Archipelago » qemu

/*

 * QEMU System Emulator block driver

 *

 * Copyright (c) 2003 Fabrice Bellard

 *

 * Permission is hereby granted, free of charge, to any person obtaining a copy

 * of this software and associated documentation files (the "Software"), to deal

 * in the Software without restriction, including without limitation the rights

 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

 * copies of the Software, and to permit persons to whom the Software is

 * furnished to do so, subject to the following conditions:

 *

 * The above copyright notice and this permission notice shall be included in

 * all copies or substantial portions of the Software.

 *

 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL

 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN

 * THE SOFTWARE.

 */

#include "config-host.h"

#include "qemu-common.h"

#include "trace.h"

#include "monitor/monitor.h"

#include "block/block_int.h"

#include "block/blockjob.h"

#include "qemu/module.h"

#include "qapi/qmp/qjson.h"

#include "sysemu/sysemu.h"

#include "qemu/notify.h"

#include "block/coroutine.h"

#include "qmp-commands.h"

#include "qemu/timer.h"

#ifdef CONFIG_BSD

#include <sys/types.h>

#include <sys/stat.h>

#include <sys/ioctl.h>

#include <sys/queue.h>

#ifndef __DragonFly__

#include <sys/disk.h>

#endif

#endif

#ifdef _WIN32

#include <windows.h>

#endif

#define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */

typedef enum {

    BDRV_REQ_COPY_ON_READ = 0x1,

    BDRV_REQ_ZERO_WRITE   = 0x2,

} BdrvRequestFlags;

static void bdrv_dev_change_media_cb(BlockDriverState *bs, bool load);

static BlockDriverAIOCB *bdrv_aio_readv_em(BlockDriverState *bs,

        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,

        BlockDriverCompletionFunc *cb, void *opaque);

static BlockDriverAIOCB *bdrv_aio_writev_em(BlockDriverState *bs,

        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,

        BlockDriverCompletionFunc *cb, void *opaque);

static int coroutine_fn bdrv_co_readv_em(BlockDriverState *bs,

                                         int64_t sector_num, int nb_sectors,

                                         QEMUIOVector *iov);

static int coroutine_fn bdrv_co_writev_em(BlockDriverState *bs,

                                         int64_t sector_num, int nb_sectors,

                                         QEMUIOVector *iov);

static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,

    int64_t sector_num, int nb_sectors, QEMUIOVector *qiov,

    BdrvRequestFlags flags);

static int coroutine_fn bdrv_co_do_writev(BlockDriverState *bs,

    int64_t sector_num, int nb_sectors, QEMUIOVector *qiov,

    BdrvRequestFlags flags);

static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,

                                               int64_t sector_num,

                                               QEMUIOVector *qiov,

                                               int nb_sectors,

                                               BlockDriverCompletionFunc *cb,

                                               void *opaque,

                                               bool is_write);

static void coroutine_fn bdrv_co_do_rw(void *opaque);

static int coroutine_fn bdrv_co_do_write_zeroes(BlockDriverState *bs,

    int64_t sector_num, int nb_sectors);

static bool bdrv_exceed_bps_limits(BlockDriverState *bs, int nb_sectors,

        bool is_write, double elapsed_time, uint64_t *wait);

static bool bdrv_exceed_iops_limits(BlockDriverState *bs, bool is_write,

        double elapsed_time, uint64_t *wait);

static bool bdrv_exceed_io_limits(BlockDriverState *bs, int nb_sectors,

        bool is_write, int64_t *wait);

static QTAILQ_HEAD(, BlockDriverState) bdrv_states =

    QTAILQ_HEAD_INITIALIZER(bdrv_states);

static QLIST_HEAD(, BlockDriver) bdrv_drivers =

    QLIST_HEAD_INITIALIZER(bdrv_drivers);

/* If non-zero, use only whitelisted block drivers */

static int use_bdrv_whitelist;

#ifdef _WIN32

static int is_windows_drive_prefix(const char *filename)

{

    return (((filename[0] >= 'a' && filename[0] <= 'z') ||

             (filename[0] >= 'A' && filename[0] <= 'Z')) &&

            filename[1] == ':');

}

int is_windows_drive(const char *filename)

{

    if (is_windows_drive_prefix(filename) &&

        filename[2] == '\0')

        return 1;

    if (strstart(filename, "\\\\.\\", NULL) ||

        strstart(filename, "//./", NULL))

        return 1;

    return 0;

}

#endif

/* throttling disk I/O limits */

void bdrv_io_limits_disable(BlockDriverState *bs)

{

    bs->io_limits_enabled = false;

    while (qemu_co_queue_next(&bs->throttled_reqs));

    if (bs->block_timer) {

        qemu_del_timer(bs->block_timer);

        qemu_free_timer(bs->block_timer);

        bs->block_timer = NULL;

    }

    bs->slice_start = 0;

    bs->slice_end   = 0;

}

static void bdrv_block_timer(void *opaque)

{

    BlockDriverState *bs = opaque;

    qemu_co_queue_next(&bs->throttled_reqs);

}

void bdrv_io_limits_enable(BlockDriverState *bs)

{

    qemu_co_queue_init(&bs->throttled_reqs);

    bs->block_timer = qemu_new_timer_ns(vm_clock, bdrv_block_timer, bs);

    bs->io_limits_enabled = true;

}

bool bdrv_io_limits_enabled(BlockDriverState *bs)

{

    BlockIOLimit *io_limits = &bs->io_limits;

    return io_limits->bps[BLOCK_IO_LIMIT_READ]

         || io_limits->bps[BLOCK_IO_LIMIT_WRITE]

         || io_limits->bps[BLOCK_IO_LIMIT_TOTAL]

         || io_limits->iops[BLOCK_IO_LIMIT_READ]

         || io_limits->iops[BLOCK_IO_LIMIT_WRITE]

         || io_limits->iops[BLOCK_IO_LIMIT_TOTAL];

}

static void bdrv_io_limits_intercept(BlockDriverState *bs,

                                     bool is_write, int nb_sectors)

{

    int64_t wait_time = -1;

    if (!qemu_co_queue_empty(&bs->throttled_reqs)) {

        qemu_co_queue_wait(&bs->throttled_reqs);

    }

    /* In fact, we hope to keep each request's timing, in FIFO mode. The next

     * throttled requests will not be dequeued until the current request is

     * allowed to be serviced. So if the current request still exceeds the

     * limits, it will be inserted to the head. All requests followed it will

     * be still in throttled_reqs queue.

     */

    while (bdrv_exceed_io_limits(bs, nb_sectors, is_write, &wait_time)) {

        qemu_mod_timer(bs->block_timer,

                       wait_time + qemu_get_clock_ns(vm_clock));

        qemu_co_queue_wait_insert_head(&bs->throttled_reqs);

    }

    qemu_co_queue_next(&bs->throttled_reqs);

}

/* check if the path starts with "<protocol>:" */

static int path_has_protocol(const char *path)

{

    const char *p;

#ifdef _WIN32

    if (is_windows_drive(path) ||

        is_windows_drive_prefix(path)) {

        return 0;

    }

    p = path + strcspn(path, ":/\\");

#else

    p = path + strcspn(path, ":/");

#endif

    return *p == ':';

}

int path_is_absolute(const char *path)

{

#ifdef _WIN32

    /* specific case for names like: "\\.\d:" */

    if (is_windows_drive(path) || is_windows_drive_prefix(path)) {

        return 1;

    }

    return (*path == '/' || *path == '\\');

#else

    return (*path == '/');

#endif

}

/* if filename is absolute, just copy it to dest. Otherwise, build a

   path to it by considering it is relative to base_path. URL are

   supported. */

void path_combine(char *dest, int dest_size,

                  const char *base_path,

                  const char *filename)

{

    const char *p, *p1;

    int len;

    if (dest_size <= 0)

        return;

    if (path_is_absolute(filename)) {

        pstrcpy(dest, dest_size, filename);

    } else {

        p = strchr(base_path, ':');

        if (p)

            p++;

        else

            p = base_path;

        p1 = strrchr(base_path, '/');

#ifdef _WIN32

        {

            const char *p2;

            p2 = strrchr(base_path, '\\');

            if (!p1 || p2 > p1)

                p1 = p2;

        }

#endif

        if (p1)

            p1++;

        else

            p1 = base_path;

        if (p1 > p)

            p = p1;

        len = p - base_path;

        if (len > dest_size - 1)

            len = dest_size - 1;

        memcpy(dest, base_path, len);

        dest[len] = '\0';

        pstrcat(dest, dest_size, filename);

    }

}

void bdrv_get_full_backing_filename(BlockDriverState *bs, char *dest, size_t sz)

{

    if (bs->backing_file[0] == '\0' || path_has_protocol(bs->backing_file)) {

        pstrcpy(dest, sz, bs->backing_file);

    } else {

        path_combine(dest, sz, bs->filename, bs->backing_file);

    }

}

void bdrv_register(BlockDriver *bdrv)

{

    /* Block drivers without coroutine functions need emulation */

    if (!bdrv->bdrv_co_readv) {

        bdrv->bdrv_co_readv = bdrv_co_readv_em;

        bdrv->bdrv_co_writev = bdrv_co_writev_em;

        /* bdrv_co_readv_em()/brdv_co_writev_em() work in terms of aio, so if

         * the block driver lacks aio we need to emulate that too.

         */

        if (!bdrv->bdrv_aio_readv) {

            /* add AIO emulation layer */

            bdrv->bdrv_aio_readv = bdrv_aio_readv_em;

            bdrv->bdrv_aio_writev = bdrv_aio_writev_em;

        }

    }

    QLIST_INSERT_HEAD(&bdrv_drivers, bdrv, list);

}

/* create a new block device (by default it is empty) */

BlockDriverState *bdrv_new(const char *device_name)

{

    BlockDriverState *bs;

    bs = g_malloc0(sizeof(BlockDriverState));

    pstrcpy(bs->device_name, sizeof(bs->device_name), device_name);

    if (device_name[0] != '\0') {

        QTAILQ_INSERT_TAIL(&bdrv_states, bs, list);

    }

    bdrv_iostatus_disable(bs);

    notifier_list_init(&bs->close_notifiers);

    return bs;

}

void bdrv_add_close_notifier(BlockDriverState *bs, Notifier *notify)

{

    notifier_list_add(&bs->close_notifiers, notify);

}

BlockDriver *bdrv_find_format(const char *format_name)

{

    BlockDriver *drv1;

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {

        if (!strcmp(drv1->format_name, format_name)) {

            return drv1;

        }

    }

    return NULL;

}

static int bdrv_is_whitelisted(BlockDriver *drv, bool read_only)

{

    static const char *whitelist_rw[] = {

        CONFIG_BDRV_RW_WHITELIST

    };

    static const char *whitelist_ro[] = {

        CONFIG_BDRV_RO_WHITELIST

    };

    const char **p;

    if (!whitelist_rw[0] && !whitelist_ro[0]) {

        return 1;               /* no whitelist, anything goes */

    }

    for (p = whitelist_rw; *p; p++) {

        if (!strcmp(drv->format_name, *p)) {

            return 1;

        }

    }

    if (read_only) {

        for (p = whitelist_ro; *p; p++) {

            if (!strcmp(drv->format_name, *p)) {

                return 1;

            }

        }

    }

    return 0;

}

BlockDriver *bdrv_find_whitelisted_format(const char *format_name,

                                          bool read_only)

{

    BlockDriver *drv = bdrv_find_format(format_name);

    return drv && bdrv_is_whitelisted(drv, read_only) ? drv : NULL;

}

typedef struct CreateCo {

    BlockDriver *drv;

    char *filename;

    QEMUOptionParameter *options;

    int ret;

} CreateCo;

static void coroutine_fn bdrv_create_co_entry(void *opaque)

{

    CreateCo *cco = opaque;

    assert(cco->drv);

    cco->ret = cco->drv->bdrv_create(cco->filename, cco->options);

}

int bdrv_create(BlockDriver *drv, const char* filename,

    QEMUOptionParameter *options)

{

    int ret;

    Coroutine *co;

    CreateCo cco = {

        .drv = drv,

        .filename = g_strdup(filename),

        .options = options,

        .ret = NOT_DONE,

    };

    if (!drv->bdrv_create) {

        ret = -ENOTSUP;

        goto out;

    }

    if (qemu_in_coroutine()) {

        /* Fast-path if already in coroutine context */

        bdrv_create_co_entry(&cco);

    } else {

        co = qemu_coroutine_create(bdrv_create_co_entry);

        qemu_coroutine_enter(co, &cco);

        while (cco.ret == NOT_DONE) {

            qemu_aio_wait();

        }

    }

    ret = cco.ret;

out:

    g_free(cco.filename);

    return ret;

}

int bdrv_create_file(const char* filename, QEMUOptionParameter *options)

{

    BlockDriver *drv;

    drv = bdrv_find_protocol(filename);

    if (drv == NULL) {

        return -ENOENT;

    }

    return bdrv_create(drv, filename, options);

}

/*

 * Create a uniquely-named empty temporary file.

 * Return 0 upon success, otherwise a negative errno value.

 */

int get_tmp_filename(char *filename, int size)

{

#ifdef _WIN32

    char temp_dir[MAX_PATH];

    /* GetTempFileName requires that its output buffer (4th param)

       have length MAX_PATH or greater.  */

    assert(size >= MAX_PATH);

    return (GetTempPath(MAX_PATH, temp_dir)

            && GetTempFileName(temp_dir, "qem", 0, filename)

            ? 0 : -GetLastError());

#else

    int fd;

    const char *tmpdir;

    tmpdir = getenv("TMPDIR");

    if (!tmpdir)

        tmpdir = "/tmp";

    if (snprintf(filename, size, "%s/vl.XXXXXX", tmpdir) >= size) {

        return -EOVERFLOW;

    }

    fd = mkstemp(filename);

    if (fd < 0) {

        return -errno;

    }

    if (close(fd) != 0) {

        unlink(filename);

        return -errno;

    }

    return 0;

#endif

}

/*

 * Detect host devices. By convention, /dev/cdrom[N] is always

 * recognized as a host CDROM.

 */

static BlockDriver *find_hdev_driver(const char *filename)

{

    int score_max = 0, score;

    BlockDriver *drv = NULL, *d;

    QLIST_FOREACH(d, &bdrv_drivers, list) {

        if (d->bdrv_probe_device) {

            score = d->bdrv_probe_device(filename);

            if (score > score_max) {

                score_max = score;

                drv = d;

            }

        }

    }

    return drv;

}

BlockDriver *bdrv_find_protocol(const char *filename)

{

    BlockDriver *drv1;

    char protocol[128];

    int len;

    const char *p;

    /* TODO Drivers without bdrv_file_open must be specified explicitly */

    /*

     * XXX(hch): we really should not let host device detection

     * override an explicit protocol specification, but moving this

     * later breaks access to device names with colons in them.

     * Thanks to the brain-dead persistent naming schemes on udev-

     * based Linux systems those actually are quite common.

     */

    drv1 = find_hdev_driver(filename);

    if (drv1) {

        return drv1;

    }

    if (!path_has_protocol(filename)) {

        return bdrv_find_format("file");

    }

    p = strchr(filename, ':');

    assert(p != NULL);

    len = p - filename;

    if (len > sizeof(protocol) - 1)

        len = sizeof(protocol) - 1;

    memcpy(protocol, filename, len);

    protocol[len] = '\0';

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {

        if (drv1->protocol_name &&

            !strcmp(drv1->protocol_name, protocol)) {

            return drv1;

        }

    }

    return NULL;

}

static int find_image_format(BlockDriverState *bs, const char *filename,

                             BlockDriver **pdrv)

{

    int score, score_max;

    BlockDriver *drv1, *drv;

    uint8_t buf[2048];

    int ret = 0;

    /* Return the raw BlockDriver * to scsi-generic devices or empty drives */

    if (bs->sg || !bdrv_is_inserted(bs) || bdrv_getlength(bs) == 0) {

        drv = bdrv_find_format("raw");

        if (!drv) {

            ret = -ENOENT;

        }

        *pdrv = drv;

        return ret;

    }

    ret = bdrv_pread(bs, 0, buf, sizeof(buf));

    if (ret < 0) {

        *pdrv = NULL;

        return ret;

    }

    score_max = 0;

    drv = NULL;

    QLIST_FOREACH(drv1, &bdrv_drivers, list) {

        if (drv1->bdrv_probe) {

            score = drv1->bdrv_probe(buf, ret, filename);

            if (score > score_max) {

                score_max = score;

                drv = drv1;

            }

        }

    }

    if (!drv) {

        ret = -ENOENT;

    }

    *pdrv = drv;

    return ret;

}

/**

 * Set the current 'total_sectors' value

 */

static int refresh_total_sectors(BlockDriverState *bs, int64_t hint)

{

    BlockDriver *drv = bs->drv;

    /* Do not attempt drv->bdrv_getlength() on scsi-generic devices */

    if (bs->sg)

        return 0;

    /* query actual device if possible, otherwise just trust the hint */

    if (drv->bdrv_getlength) {

        int64_t length = drv->bdrv_getlength(bs);

        if (length < 0) {

            return length;

        }

        hint = length >> BDRV_SECTOR_BITS;

    }

    bs->total_sectors = hint;

    return 0;

}

/**

 * Set open flags for a given discard mode

 *

 * Return 0 on success, -1 if the discard mode was invalid.

 */

int bdrv_parse_discard_flags(const char *mode, int *flags)

{

    *flags &= ~BDRV_O_UNMAP;

    if (!strcmp(mode, "off") || !strcmp(mode, "ignore")) {

        /* do nothing */

    } else if (!strcmp(mode, "on") || !strcmp(mode, "unmap")) {

        *flags |= BDRV_O_UNMAP;

    } else {

        return -1;

    }

    return 0;

}

/**

 * Set open flags for a given cache mode

 *

 * Return 0 on success, -1 if the cache mode was invalid.

 */

int bdrv_parse_cache_flags(const char *mode, int *flags)

{

    *flags &= ~BDRV_O_CACHE_MASK;

    if (!strcmp(mode, "off") || !strcmp(mode, "none")) {

        *flags |= BDRV_O_NOCACHE | BDRV_O_CACHE_WB;

    } else if (!strcmp(mode, "directsync")) {

        *flags |= BDRV_O_NOCACHE;

    } else if (!strcmp(mode, "writeback")) {

        *flags |= BDRV_O_CACHE_WB;

    } else if (!strcmp(mode, "unsafe")) {

        *flags |= BDRV_O_CACHE_WB;

        *flags |= BDRV_O_NO_FLUSH;

    } else if (!strcmp(mode, "writethrough")) {

        /* this is the default */

    } else {

        return -1;

    }

    return 0;

}

/**

 * The copy-on-read flag is actually a reference count so multiple users may

 * use the feature without worrying about clobbering its previous state.

 * Copy-on-read stays enabled until all users have called to disable it.

 */

void bdrv_enable_copy_on_read(BlockDriverState *bs)

{

    bs->copy_on_read++;

}

void bdrv_disable_copy_on_read(BlockDriverState *bs)

{

    assert(bs->copy_on_read > 0);

    bs->copy_on_read--;

}

static int bdrv_open_flags(BlockDriverState *bs, int flags)

{

    int open_flags = flags | BDRV_O_CACHE_WB;

    /*

     * Clear flags that are internal to the block layer before opening the

     * image.

     */

    open_flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);

    /*

     * Snapshots should be writable.

     */

    if (bs->is_temporary) {

        open_flags |= BDRV_O_RDWR;

    }

    return open_flags;

}

/*

 * Common part for opening disk images and files

 *

 * Removes all processed options from *options.

 */

static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,

    QDict *options, int flags, BlockDriver *drv)

{

    int ret, open_flags;

    const char *filename;

    assert(drv != NULL);

    assert(bs->file == NULL);

    assert(options != NULL && bs->options != options);

    if (file != NULL) {

        filename = file->filename;

    } else {

        filename = qdict_get_try_str(options, "filename");

    }

    trace_bdrv_open_common(bs, filename ?: "", flags, drv->format_name);

    /* bdrv_open() with directly using a protocol as drv. This layer is already

     * opened, so assign it to bs (while file becomes a closed BlockDriverState)

     * and return immediately. */

    if (file != NULL && drv->bdrv_file_open) {

        bdrv_swap(file, bs);

        return 0;

    }

    bs->open_flags = flags;

    bs->buffer_alignment = 512;

    open_flags = bdrv_open_flags(bs, flags);

    bs->read_only = !(open_flags & BDRV_O_RDWR);

    if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, bs->read_only)) {

        return -ENOTSUP;

    }

    assert(bs->copy_on_read == 0); /* bdrv_new() and bdrv_close() make it so */

    if (!bs->read_only && (flags & BDRV_O_COPY_ON_READ)) {

        bdrv_enable_copy_on_read(bs);

    }

    if (filename != NULL) {

        pstrcpy(bs->filename, sizeof(bs->filename), filename);

    } else {

        bs->filename[0] = '\0';

    }

    bs->drv = drv;

    bs->opaque = g_malloc0(drv->instance_size);

    bs->enable_write_cache = !!(flags & BDRV_O_CACHE_WB);

    /* Open the image, either directly or using a protocol */

    if (drv->bdrv_file_open) {

        assert(file == NULL);

        assert(drv->bdrv_parse_filename || filename != NULL);

        ret = drv->bdrv_file_open(bs, options, open_flags);

    } else {

        if (file == NULL) {

            qerror_report(ERROR_CLASS_GENERIC_ERROR, "Can't use '%s' as a "

                          "block driver for the protocol level",

                          drv->format_name);

            ret = -EINVAL;

            goto free_and_fail;

        }

        assert(file != NULL);

        bs->file = file;

        ret = drv->bdrv_open(bs, options, open_flags);

    }

    if (ret < 0) {

        goto free_and_fail;

    }

    ret = refresh_total_sectors(bs, bs->total_sectors);

    if (ret < 0) {

        goto free_and_fail;

    }

#ifndef _WIN32

    if (bs->is_temporary) {

        assert(filename != NULL);

        unlink(filename);

    }

#endif

    return 0;

free_and_fail:

    bs->file = NULL;

    g_free(bs->opaque);

    bs->opaque = NULL;

    bs->drv = NULL;

    return ret;

}

/*

 * Opens a file using a protocol (file, host_device, nbd, ...)

 *

 * options is a QDict of options to pass to the block drivers, or NULL for an

 * empty set of options. The reference to the QDict belongs to the block layer

 * after the call (even on failure), so if the caller intends to reuse the

 * dictionary, it needs to use QINCREF() before calling bdrv_file_open.

 */

int bdrv_file_open(BlockDriverState **pbs, const char *filename,

                   QDict *options, int flags)

{

    BlockDriverState *bs;

    BlockDriver *drv;

    const char *drvname;

    int ret;

    /* NULL means an empty set of options */

    if (options == NULL) {

        options = qdict_new();

    }

    bs = bdrv_new("");

    bs->options = options;

    options = qdict_clone_shallow(options);

    /* Fetch the file name from the options QDict if necessary */

    if (!filename) {

        filename = qdict_get_try_str(options, "filename");

    } else if (filename && !qdict_haskey(options, "filename")) {

        qdict_put(options, "filename", qstring_from_str(filename));

    } else {

        qerror_report(ERROR_CLASS_GENERIC_ERROR, "Can't specify 'file' and "

                      "'filename' options at the same time");

        ret = -EINVAL;

        goto fail;

    }

    /* Find the right block driver */

    drvname = qdict_get_try_str(options, "driver");

    if (drvname) {

        drv = bdrv_find_whitelisted_format(drvname, !(flags & BDRV_O_RDWR));

        qdict_del(options, "driver");

    } else if (filename) {

        drv = bdrv_find_protocol(filename);

    } else {

        qerror_report(ERROR_CLASS_GENERIC_ERROR,

                      "Must specify either driver or file");

        drv = NULL;

    }

    if (!drv) {

        ret = -ENOENT;

        goto fail;

    }

    /* Parse the filename and open it */

    if (drv->bdrv_parse_filename && filename) {

        Error *local_err = NULL;

        drv->bdrv_parse_filename(filename, options, &local_err);

        if (error_is_set(&local_err)) {

            qerror_report_err(local_err);

            error_free(local_err);

            ret = -EINVAL;

            goto fail;

        }

        qdict_del(options, "filename");

    } else if (!drv->bdrv_parse_filename && !filename) {

        qerror_report(ERROR_CLASS_GENERIC_ERROR,

                      "The '%s' block driver requires a file name",

                      drv->format_name);

        ret = -EINVAL;

        goto fail;

    }

    ret = bdrv_open_common(bs, NULL, options, flags, drv);

    if (ret < 0) {

        goto fail;

    }

    /* Check if any unknown options were used */

    if (qdict_size(options) != 0) {

        const QDictEntry *entry = qdict_first(options);

        qerror_report(ERROR_CLASS_GENERIC_ERROR, "Block protocol '%s' doesn't "

                      "support the option '%s'",

                      drv->format_name, entry->key);

        ret = -EINVAL;

        goto fail;

    }

    QDECREF(options);

    bs->growable = 1;

    *pbs = bs;

    return 0;

fail:

    QDECREF(options);

    if (!bs->drv) {

        QDECREF(bs->options);

    }

    bdrv_delete(bs);

    return ret;

}

/*

 * Opens the backing file for a BlockDriverState if not yet open

 *

 * options is a QDict of options to pass to the block drivers, or NULL for an

 * empty set of options. The reference to the QDict is transferred to this

 * function (even on failure), so if the caller intends to reuse the dictionary,

 * it needs to use QINCREF() before calling bdrv_file_open.

 */

int bdrv_open_backing_file(BlockDriverState *bs, QDict *options)

{

    char backing_filename[PATH_MAX];

    int back_flags, ret;

    BlockDriver *back_drv = NULL;

    if (bs->backing_hd != NULL) {

        QDECREF(options);

        return 0;