed_header_le_to_cpu(&le_header, &s->header);

    if (s->header.magic != QED_MAGIC) {

        return -EMEDIUMTYPE;

    }

    if (s->header.features & ~QED_FEATURE_MASK) {

        /* image uses unsupported feature bits */

        char buf[64];

        snprintf(buf, sizeof(buf), "%" PRIx64,

            s->header.features & ~QED_FEATURE_MASK);

        qerror_report(QERR_UNKNOWN_BLOCK_FORMAT_FEATURE,

            bs->device_name, "QED", buf);

        return -ENOTSUP;

    }

    if (!qed_is_cluster_size_valid(s->header.cluster_size)) {

        return -EINVAL;

    }

    /* Round down file size to the last cluster */

    file_size = bdrv_getlength(bs->file);

    if (file_size < 0) {

        return file_size;

    }

    s->file_size = qed_start_of_cluster(s, file_size);

    if (!qed_is_table_size_valid(s->header.table_size)) {

        return -EINVAL;

    }

    if (!qed_is_image_size_valid(s->header.image_size,

                                 s->header.cluster_size,

                                 s->header.table_size)) {

        return -EINVAL;

    }

    if (!qed_check_table_offset(s, s->header.l1_table_offset)) {

        return -EINVAL;

    }

    s->table_nelems = (s->header.cluster_size * s->header.table_size) /

                      sizeof(uint64_t);

    s->l2_shift = ffs(s->header.cluster_size) - 1;

    s->l2_mask = s->table_nelems - 1;

    s->l1_shift = s->l2_shift + ffs(s->table_nelems) - 1;

    if ((s->header.features & QED_F_BACKING_FILE)) {

        if ((uint64_t)s->header.backing_filename_offset +

            s->header.backing_filename_size >

            s->header.cluster_size * s->header.header_size) {

            return -EINVAL;

        }

        ret = qed_read_string(bs->file, s->header.backing_filename_offset,

                              s->header.backing_filename_size, bs->backing_file,

                              sizeof(bs->backing_file));

        if (ret < 0) {

            return ret;

        }

        if (s->header.features & QED_F_BACKING_FORMAT_NO_PROBE) {

            pstrcpy(bs->backing_format, sizeof(bs->backing_format), "raw");

        }

    }

    /* Reset unknown autoclear feature bits.  This is a backwards

     * compatibility mechanism that allows images to be opened by older

     * programs, which "knock out" unknown feature bits.  When an image is

     * opened by a newer program again it can detect that the autoclear

     * feature is no longer valid.

     */

    if ((s->header.autoclear_features & ~QED_AUTOCLEAR_FEATURE_MASK) != 0 &&

        !bdrv_is_read_only(bs->file) && !(flags & BDRV_O_INCOMING)) {

        s->header.autoclear_features &= QED_AUTOCLEAR_FEATURE_MASK;

        ret = qed_write_header_sync(s);

        if (ret) {

            return ret;

        }

        /* From here on only known autoclear feature bits are valid */

        bdrv_flush(bs->file);

    }

    s->l1_table = qed_alloc_table(s);

    qed_init_l2_cache(&s->l2_cache);

    ret = qed_read_l1_table_sync(s);

    if (ret) {

        goto out;

    }

    /* If image was not closed cleanly, check consistency */

    if (!(flags & BDRV_O_CHECK) && (s->header.features & QED_F_NEED_CHECK)) {

        /* Read-only images cannot be fixed.  There is no risk of corruption

         * since write operations are not possible.  Therefore, allow

         * potentially inconsistent images to be opened read-only.  This can

         * aid data recovery from an otherwise inconsistent image.

         */

        if (!bdrv_is_read_only(bs->file) &&

            !(flags & BDRV_O_INCOMING)) {

            BdrvCheckResult result = {0};

            ret = qed_check(s, &result, true);

            if (ret) {

                goto out;

            }

        }

    }

    s->need_check_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,

                                            qed_need_check_timer_cb, s);

out:

    if (ret) {

        qed_free_l2_cache(&s->l2_cache);

        qemu_vfree(s->l1_table);

    }

    return ret;

}

/* We have nothing to do for QED reopen, stubs just return

 * success */

static int bdrv_qed_reopen_prepare(BDRVReopenState *state,

                                   BlockReopenQueue *queue, Error **errp)

{

    return 0;

}

static void bdrv_qed_close(BlockDriverState *bs)

{

    BDRVQEDState *s = bs->opaque;

    qed_cancel_need_check_timer(s);

    timer_free(s->need_check_timer);

    /* Ensure writes reach stable storage */

    bdrv_flush(bs->file);

    /* Clean shutdown, no check required on next open */

    if (s->header.features & QED_F_NEED_CHECK) {

        s->header.features &= ~QED_F_NEED_CHECK;

        qed_write_header_sync(s);

    }

    qed_free_l2_cache(&s->l2_cache);

    qemu_vfree(s->l1_table);

}

static int qed_create(const char *filename, uint32_t cluster_size,

                      uint64_t image_size, uint32_t table_size,

                      const char *backing_file, const char *backing_fmt)

{

    QEDHeader header = {

        .magic = QED_MAGIC,

        .cluster_size = cluster_size,

        .table_size = table_size,

        .header_size = 1,

        .features = 0,

        .compat_features = 0,

        .l1_table_offset = cluster_size,

        .image_size = image_size,

    };

    QEDHeader le_header;

    uint8_t *l1_table = NULL;

    size_t l1_size = header.cluster_size * header.table_size;

    Error *local_err = NULL;

    int ret = 0;

    BlockDriverState *bs = NULL;

    ret = bdrv_create_file(filename, NULL, &local_err);

    if (ret < 0) {

        qerror_report_err(local_err);

        error_free(local_err);

        return ret;

    }

    ret = bdrv_file_open(&bs, filename, NULL, BDRV_O_RDWR | BDRV_O_CACHE_WB,

                         &local_err);

    if (ret < 0) {

        qerror_report_err(local_err);

        error_free(local_err);

        return ret;

    }

    /* File must start empty and grow, check truncate is supported */

    ret = bdrv_truncate(bs, 0);

    if (ret < 0) {

        goto out;

    }

    if (backing_file) {

        header.features |= QED_F_BACKING_FILE;

        header.backing_filename_offset = sizeof(le_header);

        header.backing_filename_size = strlen(backing_file);

        if (qed_fmt_is_raw(backing_fmt)) {

            header.features |= QED_F_BACKING_FORMAT_NO_PROBE;

        }

    }

    qed_header_cpu_to_le(&header, &le_header);

    ret = bdrv_pwrite(bs, 0, &le_header, sizeof(le_header));

    if (ret < 0) {

        goto out;

    }

    ret = bdrv_pwrite(bs, sizeof(le_header), backing_file,

                      header.backing_filename_size);

    if (ret < 0) {

        goto out;

    }

    l1_table = g_malloc0(l1_size);

    ret = bdrv_pwrite(bs, header.l1_table_offset, l1_table, l1_size);

    if (ret < 0) {

        goto out;

    }

    ret = 0; /* success */

out:

    g_free(l1_table);

    bdrv_unref(bs);

    return ret;

}

static int bdrv_qed_create(const char *filename, QEMUOptionParameter *options,

                           Error **errp)

{

    uint64_t image_size = 0;

    uint32_t cluster_size = QED_DEFAULT_CLUSTER_SIZE;

    uint32_t table_size = QED_DEFAULT_TABLE_SIZE;

    const char *backing_file = NULL;

    const char *backing_fmt = NULL;

    while (options && options->name) {

        if (!strcmp(options->name, BLOCK_OPT_SIZE)) {

            image_size = options->value.n;

        } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {

            backing_file = options->value.s;

        } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FMT)) {

            backing_fmt = options->value.s;

        } else if (!strcmp(options->name, BLOCK_OPT_CLUSTER_SIZE)) {

            if (options->value.n) {

                cluster_size = options->value.n;

            }

        } else if (!strcmp(options->name, BLOCK_OPT_TABLE_SIZE)) {

            if (options->value.n) {

                table_size = options->value.n;

            }

        }

        options++;

    }

    if (!qed_is_cluster_size_valid(cluster_size)) {

        fprintf(stderr, "QED cluster size must be within range [%u, %u] and power of 2\n",

                QED_MIN_CLUSTER_SIZE, QED_MAX_CLUSTER_SIZE);

        return -EINVAL;

    }

    if (!qed_is_table_size_valid(table_size)) {

        fprintf(stderr, "QED table size must be within range [%u, %u] and power of 2\n",

                QED_MIN_TABLE_SIZE, QED_MAX_TABLE_SIZE);

        return -EINVAL;

    }

    if (!qed_is_image_size_valid(image_size, cluster_size, table_size)) {

        fprintf(stderr, "QED image size must be a non-zero multiple of "

                        "cluster size and less than %" PRIu64 " bytes\n",

                qed_max_image_size(cluster_size, table_size));

        return -EINVAL;

    }

    return qed_create(filename, cluster_size, image_size, table_size,

                      backing_file, backing_fmt);

}

typedef struct {

    BlockDriverState *bs;

    Coroutine *co;

    uint64_t pos;

    int64_t status;

    int *pnum;

} QEDIsAllocatedCB;

static void qed_is_allocated_cb(void *opaque, int ret, uint64_t offset, size_t len)

{

    QEDIsAllocatedCB *cb = opaque;

    BDRVQEDState *s = cb->bs->opaque;

    *cb->pnum = len / BDRV_SECTOR_SIZE;

    switch (ret) {

    case QED_CLUSTER_FOUND:

        offset |= qed_offset_into_cluster(s, cb->pos);

        cb->status = BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | offset;

        break;

    case QED_CLUSTER_ZERO:

        cb->status = BDRV_BLOCK_ZERO;

        break;

    case QED_CLUSTER_L2:

    case QED_CLUSTER_L1:

        cb->status = 0;

        break;

    default:

        assert(ret < 0);

        cb->status = ret;

        break;

    }

    if (cb->co) {

        qemu_coroutine_enter(cb->co, NULL);

    }

}

static int64_t coroutine_fn bdrv_qed_co_get_block_status(BlockDriverState *bs,

                                                 int64_t sector_num,

                                                 int nb_sectors, int *pnum)

{

    BDRVQEDState *s = bs->opaque;

    size_t len = (size_t)nb_sectors * BDRV_SECTOR_SIZE;

    QEDIsAllocatedCB cb = {

        .bs = bs,

        .pos = (uint64_t)sector_num * BDRV_SECTOR_SIZE,

        .status = BDRV_BLOCK_OFFSET_MASK,

        .pnum = pnum,

    };

    QEDRequest request = { .l2_table = NULL };

    qed_find_cluster(s, &request, cb.pos, len, qed_is_allocated_cb, &cb);

    /* Now sleep if the callback wasn't invoked immediately */

    while (cb.status == BDRV_BLOCK_OFFSET_MASK) {

        cb.co = qemu_coroutine_self();

        qemu_coroutine_yield();

    }

    qed_unref_l2_cache_entry(request.l2_table);

    return cb.status;

}

static int bdrv_qed_make_empty(BlockDriverState *bs)

{

    return -ENOTSUP;

}

static BDRVQEDState *acb_to_s(QEDAIOCB *acb)

{

    return acb->common.bs->opaque;

}

/**

 * Read from the backing file or zero-fill if no backing file

 *

 * @s:          QED state

 * @pos:        Byte position in device

 * @qiov:       Destination I/O vector

 * @cb:         Completion function

 * @opaque:     User data for completion function

 *

 * This function reads qiov->size bytes starting at pos from the backing file.

 * If there is no backing file then zeroes are read.

 */

static void qed_read_backing_file(BDRVQEDState *s, uint64_t pos,

                                  QEMUIOVector *qiov,

                                  BlockDriverCompletionFunc *cb, void *opaque)

{

    uint64_t backing_length = 0;

    size_t size;

    /* If there is a backing file, get its length.  Treat the absence of a

     * backing file like a zero length backing file.

     */

    if (s->bs->backing_hd) {

        int64_t l = bdrv_getlength(s->bs->backing_hd);

        if (l < 0) {

            cb(opaque, l);

            return;

        }

        backing_length = l;

    }

    /* Zero all sectors if reading beyond the end of the backing file */

    if (pos >= backing_length ||

        pos + qiov->size > backing_length) {

        qemu_iovec_memset(qiov, 0, 0, qiov->size);

    }

    /* Complete now if there are no backing file sectors to read */

    if (pos >= backing_length) {

        cb(opaque, 0);

        return;

    }

    /* If the read straddles the end of the backing file, shorten it */

    size = MIN((uint64_t)backing_length - pos, qiov->size);

    BLKDBG_EVENT(s->bs->file, BLKDBG_READ_BACKING_AIO);

    bdrv_aio_readv(s->bs->backing_hd, pos / BDRV_SECTOR_SIZE,

                   qiov, size / BDRV_SECTOR_SIZE, cb, opaque);

}

typedef struct {

    GenericCB gencb;

    BDRVQEDState *s;

    QEMUIOVector qiov;

    struct iovec iov;

    uint64_t offset;

} CopyFromBackingFileCB;

static void qed_copy_from_backing_file_cb(void *opaque, int ret)

{

    CopyFromBackingFileCB *copy_cb = opaque;

    qemu_vfree(copy_cb->iov.iov_base);

    gencb_complete(&copy_cb->gencb, ret);

}

static void qed_copy_from_backing_file_write(void *opaque, int ret)

{

    CopyFromBackingFileCB *copy_cb = opaque;

    BDRVQEDState *s = copy_cb->s;

    if (ret) {

        qed_copy_from_backing_file_cb(copy_cb, ret);

        return;

    }

    BLKDBG_EVENT(s->bs->file, BLKDBG_COW_WRITE);

    bdrv_aio_writev(s->bs->file, copy_cb->offset / BDRV_SECTOR_SIZE,

                    &copy_cb->qiov, copy_cb->qiov.size / BDRV_SECTOR_SIZE,

                    qed_copy_from_backing_file_cb, copy_cb);

}

/**

 * Copy data from backing file into the image

 *

 * @s:          QED state

 * @pos:        Byte position in device

 * @len:        Number of bytes

 * @offset:     Byte offset in image file

 * @cb:         Completion function

 * @opaque:     User data for completion function

 */

static void qed_copy_from_backing_file(BDRVQEDState *s, uint64_t pos,

                                       uint64_t len, uint64_t offset,

                                       BlockDriverCompletionFunc *cb,

                                       void *opaque)

{

    CopyFromBackingFileCB *copy_cb;

    /* Skip copy entirely if there is no work to do */

    if (len == 0) {

        cb(opaque, 0);

        return;

    }

    copy_cb = gencb_alloc(sizeof(*copy_cb), cb, opaque);

    copy_cb->s = s;

    copy_cb->offset = offset;

    copy_cb->iov.iov_base = qemu_blockalign(s->bs, len);

    copy_cb->iov.iov_len = len;

    qemu_iovec_init_external(&copy_cb->qiov, &copy_cb->iov, 1);

    qed_read_backing_file(s, pos, &copy_cb->qiov,

                          qed_copy_from_backing_file_write, copy_cb);

}

/**

 * Link one or more contiguous clusters into a table

 *

 * @s:              QED state

 * @table:          L2 table

 * @index:          First cluster index

 * @n:              Number of contiguous clusters

 * @cluster:        First cluster offset

 *

 * The cluster offset may be an allocated byte offset in the image file, the

 * zero cluster marker, or the unallocated cluster marker.

 */

static void qed_update_l2_table(BDRVQEDState *s, QEDTable *table, int index,

                                unsigned int n, uint64_t cluster)

{

    int i;

    for (i = index; i < index + n; i++) {

        table->offsets[i] = cluster;

        if (!qed_offset_is_unalloc_cluster(cluster) &&

            !qed_offset_is_zero_cluster(cluster)) {

            cluster += s->header.cluster_size;

        }

    }

}

static void qed_aio_complete_bh(void *opaque)

{

    QEDAIOCB *acb = opaque;

    BlockDriverCompletionFunc *cb = acb->common.cb;

    void *user_opaque = acb->common.opaque;

    int ret = acb->bh_ret;

    bool *finished = acb->finished;

    qemu_bh_delete(acb->bh);

    qemu_aio_release(acb);

    /* Invoke callback */

    cb(user_opaque, ret);

    /* Signal cancel completion */

    if (finished) {

        *finished = true;

    }

}

static void qed_aio_complete(QEDAIOCB *acb, int ret)

{

    BDRVQEDState *s = acb_to_s(acb);

    trace_qed_aio_complete(s, acb, ret);

    /* Free resources */

    qemu_iovec_destroy(&acb->cur_qiov);

    qed_unref_l2_cache_entry(acb->request.l2_table);

    /* Free the buffer we may have allocated for zero writes */

    if (acb->flags & QED_AIOCB_ZERO) {

        qemu_vfree(acb->qiov->iov[0].iov_base);

        acb->qiov->iov[0].iov_base = NULL;

    }

    /* Arrange for a bh to invoke the completion function */

    acb->bh_ret = ret;

    acb->bh = qemu_bh_new(qed_aio_complete_bh, acb);

    qemu_bh_schedule(acb->bh);

    /* Start next allocating write request waiting behind this one.  Note that

     * requests enqueue themselves when they first hit an unallocated cluster