Archipelago » qemu

/*

 *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>

 *

 *  Network Block Device

 *

 *  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; under version 2 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 <http://www.gnu.org/licenses/>.

 */

#include "block/nbd.h"

#include "block/block.h"

#include "block/coroutine.h"

#include <errno.h>

#include <string.h>

#ifndef _WIN32

#include <sys/ioctl.h>

#endif

#if defined(__sun__) || defined(__HAIKU__)

#include <sys/ioccom.h>

#endif

#include <ctype.h>

#include <inttypes.h>

#ifdef __linux__

#include <linux/fs.h>

#endif

#include "qemu/sockets.h"

#include "qemu/queue.h"

//#define DEBUG_NBD

#ifdef DEBUG_NBD

#define TRACE(msg, ...) do { \

    LOG(msg, ## __VA_ARGS__); \

} while(0)

#else

#define TRACE(msg, ...) \

    do { } while (0)

#endif

#define LOG(msg, ...) do { \

    fprintf(stderr, "%s:%s():L%d: " msg "\n", \

            __FILE__, __FUNCTION__, __LINE__, ## __VA_ARGS__); \

} while(0)

/* This is all part of the "official" NBD API */

#define NBD_REQUEST_SIZE        (4 + 4 + 8 + 8 + 4)

#define NBD_REPLY_SIZE          (4 + 4 + 8)

#define NBD_REQUEST_MAGIC       0x25609513

#define NBD_REPLY_MAGIC         0x67446698

#define NBD_OPTS_MAGIC          0x49484156454F5054LL

#define NBD_CLIENT_MAGIC        0x0000420281861253LL

#define NBD_SET_SOCK            _IO(0xab, 0)

#define NBD_SET_BLKSIZE         _IO(0xab, 1)

#define NBD_SET_SIZE            _IO(0xab, 2)

#define NBD_DO_IT               _IO(0xab, 3)

#define NBD_CLEAR_SOCK          _IO(0xab, 4)

#define NBD_CLEAR_QUE           _IO(0xab, 5)

#define NBD_PRINT_DEBUG         _IO(0xab, 6)

#define NBD_SET_SIZE_BLOCKS     _IO(0xab, 7)

#define NBD_DISCONNECT          _IO(0xab, 8)

#define NBD_SET_TIMEOUT         _IO(0xab, 9)

#define NBD_SET_FLAGS           _IO(0xab, 10)

#define NBD_OPT_EXPORT_NAME     (1 << 0)

/* Definitions for opaque data types */

typedef struct NBDRequest NBDRequest;

struct NBDRequest {

    QSIMPLEQ_ENTRY(NBDRequest) entry;

    NBDClient *client;

    uint8_t *data;

};

struct NBDExport {

    int refcount;

    void (*close)(NBDExport *exp);

    BlockDriverState *bs;

    char *name;

    off_t dev_offset;

    off_t size;

    uint32_t nbdflags;

    QTAILQ_HEAD(, NBDClient) clients;

    QSIMPLEQ_HEAD(, NBDRequest) requests;

    QTAILQ_ENTRY(NBDExport) next;

};

static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);

struct NBDClient {

    int refcount;

    void (*close)(NBDClient *client);

    NBDExport *exp;

    int sock;

    Coroutine *recv_coroutine;

    CoMutex send_lock;

    Coroutine *send_coroutine;

    QTAILQ_ENTRY(NBDClient) next;

    int nb_requests;

    bool closing;

};

/* That's all folks */

ssize_t nbd_wr_sync(int fd, void *buffer, size_t size, bool do_read)

{

    size_t offset = 0;

    int err;

    if (qemu_in_coroutine()) {

        if (do_read) {

            return qemu_co_recv(fd, buffer, size);

        } else {

            return qemu_co_send(fd, buffer, size);

        }

    }

    while (offset < size) {

        ssize_t len;

        if (do_read) {

            len = qemu_recv(fd, buffer + offset, size - offset, 0);

        } else {

            len = send(fd, buffer + offset, size - offset, 0);

        }

        if (len < 0) {

            err = socket_error();

            /* recoverable error */

            if (err == EINTR || (offset > 0 && err == EAGAIN)) {

                continue;

            }

            /* unrecoverable error */

            return -err;

        }

        /* eof */

        if (len == 0) {

            break;

        }

        offset += len;

    }

    return offset;

}

static ssize_t read_sync(int fd, void *buffer, size_t size)

{

    /* Sockets are kept in blocking mode in the negotiation phase.  After

     * that, a non-readable socket simply means that another thread stole

     * our request/reply.  Synchronization is done with recv_coroutine, so

     * that this is coroutine-safe.

     */

    return nbd_wr_sync(fd, buffer, size, true);

}

static ssize_t write_sync(int fd, void *buffer, size_t size)

{

    int ret;

    do {

        /* For writes, we do expect the socket to be writable.  */

        ret = nbd_wr_sync(fd, buffer, size, false);

    } while (ret == -EAGAIN);

    return ret;

}

static void combine_addr(char *buf, size_t len, const char* address,

                         uint16_t port)

{

    /* If the address-part contains a colon, it's an IPv6 IP so needs [] */

    if (strstr(address, ":")) {

        snprintf(buf, len, "[%s]:%u", address, port);

    } else {

        snprintf(buf, len, "%s:%u", address, port);

    }

}

int tcp_socket_outgoing_opts(QemuOpts *opts)

{

    Error *local_err = NULL;

    int fd = inet_connect_opts(opts, &local_err, NULL, NULL);

    if (local_err != NULL) {

        qerror_report_err(local_err);

        error_free(local_err);

    }

    return fd;

}

int tcp_socket_incoming(const char *address, uint16_t port)

{

    char address_and_port[128];

    combine_addr(address_and_port, 128, address, port);

    return tcp_socket_incoming_spec(address_and_port);

}

int tcp_socket_incoming_spec(const char *address_and_port)

{

    Error *local_err = NULL;

    int fd = inet_listen(address_and_port, NULL, 0, SOCK_STREAM, 0, &local_err);

    if (local_err != NULL) {

        qerror_report_err(local_err);

        error_free(local_err);

    }

    return fd;

}

int unix_socket_incoming(const char *path)

{

    Error *local_err = NULL;

    int fd = unix_listen(path, NULL, 0, &local_err);

    if (local_err != NULL) {

        qerror_report_err(local_err);

        error_free(local_err);

    }

    return fd;

}

int unix_socket_outgoing(const char *path)

{

    Error *local_err = NULL;

    int fd = unix_connect(path, &local_err);

    if (local_err != NULL) {

        qerror_report_err(local_err);

        error_free(local_err);

    }

    return fd;

}

/* Basic flow for negotiation

   Server         Client

   Negotiate

   or

   Server         Client

   Negotiate #1

                  Option

   Negotiate #2

   ----

   followed by

   Server         Client

                  Request

   Response

                  Request

   Response

                  ...

   ...

                  Request (type == 2)

*/

static int nbd_receive_options(NBDClient *client)

{

    int csock = client->sock;

    char name[256];

    uint32_t tmp, length;

    uint64_t magic;

    int rc;

    /* Client sends:

        [ 0 ..   3]   reserved (0)

        [ 4 ..  11]   NBD_OPTS_MAGIC

        [12 ..  15]   NBD_OPT_EXPORT_NAME

        [16 ..  19]   length

        [20 ..  xx]   export name (length bytes)

     */

    rc = -EINVAL;

    if (read_sync(csock, &tmp, sizeof(tmp)) != sizeof(tmp)) {

        LOG("read failed");

        goto fail;

    }

    TRACE("Checking reserved");

    if (tmp != 0) {

        LOG("Bad reserved received");

        goto fail;

    }

    if (read_sync(csock, &magic, sizeof(magic)) != sizeof(magic)) {

        LOG("read failed");

        goto fail;

    }

    TRACE("Checking reserved");

    if (magic != be64_to_cpu(NBD_OPTS_MAGIC)) {

        LOG("Bad magic received");

        goto fail;

    }

    if (read_sync(csock, &tmp, sizeof(tmp)) != sizeof(tmp)) {

        LOG("read failed");

        goto fail;

    }

    TRACE("Checking option");

    if (tmp != be32_to_cpu(NBD_OPT_EXPORT_NAME)) {

        LOG("Bad option received");

        goto fail;

    }

    if (read_sync(csock, &length, sizeof(length)) != sizeof(length)) {

        LOG("read failed");

        goto fail;

    }

    TRACE("Checking length");

    length = be32_to_cpu(length);

    if (length > 255) {

        LOG("Bad length received");

        goto fail;

    }

    if (read_sync(csock, name, length) != length) {

        LOG("read failed");

        goto fail;

    }

    name[length] = '\0';

    client->exp = nbd_export_find(name);

    if (!client->exp) {

        LOG("export not found");

        goto fail;

    }

    QTAILQ_INSERT_TAIL(&client->exp->clients, client, next);

    nbd_export_get(client->exp);

    TRACE("Option negotiation succeeded.");

    rc = 0;

fail:

    return rc;

}

static int nbd_send_negotiate(NBDClient *client)

{

    int csock = client->sock;

    char buf[8 + 8 + 8 + 128];

    int rc;

    const int myflags = (NBD_FLAG_HAS_FLAGS | NBD_FLAG_SEND_TRIM |

                         NBD_FLAG_SEND_FLUSH | NBD_FLAG_SEND_FUA);

    /* Negotiation header without options:

        [ 0 ..   7]   passwd       ("NBDMAGIC")

        [ 8 ..  15]   magic        (NBD_CLIENT_MAGIC)

        [16 ..  23]   size

        [24 ..  25]   server flags (0)

        [24 ..  27]   export flags

        [28 .. 151]   reserved     (0)

       Negotiation header with options, part 1:

        [ 0 ..   7]   passwd       ("NBDMAGIC")

        [ 8 ..  15]   magic        (NBD_OPTS_MAGIC)

        [16 ..  17]   server flags (0)

       part 2 (after options are sent):

        [18 ..  25]   size

        [26 ..  27]   export flags

        [28 .. 151]   reserved     (0)

     */

    qemu_set_block(csock);

    rc = -EINVAL;

    TRACE("Beginning negotiation.");

    memset(buf, 0, sizeof(buf));

    memcpy(buf, "NBDMAGIC", 8);

    if (client->exp) {

        assert ((client->exp->nbdflags & ~65535) == 0);

        cpu_to_be64w((uint64_t*)(buf + 8), NBD_CLIENT_MAGIC);

        cpu_to_be64w((uint64_t*)(buf + 16), client->exp->size);

        cpu_to_be16w((uint16_t*)(buf + 26), client->exp->nbdflags | myflags);

    } else {

        cpu_to_be64w((uint64_t*)(buf + 8), NBD_OPTS_MAGIC);

    }

    if (client->exp) {

        if (write_sync(csock, buf, sizeof(buf)) != sizeof(buf)) {

            LOG("write failed");

            goto fail;

        }

    } else {

        if (write_sync(csock, buf, 18) != 18) {

            LOG("write failed");

            goto fail;

        }

        rc = nbd_receive_options(client);

        if (rc < 0) {

            LOG("option negotiation failed");

            goto fail;

        }

        assert ((client->exp->nbdflags & ~65535) == 0);

        cpu_to_be64w((uint64_t*)(buf + 18), client->exp->size);

        cpu_to_be16w((uint16_t*)(buf + 26), client->exp->nbdflags | myflags);

        if (write_sync(csock, buf + 18, sizeof(buf) - 18) != sizeof(buf) - 18) {

            LOG("write failed");

            goto fail;

        }

    }

    TRACE("Negotiation succeeded.");

    rc = 0;

fail:

    qemu_set_nonblock(csock);

    return rc;

}

int nbd_receive_negotiate(int csock, const char *name, uint32_t *flags,

                          off_t *size, size_t *blocksize)

{

    char buf[256];

    uint64_t magic, s;

    uint16_t tmp;

    int rc;

    TRACE("Receiving negotiation.");

    qemu_set_block(csock);

    rc = -EINVAL;

    if (read_sync(csock, buf, 8) != 8) {

        LOG("read failed");

        goto fail;

    }

    buf[8] = '\0';

    if (strlen(buf) == 0) {

        LOG("server connection closed");

        goto fail;

    }

    TRACE("Magic is %c%c%c%c%c%c%c%c",

          qemu_isprint(buf[0]) ? buf[0] : '.',

          qemu_isprint(buf[1]) ? buf[1] : '.',

          qemu_isprint(buf[2]) ? buf[2] : '.',

          qemu_isprint(buf[3]) ? buf[3] : '.',

          qemu_isprint(buf[4]) ? buf[4] : '.',

          qemu_isprint(buf[5]) ? buf[5] : '.',

          qemu_isprint(buf[6]) ? buf[6] : '.',

          qemu_isprint(buf[7]) ? buf[7] : '.');

    if (memcmp(buf, "NBDMAGIC", 8) != 0) {

        LOG("Invalid magic received");

        goto fail;

    }

    if (read_sync(csock, &magic, sizeof(magic)) != sizeof(magic)) {

        LOG("read failed");

        goto fail;

    }

    magic = be64_to_cpu(magic);

    TRACE("Magic is 0x%" PRIx64, magic);

    if (name) {

        uint32_t reserved = 0;

        uint32_t opt;

        uint32_t namesize;

        TRACE("Checking magic (opts_magic)");

        if (magic != NBD_OPTS_MAGIC) {

            LOG("Bad magic received");

            goto fail;

        }

        if (read_sync(csock, &tmp, sizeof(tmp)) != sizeof(tmp)) {

            LOG("flags read failed");

            goto fail;

        }

        *flags = be16_to_cpu(tmp) << 16;

        /* reserved for future use */

        if (write_sync(csock, &reserved, sizeof(reserved)) !=

            sizeof(reserved)) {

            LOG("write failed (reserved)");

            goto fail;

        }

        /* write the export name */

        magic = cpu_to_be64(magic);

        if (write_sync(csock, &magic, sizeof(magic)) != sizeof(magic)) {

            LOG("write failed (magic)");

            goto fail;

        }

        opt = cpu_to_be32(NBD_OPT_EXPORT_NAME);

        if (write_sync(csock, &opt, sizeof(opt)) != sizeof(opt)) {

            LOG("write failed (opt)");

            goto fail;

        }

        namesize = cpu_to_be32(strlen(name));

        if (write_sync(csock, &namesize, sizeof(namesize)) !=

            sizeof(namesize)) {

            LOG("write failed (namesize)");

            goto fail;

        }

        if (write_sync(csock, (char*)name, strlen(name)) != strlen(name)) {

            LOG("write failed (name)");

            goto fail;

        }

    } else {

        TRACE("Checking magic (cli_magic)");

        if (magic != NBD_CLIENT_MAGIC) {

            LOG("Bad magic received");

            goto fail;

        }

    }

    if (read_sync(csock, &s, sizeof(s)) != sizeof(s)) {

        LOG("read failed");

        goto fail;

    }

    *size = be64_to_cpu(s);

    *blocksize = 1024;

    TRACE("Size is %" PRIu64, *size);

    if (!name) {

        if (read_sync(csock, flags, sizeof(*flags)) != sizeof(*flags)) {

            LOG("read failed (flags)");

            goto fail;

        }

        *flags = be32_to_cpup(flags);

    } else {

        if (read_sync(csock, &tmp, sizeof(tmp)) != sizeof(tmp)) {

            LOG("read failed (tmp)");

            goto fail;

        }

        *flags |= be32_to_cpu(tmp);

    }

    if (read_sync(csock, &buf, 124) != 124) {

        LOG("read failed (buf)");

        goto fail;

    }

    rc = 0;

fail:

    qemu_set_nonblock(csock);

    return rc;

}

#ifdef __linux__

int nbd_init(int fd, int csock, uint32_t flags, off_t size, size_t blocksize)

{

    TRACE("Setting NBD socket");

    if (ioctl(fd, NBD_SET_SOCK, csock) < 0) {

        int serrno = errno;

        LOG("Failed to set NBD socket");

        return -serrno;

    }

    TRACE("Setting block size to %lu", (unsigned long)blocksize);

    if (ioctl(fd, NBD_SET_BLKSIZE, blocksize) < 0) {

        int serrno = errno;

        LOG("Failed setting NBD block size");

        return -serrno;

    }

        TRACE("Setting size to %zd block(s)", (size_t)(size / blocksize));

    if (ioctl(fd, NBD_SET_SIZE_BLOCKS, size / blocksize) < 0) {

        int serrno = errno;

        LOG("Failed setting size (in blocks)");

        return -serrno;

    }

    if (ioctl(fd, NBD_SET_FLAGS, flags) < 0) {

        if (errno == ENOTTY) {

            int read_only = (flags & NBD_FLAG_READ_ONLY) != 0;

            TRACE("Setting readonly attribute");

            if (ioctl(fd, BLKROSET, (unsigned long) &read_only) < 0) {

                int serrno = errno;

                LOG("Failed setting read-only attribute");

                return -serrno;

            }

        } else {

            int serrno = errno;

            LOG("Failed setting flags");

            return -serrno;

        }

    }

    TRACE("Negotiation ended");

    return 0;

}

int nbd_disconnect(int fd)

{

    ioctl(fd, NBD_CLEAR_QUE);

    ioctl(fd, NBD_DISCONNECT);

    ioctl(fd, NBD_CLEAR_SOCK);

    return 0;

}

int nbd_client(int fd)

{

    int ret;

    int serrno;

    TRACE("Doing NBD loop");

    ret = ioctl(fd, NBD_DO_IT);

    if (ret < 0 && errno == EPIPE) {

        /* NBD_DO_IT normally returns EPIPE when someone has disconnected

         * the socket via NBD_DISCONNECT.  We do not want to return 1 in

         * that case.

         */

        ret = 0;

    }

    serrno = errno;

    TRACE("NBD loop returned %d: %s", ret, strerror(serrno));

    TRACE("Clearing NBD queue");

    ioctl(fd, NBD_CLEAR_QUE);

    TRACE("Clearing NBD socket");

    ioctl(fd, NBD_CLEAR_SOCK);

    errno = serrno;

    return ret;

}

#else

int nbd_init(int fd, int csock, uint32_t flags, off_t size, size_t blocksize)

{

    return -ENOTSUP;

}

int nbd_disconnect(int fd)

{

    return -ENOTSUP;

}

int nbd_client(int fd)

{

    return -ENOTSUP;

}

#endif

ssize_t nbd_send_request(int csock, struct nbd_request *request)

{

    uint8_t buf[NBD_REQUEST_SIZE];

    ssize_t ret;

    cpu_to_be32w((uint32_t*)buf, NBD_REQUEST_MAGIC);

    cpu_to_be32w((uint32_t*)(buf + 4), request->type);

    cpu_to_be64w((uint64_t*)(buf + 8), request->handle);

    cpu_to_be64w((uint64_t*)(buf + 16), request->from);

    cpu_to_be32w((uint32_t*)(buf + 24), request->len);

    TRACE("Sending request to client: "

          "{ .from = %" PRIu64", .len = %u, .handle = %" PRIu64", .type=%i}",

          request->from, request->len, request->handle, request->type);

    ret = write_sync(csock, buf, sizeof(buf));

    if (ret < 0) {

        return ret;

    }

    if (ret != sizeof(buf)) {

        LOG("writing to socket failed");

        return -EINVAL;

    }

    return 0;

}

static ssize_t nbd_receive_request(int csock, struct nbd_request *request)

{

    uint8_t buf[NBD_REQUEST_SIZE];

    uint32_t magic;

    ssize_t ret;

    ret = read_sync(csock, buf, sizeof(buf));

    if (ret < 0) {

        return ret;

    }

    if (ret != sizeof(buf)) {

        LOG("read failed");

        return -EINVAL;

    }

    /* Request

       [ 0 ..  3]   magic   (NBD_REQUEST_MAGIC)

       [ 4 ..  7]   type    (0 == READ, 1 == WRITE)

       [ 8 .. 15]   handle

       [16 .. 23]   from

       [24 .. 27]   len

     */

    magic = be32_to_cpup((uint32_t*)buf);

    request->type  = be32_to_cpup((uint32_t*)(buf + 4));

    request->handle = be64_to_cpup((uint64_t*)(buf + 8));

    request->from  = be64_to_cpup((uint64_t*)(buf + 16));

    request->len   = be32_to_cpup((uint32_t*)(buf + 24));

    TRACE("Got request: "

          "{ magic = 0x%x, .type = %d, from = %" PRIu64" , len = %u }",

          magic, request->type, request->from, request->len);

    if (magic != NBD_REQUEST_MAGIC) {

        LOG("invalid magic (got 0x%x)", magic);

        return -EINVAL;

    }

    return 0;

}

ssize_t nbd_receive_reply(int csock, struct nbd_reply *reply)

{

    uint8_t buf[NBD_REPLY_SIZE];

    uint32_t magic;

    ssize_t ret;

    ret = read_sync(csock, buf, sizeof(buf));

    if (ret < 0) {

        return ret;

    }

    if (ret != sizeof(buf)) {

        LOG("read failed");

        return -EINVAL;

    }

    /* Reply

       [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)

       [ 4 ..  7]    error   (0 == no error)

       [ 7 .. 15]    handle

     */

    magic = be32_to_cpup((uint32_t*)buf);

    reply->error  = be32_to_cpup((uint32_t*)(buf + 4));

    reply->handle = be64_to_cpup((uint64_t*)(buf + 8));

    TRACE("Got reply: "

          "{ magic = 0x%x, .error = %d, handle = %" PRIu64" }",

          magic, reply->error, reply->handle);

    if (magic != NBD_REPLY_MAGIC) {

        LOG("invalid magic (got 0x%x)", magic);

        return -EINVAL;

    }

    return 0;

}

static ssize_t nbd_send_reply(int csock, struct nbd_reply *reply)

{

    uint8_t buf[NBD_REPLY_SIZE];

    ssize_t ret;

    /* Reply

       [ 0 ..  3]    magic   (NBD_REPLY_MAGIC)

       [ 4 ..  7]    error   (0 == no error)

       [ 7 .. 15]    handle

     */

    cpu_to_be32w((uint32_t*)buf, NBD_REPLY_MAGIC);

    cpu_to_be32w((uint32_t*)(buf + 4), reply->error);

    cpu_to_be64w((uint64_t*)(buf + 8), reply->handle);

    TRACE("Sending response to client");

    ret = write_sync(csock, buf, sizeof(buf));

    if (ret < 0) {

        return ret;

    }

    if (ret != sizeof(buf)) {

        LOG("writing to socket failed");

        return -EINVAL;

    }

    return 0;

}

#define MAX_NBD_REQUESTS 16

void nbd_client_get(NBDClient *client)

{

    client->refcount++;

}

void nbd_client_put(NBDClient *client)

{

    if (--client->refcount == 0) {

        /* The last reference should be dropped by client->close,

         * which is called by nbd_client_close.

         */

        assert(client->closing);

        qemu_set_fd_handler2(client->sock, NULL, NULL, NULL, NULL);

        close(client->sock);

        client->sock = -1;

        if (client->exp) {

            QTAILQ_REMOVE(&client->exp->clients, client, next);

            nbd_export_put(client->exp);

        }

        g_free(client);

    }

}

void nbd_client_close(NBDClient *client)

{

    if (client->closing) {

        return;

    }

    client->closing = true;

    /* Force requests to finish.  They will drop their own references,

     * then we'll close the socket and free the NBDClient.

     */

    shutdown(client->sock, 2);

    /* Also tell the client, so that they release their reference.  */

    if (client->close) {

        client->close(client);

    }

}

static NBDRequest *nbd_request_get(NBDClient *client)

{

    NBDRequest *req;

    NBDExport *exp = client->exp;

    assert(client->nb_requests <= MAX_NBD_REQUESTS - 1);

    client->nb_requests++;

    if (QSIMPLEQ_EMPTY(&exp->requests)) {

        req = g_malloc0(sizeof(NBDRequest));

        req->data = qemu_blockalign(exp->bs, NBD_BUFFER_SIZE);

    } else {

        req = QSIMPLEQ_FIRST(&exp->requests);

        QSIMPLEQ_REMOVE_HEAD(&exp->requests, entry);

    }

    nbd_client_get(client);

    req->client = client;

    return req;

}

static void nbd_request_put(NBDRequest *req)

{

    NBDClient *client = req->client;

    QSIMPLEQ_INSERT_HEAD(&client->exp->requests, req, entry);

    if (client->nb_requests-- == MAX_NBD_REQUESTS) {