[Qemu-devel] Re: [PATCH 3/3] block/nbd: Make the NBD block device use the AIO interface

[Kevin Wolf <kwolf@redhat.com>]

Am 18.02.2011 13:55, schrieb Nick Thomas:
> This preserves the previous behaviour where the NBD server is
> unavailable or goes away during guest execution, but switches the
> NBD backend to present the AIO interface instead of the sync IO
> interface.
> 
> We also split write requests into 1 MiB blocks (minus request size).
> This is a hard limit in the NBD servers (including qemu-nbd), but
> never seemed to come up with the previous backend code.
> ---
>  block/nbd.c |  555 ++++++++++++++++++++++++++++++++++++++++++++++++++---------
>  1 files changed, 470 insertions(+), 85 deletions(-)
> 
> diff --git a/block/nbd.c b/block/nbd.c
> index c8dc763..e7b1f7e 100644
> --- a/block/nbd.c
> +++ b/block/nbd.c
> @@ -1,11 +1,12 @@
>  /*
> - * QEMU Block driver for  NBD
> + * QEMU Block driver for  NBD - asynchronous IO
>   *
>   * Copyright (C) 2008 Bull S.A.S.
>   *     Author: Laurent Vivier <Laurent.Vivier@bull.net>
>   *
>   * Some parts:
>   *    Copyright (C) 2007 Anthony Liguori <anthony@codemonkey.ws>
> + *    Copyright (C) 2011 Nick Thomas <nick@bytemark.co.uk>
>   *
>   * Permission is hereby granted, free of charge, to any person obtaining a copy
>   * of this software and associated documentation files (the "Software"), to deal
> @@ -27,66 +28,135 @@
>   */
>  
>  #include "qemu-common.h"
> +#include "qemu_socket.h"
>  #include "nbd.h"
>  #include "module.h"
>  
>  #include <sys/types.h>
>  #include <unistd.h>
>  
> -#define EN_OPTSTR ":exportname="
> +#define EN_OPTSTR      ":exportname="
> +#define SECTOR_SIZE    512
> +
> +/* 1MiB minus request header size */
> +#define MAX_NBD_WRITE      ((1024*1024) - (4 + 4 + 8 + 8 + 4))

-EMAGIC :-)

I think we can add an uint32_t magic to struct nbd_request, make it
packed and then replace this magic "4 + 4 + 8 + 8 + 4" thing by
"sizeof(struct nbd_request)" everywhere.

> +
> +/* #define DEBUG_NBD */
> +
> +#if defined(DEBUG_NBD)
> +#define logout(fmt, ...) \
> +                fprintf(stderr, "nbd\t%-24s" fmt, __func__, ##__VA_ARGS__)
> +#else
> +#define logout(fmt, ...) ((void)0)
> +#endif
> +
> +
> +typedef struct NBDAIOCB NBDAIOCB;
> +
> +typedef struct AIOReq {
> +    NBDAIOCB *aiocb;
> +    off_t iov_offset; /* Where on the iov does this req start? */
> +    off_t offset;     /* Starting point of the read */
> +
> +    size_t data_len;
> +    uint8_t flags;
> +    uint64_t handle;
> +
> +    QLIST_ENTRY(AIOReq) outstanding_aio_siblings;
> +    QLIST_ENTRY(AIOReq) aioreq_siblings;
> +} AIOReq;
> +
>  
>  typedef struct BDRVNBDState {
>      int sock;
>      off_t size;
>      size_t blocksize;
> +
> +    /* Filled in by nbd_config. Store host_spec because DNS may change  */
> +    bool tcp_conn;      /* True, we use TCP. False, UNIX domain sockets */

Should it be an enum then?

> +    char *export_name;  /* An NBD server may export several devices     */
> +    char *host_spec;    /* Path to socket (UNIX) or hostname/IP (TCP)   */
> +    uint16_t tcp_port;

Is this part really related to AIO? If not, it should be a patch of its own.

> +
> +    /* We use these for asynchronous I/O */
> +    uint64_t aioreq_seq_num;
> +    QLIST_HEAD(outstanding_aio_head, AIOReq) outstanding_aio_head;
>  } BDRVNBDState;
>  
> -static int nbd_open(BlockDriverState *bs, const char* filename, int flags)
> +enum AIOCBState {
> +    AIOCB_WRITE_UDATA,
> +    AIOCB_READ_UDATA,
> +};
> +
> +struct NBDAIOCB {
> +    BlockDriverAIOCB common;
> +
> +    QEMUIOVector *qiov;
> +
> +    int64_t sector_num;
> +    int nb_sectors;
> +
> +    int ret;
> +    enum AIOCBState aiocb_type;
> +
> +    QEMUBH *bh;
> +    void (*aio_done_func)(NBDAIOCB *);
> +
> +    int canceled;

bool?

> +
> +    QLIST_HEAD(aioreq_head, AIOReq) aioreq_head;
> +};
> +
> +static inline int free_aio_req(BDRVNBDState *s, AIOReq *aio_req)
>  {
> -    BDRVNBDState *s = bs->opaque;
> -    uint32_t nbdflags;
> +    NBDAIOCB *acb = aio_req->aiocb;
> +    QLIST_REMOVE(aio_req, outstanding_aio_siblings);
> +    QLIST_REMOVE(aio_req, aioreq_siblings);
> +    qemu_free(aio_req);
>  
> +    return !QLIST_EMPTY(&acb->aioreq_head);
> +}
> +
> +static int nbd_config(BDRVNBDState *s, const char* filename, int flags)
> +{
>      char *file;
> -    char *name;
> -    const char *host;
> +    char *export_name;
> +    const char *host_spec;
>      const char *unixpath;
> -    int sock;
> -    off_t size;
> -    size_t blocksize;
> -    int ret;
>      int err = -EINVAL;
>  
>      file = qemu_strdup(filename);
>  
> -    name = strstr(file, EN_OPTSTR);
> -    if (name) {
> -        if (name[strlen(EN_OPTSTR)] == 0) {
> +    export_name = strstr(file, EN_OPTSTR);
> +    if (export_name) {
> +        if (export_name[strlen(EN_OPTSTR)] == 0) {
>              goto out;
>          }
> -        name[0] = 0;
> -        name += strlen(EN_OPTSTR);
> +        export_name[0] = 0; /* truncate 'file' */
> +        export_name += strlen(EN_OPTSTR);
> +        s->export_name = qemu_strdup(export_name);
>      }
>  
> -    if (!strstart(file, "nbd:", &host)) {
> +    /* extract the host_spec - fail if it's not nbd:* */
> +    if (!strstart(file, "nbd:", &host_spec)) {
>          goto out;
>      }
>  
> -    if (strstart(host, "unix:", &unixpath)) {
> -
> -        if (unixpath[0] != '/') {
> +    /* are we a UNIX or TCP socket? */
> +    if (strstart(host_spec, "unix:", &unixpath)) {
> +        if (unixpath[0] != '/') { /* We demand  an absolute path*/
>              goto out;
>          }
> -
> -        sock = unix_socket_outgoing(unixpath);
> -
> +        s->tcp_conn = false;
> +        s->host_spec = qemu_strdup(unixpath);
>      } else {
> +        /* We should have an <IPv4 address>:<port> string to split up */
>          uint16_t port = NBD_DEFAULT_PORT;
>          char *p, *r;
>          char hostname[128];
>  
> -        pstrcpy(hostname, 128, host);
> -
> -        p = strchr(hostname, ':');
> +        pstrcpy(hostname, 128, host_spec);
> +        p = strchr(hostname, ':'); /* FIXME: IPv6 */
>          if (p != NULL) {
>              *p = '\0';
>              p++;
> @@ -96,121 +166,436 @@ static int nbd_open(BlockDriverState *bs, const char* filename, int flags)
>                  goto out;
>              }
>          }
> +        s->tcp_conn = true;
> +        s->host_spec = qemu_strdup(hostname);
> +        s->tcp_port = port;
> +    }
> +
> +    err = 0;
> +
> +out:
> +    qemu_free(file);
> +    if (err != 0) {
> +        if (s->export_name != NULL) {
> +            qemu_free(s->export_name);
> +        }
> +        if (s->host_spec != NULL) {
> +            qemu_free(s->host_spec);
> +        }

free (and qemu_free) works just fine with NULL. It's defined to do
nothing in this case.

> +    }
> +    return err;
> +}
> +
> +static void aio_read_response(void *opaque)
> +{
> +    BDRVNBDState *s = opaque;
> +    struct nbd_reply reply;
> +
> +    AIOReq *aio_req = NULL;
> +    NBDAIOCB *acb;
> +    int rest;
> +
> +    if (QLIST_EMPTY(&s->outstanding_aio_head)) {
> +        return;
> +    }
> +
> +    /* read the header */
> +    if (nbd_receive_reply(s->sock, &reply) == -1) {
> +        logout("Failed to read response from socket\n");
> +        /* Having failed to read the reply header, we can't know which
> +         * aio_req this corresponds to - so we can't signal a failure.
> +         */
> +        return;
> +    }
> +
> +    /* find the right aio_req from the outstanding_aio list */
> +    QLIST_FOREACH(aio_req, &s->outstanding_aio_head, outstanding_aio_siblings) {
> +        if (aio_req->handle == reply.handle) {
> +            break;
> +        }
> +    }
> +
> +    if (!aio_req) {
> +        logout("cannot find aio_req for handle %lu\n", reply.handle);
> +        return;
> +    }
> +
> +    acb = aio_req->aiocb;
> +
> +    if (acb->aiocb_type == AIOCB_READ_UDATA) {
> +        off_t offset = 0;
> +        int ret = 0;
> +        size_t total = aio_req->data_len;
> +
> +        while (offset < total) {
> +            ret = nbd_wr_aio(s->sock, acb->qiov->iov, total - offset, offset,
> +                             true);
> +
> +            if (ret == -1) {
> +                logout("Error reading from NBD server: %i (%s)\n",
> +                       errno, strerror(errno));

I think you need to call the AIO callback with ret = -errno here.

> +                return;
> +            }
> +
> +            offset += ret;
> +        }
> +    }
>  
> -        sock = tcp_socket_outgoing(hostname, port);
> +    if (reply.error != 0) {
> +        acb->ret = -EIO;
> +        logout("NBD request resulted in error %i\n", reply.error);
>      }
>  
> +    rest = free_aio_req(s, aio_req);
> +    if (!rest) {
> +        acb->aio_done_func(acb);
> +    }
> +
> +    return;
> +}
> +
> +static int aio_flush_request(void *opaque)
> +{
> +    BDRVNBDState *s = opaque;
> +
> +    return !QLIST_EMPTY(&s->outstanding_aio_head);
> +}
> +
> +/*
> + * Connect to the NBD server specified in the state object
> + */
> +static int nbd_establish_connection(BlockDriverState *bs)
> +{
> +    BDRVNBDState *s = bs->opaque;
> +    int sock;
> +    int ret;
> +    off_t size;
> +    size_t blocksize;
> +    uint32_t nbdflags;
> +
> +    if (s->tcp_conn == true) {
> +        sock = tcp_socket_outgoing(s->host_spec, s->tcp_port);
> +    } else {
> +        sock = unix_socket_outgoing(s->host_spec);
> +    }
> +
> +    /* Failed to establish connection */
>      if (sock == -1) {
> -        err = -errno;
> -        goto out;
> +        logout("Failed to establish connection to NBD server\n");
> +        return -errno;
>      }
>  
> -    ret = nbd_receive_negotiate(sock, name, &nbdflags, &size, &blocksize);
> +    /* NBD handshake */
> +    ret = nbd_receive_negotiate(sock, s->export_name, &nbdflags, &size,
> +                                &blocksize);
>      if (ret == -1) {
> -        err = -errno;
> -        goto out;
> +        logout("Failed to negotiate with the NBD server\n");
> +        closesocket(sock);
> +        return -errno;
>      }
>  
> +    /* Now that we're connected, set the socket to be non-blocking */
> +    socket_set_nonblock(sock);
> +
>      s->sock = sock;
>      s->size = size;
>      s->blocksize = blocksize;
> -    err = 0;
>  
> -out:
> -    qemu_free(file);
> -    return err;
> +    /* Response handler. This is called when there is data to read */
> +    qemu_aio_set_fd_handler(sock, aio_read_response, NULL, aio_flush_request,
> +                            NULL, s);
> +    logout("Established connection with NBD server\n");
> +    return 0;
>  }
>  
> -static int nbd_read(BlockDriverState *bs, int64_t sector_num,
> -                    uint8_t *buf, int nb_sectors)
> +static void nbd_teardown_connection(BlockDriverState *bs)
>  {
> +    /* Send the final packet to the NBD server and close the socket */
>      BDRVNBDState *s = bs->opaque;
>      struct nbd_request request;
> -    struct nbd_reply reply;
>  
> -    request.type = NBD_CMD_READ;
> +    request.type = NBD_CMD_DISC;
>      request.handle = (uint64_t)(intptr_t)bs;
> -    request.from = sector_num * 512;;
> -    request.len = nb_sectors * 512;
> +    request.from = 0;
> +    request.len = 0;
> +    nbd_send_request(s->sock, &request);
>  
> -    if (nbd_send_request(s->sock, &request) == -1)
> -        return -errno;
> +    qemu_aio_set_fd_handler(s->sock, NULL, NULL, NULL, NULL, NULL);
> +    closesocket(s->sock);
> +    logout("Connection to NBD server closed\n");
> +    return;
> +}
>  
> -    if (nbd_receive_reply(s->sock, &reply) == -1)
> -        return -errno;
> +static int nbd_open(BlockDriverState *bs, const char* filename, int flags)
> +{
> +    BDRVNBDState *s = bs->opaque;
> +    int result;
>  
> -    if (reply.error !=0)
> -        return -reply.error;
> +    /* Pop the config into our state object. Exit if invalid. */
> +    result = nbd_config(s, filename, flags);
>  
> -    if (reply.handle != request.handle)
> -        return -EIO;
> +    if (result != 0) {
> +        return result;
> +    }
>  
> -    if (nbd_wr_sync(s->sock, buf, request.len, 1) != request.len)
> -        return -EIO;
> +    QLIST_INIT(&s->outstanding_aio_head);
> +
> +    /* establish TCP connection, return error if it fails
> +     * TODO: Configurable retry-until-timeout behaviour.
> +     */
> +    result = nbd_establish_connection(bs);
> +    if (result != 0) {
> +        return result;
> +    }
>  
>      return 0;
>  }
>  
> -static int nbd_write(BlockDriverState *bs, int64_t sector_num,
> -                     const uint8_t *buf, int nb_sectors)
> +static void nbd_close(BlockDriverState *bs)
>  {
> +    nbd_teardown_connection(bs);
>      BDRVNBDState *s = bs->opaque;
> +
> +    if (s->export_name != NULL) {
> +        qemu_free(s->export_name);
> +    }
> +    if (s->host_spec != NULL) {
> +        qemu_free(s->host_spec);
> +    }
> +
> +    return;
> +}
> +
> +static int add_aio_request(BDRVNBDState *s, AIOReq *aio_req, QEMUIOVector *qiov,
> +                           enum AIOCBState aiocb_type)
> +{
>      struct nbd_request request;
> -    struct nbd_reply reply;
>  
> -    request.type = NBD_CMD_WRITE;
> -    request.handle = (uint64_t)(intptr_t)bs;
> -    request.from = sector_num * 512;;
> -    request.len = nb_sectors * 512;
> +    request.from = aio_req->offset;
> +    request.len = aio_req->data_len;
> +    request.handle = aio_req->handle;
> +
> +    if (aiocb_type == AIOCB_READ_UDATA) {
> +        request.type = NBD_CMD_READ;
> +    } else {
> +        request.type = NBD_CMD_WRITE;
> +    }
>  
> -    if (nbd_send_request(s->sock, &request) == -1)
> +    /* Write the request to the socket. Header first. */
> +    if (nbd_send_request(s->sock, &request) == -1) {
> +        /* TODO: retry handling. This leads to -EIO and request cancellation */
> +        logout("writing request header to server failed\n");
>          return -errno;
> +    }
>  
> -    if (nbd_wr_sync(s->sock, (uint8_t*)buf, request.len, 0) != request.len)
> -        return -EIO;
> +    /* If this is a write, send the data too */
> +    if (aiocb_type == AIOCB_WRITE_UDATA) {
> +        int ret = 0;
> +        off_t offset = 0;
> +        size_t total = aio_req->data_len;
> +
> +        while (offset < total) {
> +            ret = nbd_wr_aio(s->sock, qiov->iov, total - offset,
> +                             offset + aio_req->iov_offset, false);
> +            if (ret == -1) {
> +                logout("Error writing request data to NBD server: %i (%s)\n",
> +                       errno, strerror(errno));
> +                return -EIO;

-errno

Or maybe nbd_wr_aio should already return -errno so that you can return
ret here.

> +            }
>  
> -    if (nbd_receive_reply(s->sock, &reply) == -1)
> -        return -errno;
> +            offset += ret;
> +        }
> +    }
>  
> -    if (reply.error !=0)
> -        return -reply.error;
> +    return 0;
> +}
> +
> +static inline AIOReq *alloc_aio_req(BDRVNBDState *s, NBDAIOCB *acb,
> +                                    size_t data_len,
> +                                    off_t offset,
> +                                    off_t iov_offset)
> +{
> +    AIOReq *aio_req;
>  
> -    if (reply.handle != request.handle)
> +    aio_req = qemu_malloc(sizeof(*aio_req));
> +    aio_req->aiocb = acb;
> +    aio_req->iov_offset = iov_offset;
> +    aio_req->offset = offset;
> +    aio_req->data_len = data_len;
> +    aio_req->handle = s->aioreq_seq_num++; /* FIXME: Trivially guessable */
> +
> +    QLIST_INSERT_HEAD(&s->outstanding_aio_head, aio_req,
> +                      outstanding_aio_siblings);
> +    QLIST_INSERT_HEAD(&acb->aioreq_head, aio_req, aioreq_siblings);
> +
> +    return aio_req;
> +}
> +
> +static void nbd_finish_aiocb(NBDAIOCB *acb)
> +{
> +    if (!acb->canceled) {
> +        acb->common.cb(acb->common.opaque, acb->ret);
> +    }
> +    qemu_aio_release(acb);
> +}
> +
> +
> +static void nbd_aio_cancel(BlockDriverAIOCB *blockacb)
> +{
> +    NBDAIOCB *acb = (NBDAIOCB *)blockacb;
> +
> +    /*
> +     * We cannot cancel the requests which are already sent to
> +     * the servers, so we just complete the request with -EIO here.
> +     */
> +    acb->common.cb(acb->common.opaque, -EIO);
> +    acb->canceled = 1;
> +}

I think you need to check for acb->canceled before you write to the
associated buffer when receiving the reply for a read request. The
buffer might not exist any more after the request is cancelled.



> +
> +static AIOPool nbd_aio_pool = {
> +    .aiocb_size = sizeof(NBDAIOCB),
> +    .cancel = nbd_aio_cancel,
> +};
> +
> +static NBDAIOCB *nbd_aio_setup(BlockDriverState *bs, QEMUIOVector *qiov,
> +                                   int64_t sector_num, int nb_sectors,
> +                                   BlockDriverCompletionFunc *cb, void *opaque)
> +{
> +    NBDAIOCB *acb;
> +
> +    acb = qemu_aio_get(&nbd_aio_pool, bs, cb, opaque);
> +
> +    acb->qiov = qiov;
> +
> +    acb->sector_num = sector_num;
> +    acb->nb_sectors = nb_sectors;
> +
> +    acb->aio_done_func = NULL;
> +    acb->canceled = 0;
> +    acb->bh = NULL;
> +    acb->ret = 0;
> +    QLIST_INIT(&acb->aioreq_head);
> +    return acb;
> +}
> +
> +static int nbd_schedule_bh(QEMUBHFunc *cb, NBDAIOCB *acb)
> +{
> +    if (acb->bh) {
> +        logout("bug: %d %d\n", acb->aiocb_type, acb->aiocb_type);
>          return -EIO;
> +    }
> +
> +    acb->bh = qemu_bh_new(cb, acb);
> +    if (!acb->bh) {
> +        logout("oom: %d %d\n", acb->aiocb_type, acb->aiocb_type);
> +        return -EIO;
> +    }
> +
> +    qemu_bh_schedule(acb->bh);
>  
>      return 0;
>  }
>  
> -static void nbd_close(BlockDriverState *bs)
> +/*
> + * Send I/O requests to the server.
> + *
> + * This function sends requests to the server, links the requests to
> + * the outstanding_list in BDRVNBDState, and exits without waiting for
> + * the response.  The responses are received in the `aio_read_response'
> + * function which is called from the main loop as a fd handler.
> + * If this is a write request and it's >1MB, split it into multiple AIOReqs
> + */
> +static void nbd_readv_writev_bh_cb(void *p)
>  {
> -    BDRVNBDState *s = bs->opaque;
> -    struct nbd_request request;
> +    NBDAIOCB *acb = p;
> +    int ret = 0;
>  
> -    request.type = NBD_CMD_DISC;
> -    request.handle = (uint64_t)(intptr_t)bs;
> -    request.from = 0;
> -    request.len = 0;
> -    nbd_send_request(s->sock, &request);
> +    size_t len, done = 0;
> +    size_t total = acb->nb_sectors * SECTOR_SIZE;
> +
> +    /* Where the read/write starts from */
> +    size_t offset = acb->sector_num * SECTOR_SIZE;
> +    BDRVNBDState *s = acb->common.bs->opaque;
> +
> +    AIOReq *aio_req;
>  
> -    close(s->sock);
> +    qemu_bh_delete(acb->bh);
> +    acb->bh = NULL;
> +
> +    while (done != total) {
> +        len = (total - done);
> +
> +        /* Split write requests into 1MiB segments */
> +        if(acb->aiocb_type == AIOCB_WRITE_UDATA && len > MAX_NBD_WRITE) {
> +          len = MAX_NBD_WRITE;
> +        }
> +
> +        aio_req = alloc_aio_req(s, acb, len, offset + done, done);
> +        ret = add_aio_request(s, aio_req, acb->qiov, acb->aiocb_type);
> +
> +        if (ret < 0) {
> +            free_aio_req(s, aio_req);
> +            acb->ret = -EIO;
> +            goto out;
> +        }
> +
> +        done += len;
> +    }
> +out:
> +    if (QLIST_EMPTY(&acb->aioreq_head)) {
> +        nbd_finish_aiocb(acb);
> +    }
>  }
>  
> +static BlockDriverAIOCB *nbd_aio_readv(BlockDriverState *bs,
> +        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
> +        BlockDriverCompletionFunc *cb, void *opaque)
> +{
> +    NBDAIOCB *acb;
> +    int i;
> +
> +    acb = nbd_aio_setup(bs, qiov, sector_num, nb_sectors, cb, opaque);
> +    acb->aiocb_type = AIOCB_READ_UDATA;
> +    acb->aio_done_func = nbd_finish_aiocb;
> +
> +    for (i = 0; i < qiov->niov; i++) {
> +        memset(qiov->iov[i].iov_base, 0, qiov->iov[i].iov_len);
> +    }

qemu_iovec_memset?

What is this even for? Aren't these zeros overwritten anyway?

> +
> +    nbd_schedule_bh(nbd_readv_writev_bh_cb, acb);
> +    return &acb->common;
> +}
> +
> +static BlockDriverAIOCB *nbd_aio_writev(BlockDriverState *bs,
> +        int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
> +        BlockDriverCompletionFunc *cb, void *opaque)
> +{
> +    NBDAIOCB *acb;
> +
> +    acb = nbd_aio_setup(bs, qiov, sector_num, nb_sectors, cb, opaque);
> +    acb->aiocb_type = AIOCB_WRITE_UDATA;
> +    acb->aio_done_func = nbd_finish_aiocb;
> +
> +    nbd_schedule_bh(nbd_readv_writev_bh_cb, acb);
> +    return &acb->common;
> +}
> +
> +
>  static int64_t nbd_getlength(BlockDriverState *bs)
>  {
>      BDRVNBDState *s = bs->opaque;
> -
>      return s->size;
>  }
>  
>  static BlockDriver bdrv_nbd = {
> -    .format_name	= "nbd",
> -    .instance_size	= sizeof(BDRVNBDState),
> -    .bdrv_file_open	= nbd_open,
> -    .bdrv_read		= nbd_read,
> -    .bdrv_write		= nbd_write,
> -    .bdrv_close		= nbd_close,
> -    .bdrv_getlength	= nbd_getlength,
> -    .protocol_name	= "nbd",
> +    .format_name     = "nbd",
> +    .instance_size   = sizeof(BDRVNBDState),
> +    .bdrv_file_open  = nbd_open,
> +    .bdrv_aio_readv  = nbd_aio_readv,
> +    .bdrv_aio_writev = nbd_aio_writev,
> +    .bdrv_close      = nbd_close,
> +    .bdrv_getlength  = nbd_getlength,
> +    .protocol_name   = "nbd"
>  };
>  
>  static void bdrv_nbd_init(void)

Kevin