Patchwork [v3] block: Add support for Secure Shell (ssh) block device.

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Submitter Richard W.M. Jones
Date March 26, 2013, 10:38 a.m.
Message ID <1364294326-22155-1-git-send-email-rjones@redhat.com>
Download mbox | patch
Permalink /patch/231162/
State New
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Comments

Richard W.M. Jones - March 26, 2013, 10:38 a.m.
From: "Richard W.M. Jones" <rjones@redhat.com>

  qemu-system-x86_64 -drive file=ssh://hostname/some/image

QEMU will ssh into 'hostname' and open '/some/image' which is made
available as a standard block device.

You can specify a username (ssh://user@host/...) and/or a port number
(ssh://host:port/...).

Current limitations:

- Authentication must be done without passwords or passphrases, using
  ssh-agent.  Other authentication methods are not supported. (*)

- New remote files cannot be created. (*)

- Uses a single connection, instead of concurrent AIO with multiple
  SSH connections.

(*) = potentially easy fix

This is implemented using libssh2 on the client side.  The server just
requires a regular ssh daemon with sftp-server support.  Most ssh
daemons on Unix/Linux systems will work out of the box.

Thanks: Stefan Hajnoczi, Kevin Wolf.
---
 block/Makefile.objs |   1 +
 block/ssh.c         | 750 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 configure           |  47 ++++
 qemu-doc.texi       |  40 +++
 qemu-options.hx     |  12 +
 5 files changed, 850 insertions(+)
 create mode 100644 block/ssh.c
Richard W.M. Jones - March 26, 2013, 2:21 p.m.
On Tue, Mar 26, 2013 at 10:38:46AM +0000, Richard W.M. Jones wrote:
> +    /* According to the docs, this just updates a field in the
> +     * sftp_handle structure, so there is no network traffic and it
> +     * cannot fail.
> +     */
> +    libssh2_sftp_seek64(s->sftp_handle, offset);

While I was looking at the performance of qemu+ssh vs sshfs, I noticed
that the line above has a catastrophic effect on performance, since it
causes all readahead data to be thrown away.  Ooops.  I'm reworking
the patch to make seeking more intelligent.

Rich.
Stefan Hajnoczi - March 26, 2013, 4:23 p.m.
On Tue, Mar 26, 2013 at 02:21:39PM +0000, Richard W.M. Jones wrote:
> On Tue, Mar 26, 2013 at 10:38:46AM +0000, Richard W.M. Jones wrote:
> > +    /* According to the docs, this just updates a field in the
> > +     * sftp_handle structure, so there is no network traffic and it
> > +     * cannot fail.
> > +     */
> > +    libssh2_sftp_seek64(s->sftp_handle, offset);
> 
> While I was looking at the performance of qemu+ssh vs sshfs, I noticed
> that the line above has a catastrophic effect on performance, since it
> causes all readahead data to be thrown away.  Ooops.  I'm reworking
> the patch to make seeking more intelligent.

Should libssh2 be smarter and preserve the readahead when offsets are
contiguous?

It makes sense to fix in QEMU for older libssh2 binaries, but the real
fix might be for libssh2 to be smart.

Stefan
Richard W.M. Jones - March 26, 2013, 5 p.m.
On Tue, Mar 26, 2013 at 05:23:03PM +0100, Stefan Hajnoczi wrote:
> On Tue, Mar 26, 2013 at 02:21:39PM +0000, Richard W.M. Jones wrote:
> > On Tue, Mar 26, 2013 at 10:38:46AM +0000, Richard W.M. Jones wrote:
> > > +    /* According to the docs, this just updates a field in the
> > > +     * sftp_handle structure, so there is no network traffic and it
> > > +     * cannot fail.
> > > +     */
> > > +    libssh2_sftp_seek64(s->sftp_handle, offset);
> > 
> > While I was looking at the performance of qemu+ssh vs sshfs, I noticed
> > that the line above has a catastrophic effect on performance, since it
> > causes all readahead data to be thrown away.  Ooops.  I'm reworking
> > the patch to make seeking more intelligent.
> 
> Should libssh2 be smarter and preserve the readahead when offsets are
> contiguous?
> 
> It makes sense to fix in QEMU for older libssh2 binaries, but the real
> fix might be for libssh2 to be smart.

It's quite tricky to fix at the libssh2 API level ...  However I sent
a preliminary patch upstream:

  http://www.libssh2.org/mail/libssh2-devel-archive-2013-03/0019.shtml

Rich.
Richard W.M. Jones - March 26, 2013, 7:34 p.m.
BTW, I'd be interested in tips on what software/techniques people use
for benchmarking qemu block devices.

So far I've been testing this by booting various guests and timing how
long they take to reach various stages (eg. graphical login), but this
has not proven to be reproducible.

Rich.
Stefan Hajnoczi - March 27, 2013, 6:35 a.m.
On Tue, Mar 26, 2013 at 07:34:43PM +0000, Richard W.M. Jones wrote:
> 
> BTW, I'd be interested in tips on what software/techniques people use
> for benchmarking qemu block devices.
> 
> So far I've been testing this by booting various guests and timing how
> long they take to reach various stages (eg. graphical login), but this
> has not proven to be reproducible.

Running fio inside the guest is the way to go for real benchmarking:
http://freecode.com/projects/fio

For quick benchmarks you could use one more more dd(1) processes:
dd if=/dev/vdb of=/dev/null iflag=direct bs=64k

Stefan
Anthony Liguori - March 27, 2013, 3:12 p.m.
"Richard W.M. Jones" <rjones@redhat.com> writes:

> From: "Richard W.M. Jones" <rjones@redhat.com>
>
>   qemu-system-x86_64 -drive file=ssh://hostname/some/image
>
> QEMU will ssh into 'hostname' and open '/some/image' which is made
> available as a standard block device.
>
> You can specify a username (ssh://user@host/...) and/or a port number
> (ssh://host:port/...).
>
> Current limitations:
>
> - Authentication must be done without passwords or passphrases, using
>   ssh-agent.  Other authentication methods are not supported. (*)
>
> - New remote files cannot be created. (*)
>
> - Uses a single connection, instead of concurrent AIO with multiple
>   SSH connections.
>
> (*) = potentially easy fix
>
> This is implemented using libssh2 on the client side.  The server just
> requires a regular ssh daemon with sftp-server support.  Most ssh
> daemons on Unix/Linux systems will work out of the box.
>
> Thanks: Stefan Hajnoczi, Kevin Wolf.

Curl actually supports sftp already.  In theory, we just need to add:

static BlockDriver bdrv_sftp = {
    .format_name     = "sftp",
    .protocol_name   = "sftp",

    .instance_size   = sizeof(BDRVCURLState),
    .bdrv_file_open  = curl_open,
    .bdrv_close      = curl_close,
    .bdrv_getlength  = curl_getlength,

    .bdrv_aio_readv  = curl_aio_readv,
};

To block/curl.c and it should Just Work.   Have you considered doing
this through curl?

Regards,

Anthony Liguori


> ---
>  block/Makefile.objs |   1 +
>  block/ssh.c         | 750 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  configure           |  47 ++++
>  qemu-doc.texi       |  40 +++
>  qemu-options.hx     |  12 +
>  5 files changed, 850 insertions(+)
>  create mode 100644 block/ssh.c
>
> diff --git a/block/Makefile.objs b/block/Makefile.objs
> index c067f38..6c4b5bc 100644
> --- a/block/Makefile.objs
> +++ b/block/Makefile.objs
> @@ -13,6 +13,7 @@ block-obj-$(CONFIG_LIBISCSI) += iscsi.o
>  block-obj-$(CONFIG_CURL) += curl.o
>  block-obj-$(CONFIG_RBD) += rbd.o
>  block-obj-$(CONFIG_GLUSTERFS) += gluster.o
> +block-obj-$(CONFIG_LIBSSH2) += ssh.o
>  endif
>  
>  common-obj-y += stream.o
> diff --git a/block/ssh.c b/block/ssh.c
> new file mode 100644
> index 0000000..f7b8d32
> --- /dev/null
> +++ b/block/ssh.c
> @@ -0,0 +1,750 @@
> +/*
> + * Secure Shell (ssh) backend for QEMU.
> + *
> + * Copyright (C) 2013 Red Hat Inc., Richard W.M. Jones <rjones@redhat.com>
> + *
> + * 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 <stdio.h>
> +#include <stdlib.h>
> +#include <stdarg.h>
> +
> +#ifndef _WIN32
> +#include <pwd.h>
> +#endif
> +
> +#include <libssh2.h>
> +#include <libssh2_sftp.h>
> +
> +#include "block/block_int.h"
> +#include "qemu/sockets.h"
> +#include "qemu/uri.h"
> +#include "qapi/qmp/qint.h"
> +
> +#define DEBUG_SSH 1
> +
> +#if DEBUG_SSH
> +#define DPRINTF(fmt,...)                                                \
> +    do {                                                                \
> +        fprintf(stderr, "ssh: %-15s " fmt "\n", __func__, ##__VA_ARGS__); \
> +    } while (0)
> +#else
> +#define DPRINTF(fmt,...) /* nothing */
> +#endif
> +
> +typedef struct BDRVSSHState {
> +    CoMutex lock;                     /* coroutine lock */
> +
> +    /* ssh connection */
> +    int sock;                         /* socket */
> +    LIBSSH2_SESSION *session;         /* ssh session */
> +    LIBSSH2_SFTP *sftp;               /* sftp session */
> +    LIBSSH2_SFTP_HANDLE *sftp_handle; /* sftp remote file handle */
> +
> +    /* file attributes (at open) */
> +    LIBSSH2_SFTP_ATTRIBUTES attrs;
> +} BDRVSSHState;
> +
> +/* Wrappers around error_report which make sure to dump as much
> + * information from libssh2 as possible.
> + */
> +static void
> +session_error_report(BDRVSSHState *s, const char *fs, ...)
> +{
> +    va_list args;
> +
> +    va_start(args, fs);
> +
> +    if ((s)->session) {
> +        char *ssh_err;
> +        int ssh_err_len = sizeof ssh_err;
> +        int ssh_err_code;
> +
> +        libssh2_session_last_error((s)->session,
> +                                   &ssh_err, &ssh_err_len, 0);
> +        /* This is not an errno.  See <libssh2.h>. */
> +        ssh_err_code = libssh2_session_last_errno((s)->session);
> +
> +        error_vprintf(fs, args);
> +        error_printf(": %s (libssh2 error code: %d)", ssh_err, ssh_err_code);
> +    }
> +    else {
> +        error_vprintf(fs, args);
> +    }
> +
> +    va_end(args);
> +    error_printf("\n");
> +}
> +
> +static void
> +sftp_error_report(BDRVSSHState *s, const char *fs, ...)
> +{
> +    va_list args;
> +
> +    va_start(args, fs);
> +
> +    if ((s)->sftp) {
> +        char *ssh_err;
> +        int ssh_err_len = sizeof ssh_err;
> +        int ssh_err_code;
> +        unsigned long sftp_err_code;
> +
> +        libssh2_session_last_error((s)->session,
> +                                   &ssh_err, &ssh_err_len, 0);
> +        /* This is not an errno.  See <libssh2.h>. */
> +        ssh_err_code = libssh2_session_last_errno((s)->session);
> +        /* See <libssh2_sftp.h>. */
> +        sftp_err_code = libssh2_sftp_last_error((s)->sftp);
> +
> +        error_vprintf(fs, args);
> +        error_printf(": %s (libssh2 error code: %d, sftp error code: %lu)",
> +                     ssh_err, ssh_err_code, sftp_err_code);
> +    }
> +    else {
> +        error_vprintf(fs, args);
> +    }
> +
> +    va_end(args);
> +    error_printf("\n");
> +}
> +
> +#ifndef _WIN32
> +static const char *get_username(void)
> +{
> +    struct passwd *pwd;
> +
> +    pwd = getpwuid(geteuid ());
> +    if (!pwd) {
> +        error_report("failed to get current user name");
> +        return NULL;
> +    }
> +    return pwd->pw_name;
> +}
> +#else
> +static const char *get_username(void)
> +{
> +    error_report("on Windows, specify a remote user name in the URI");
> +    return NULL;
> +}
> +#endif
> +
> +static int parse_uri(const char *filename, QDict *options)
> +{
> +    URI *uri;
> +    char *host = NULL;
> +
> +    uri = uri_parse(filename);
> +    if (!uri) {
> +        return -EINVAL;
> +    }
> +
> +    if (strcmp(uri->scheme, "ssh") != 0) {
> +        error_report("URI scheme must be 'ssh'");
> +        goto err;
> +    }
> +
> +    if (!uri->server || strcmp(uri->server, "") == 0) {
> +        error_report("missing hostname in URI");
> +        goto err;
> +    }
> +
> +    if (!uri->path || strcmp(uri->path, "") == 0) {
> +        error_report("missing remote path in URI");
> +        goto err;
> +    }
> +
> +    if(uri->user && strcmp(uri->user, "") != 0) {
> +        qdict_put(options, "user", qstring_from_str(uri->user));
> +    }
> +
> +    qdict_put(options, "host", qstring_from_str(uri->server));
> +
> +    if (uri->port) {
> +        qdict_put(options, "port", qint_from_int(uri->port));
> +    }
> +
> +    qdict_put(options, "path", qstring_from_str(uri->path));
> +
> +    uri_free(uri);
> +    free(host);
> +    return 0;
> +
> + err:
> +    uri_free(uri);
> +    free(host);
> +    return -EINVAL;
> +}
> +
> +static void ssh_parse_filename(const char *filename, QDict *options,
> +                               Error **errp)
> +{
> +    if (qdict_haskey(options, "user") ||
> +        qdict_haskey(options, "host") ||
> +        qdict_haskey(options, "port") ||
> +        qdict_haskey(options, "path")) {
> +        error_setg(errp, "user, host, port or path cannot be used at the same time as a file option");
> +        return;
> +    }
> +
> +    if (parse_uri(filename, options) < 0) {
> +        error_setg(errp, "no valid URL specified");
> +    }
> +}
> +
> +static int check_host_key(BDRVSSHState *s, const char *host, int port)
> +{
> +    int ret;
> +    const char *home;
> +    char *knh_file = NULL;
> +    LIBSSH2_KNOWNHOSTS *knh = NULL;
> +    const char *fingerprint;
> +    size_t len;
> +    int type;
> +
> +    knh = libssh2_knownhost_init(s->session);
> +    if (!knh) {
> +        ret = -EINVAL;
> +        session_error_report(s, "failed to initialize known hosts support");
> +        goto out;
> +    }
> +
> +    home = getenv("HOME");
> +    if (home) {
> +        knh_file = g_strdup_printf("%s/.ssh/known_hosts", home);
> +    } else {
> +        knh_file = g_strdup_printf("/root/.ssh/known_hosts");
> +    }
> +
> +    /* Read all known hosts from OpenSSH-style known_hosts file. */
> +    libssh2_knownhost_readfile(knh, knh_file, LIBSSH2_KNOWNHOST_FILE_OPENSSH);
> +
> +    fingerprint = libssh2_session_hostkey(s->session, &len, &type);
> +    if (fingerprint) {
> +        struct libssh2_knownhost *found;
> +        int r;
> +
> +        r = libssh2_knownhost_checkp(knh, host, port, fingerprint, len,
> +                                     LIBSSH2_KNOWNHOST_TYPE_PLAIN|
> +                                     LIBSSH2_KNOWNHOST_KEYENC_RAW,
> +                                     &found);
> +        switch (r) {
> +        case LIBSSH2_KNOWNHOST_CHECK_MATCH:
> +            /* OK */
> +            DPRINTF("host key OK: %s", found->key);
> +            break;
> +        case LIBSSH2_KNOWNHOST_CHECK_MISMATCH:
> +            ret = -EINVAL;
> +            session_error_report(s, "host key does not match the one in known_hosts (found key %s)",
> +                                 found->key);
> +            goto out;
> +        case LIBSSH2_KNOWNHOST_CHECK_NOTFOUND:
> +            ret = -EINVAL;
> +            session_error_report(s, "no host key was found in known_hosts");
> +            goto out;
> +        case LIBSSH2_KNOWNHOST_CHECK_FAILURE:
> +            ret = -EINVAL;
> +            session_error_report(s, "failure matching the host key with known_hosts");
> +            goto out;
> +        default:
> +            ret = -EINVAL;
> +            session_error_report(s, "unknown error matching the host key with known_hosts (%d)",
> +                                 r);
> +            goto out;
> +        }
> +    }
> +
> +    ret = 0;
> +
> + out:
> +    if (knh != NULL) {
> +        libssh2_knownhost_free(knh);
> +    }
> +    g_free(knh_file);
> +    return ret;
> +}
> +
> +static int authenticate(BDRVSSHState *s, const char *user)
> +{
> +    int r, ret;
> +    const char *userauthlist;
> +    LIBSSH2_AGENT *agent = NULL;
> +    struct libssh2_agent_publickey *identity;
> +    struct libssh2_agent_publickey *prev_identity = NULL;
> +
> +    userauthlist = libssh2_userauth_list(s->session, user, strlen(user));
> +    if (strstr(userauthlist, "publickey") == NULL) {
> +        ret = -EPERM;
> +        error_report("remote server does not support \"publickey\" authentication");
> +        goto out;
> +    }
> +
> +    /* Connect to ssh-agent and try each identity in turn. */
> +    agent = libssh2_agent_init(s->session);
> +    if (!agent) {
> +        ret = -EINVAL;
> +        session_error_report(s, "failed to initialize ssh-agent support");
> +        goto out;
> +    }
> +    if (libssh2_agent_connect(agent)) {
> +        ret = -ECONNREFUSED;
> +        session_error_report(s, "failed to connect to ssh-agent");
> +        goto out;
> +    }
> +    if (libssh2_agent_list_identities(agent)) {
> +        ret = -EINVAL;
> +        session_error_report(s, "failed requesting identities from ssh-agent");
> +        goto out;
> +    }
> +
> +    for(;;) {
> +        r = libssh2_agent_get_identity(agent, &identity, prev_identity);
> +        if (r == 1) {           /* end of list */
> +            break;
> +        }
> +        if (r < 0) {
> +            ret = -EINVAL;
> +            session_error_report(s, "failed to obtain identity from ssh-agent");
> +            goto out;
> +        }
> +        r = libssh2_agent_userauth(agent, user, identity);
> +        if (r == 0) {
> +            /* Authenticated! */
> +            ret = 0;
> +            goto out;
> +        }
> +        /* Failed to authenticate with this identity, try the next one. */
> +        prev_identity = identity;
> +    }
> +
> +    ret = -EPERM;
> +    error_report("failed to authenticate using publickey authentication "
> +                 "and the identities held by your ssh-agent");
> +
> + out:
> +    if (agent != NULL) {
> +        /* Note: libssh2 implementation implicitly calls
> +         * libssh2_agent_disconnect if necessary.
> +         */
> +        libssh2_agent_free(agent);
> +    }
> +
> +    return ret;
> +}
> +
> +static int connect_to_ssh(BDRVSSHState *s, QDict *options, int bdrv_flags)
> +{
> +    int r, ret, flags;
> +    Error *err = NULL;
> +    const char *host, *user, *path;
> +    int port;
> +    char *hostport = NULL;
> +
> +    host = qdict_get_str(options, "host");
> +
> +    if (qdict_haskey(options, "port")) {
> +        port = qdict_get_int(options, "port");
> +    } else {
> +        port = 22;
> +    }
> +
> +    path = qdict_get_str(options, "path");
> +
> +    if (qdict_haskey(options, "user")) {
> +        user = qdict_get_str(options, "user");
> +    } else {
> +        user = get_username();
> +        if (!user) {
> +            ret = -EINVAL;
> +            goto err;
> +        }
> +    }
> +
> +    /* Open the socket and connect. */
> +    hostport = g_strdup_printf("%s:%d", host, port);
> +    s->sock = inet_connect(hostport, &err);
> +    if (err != NULL) {
> +        ret = -errno;
> +        qerror_report_err(err);
> +        error_free(err);
> +        goto err;
> +    }
> +
> +    /* Create SSH session. */
> +    s->session = libssh2_session_init();
> +    if (!s->session) {
> +        ret = -EINVAL;
> +        session_error_report(s, "failed to initialize libssh2 session");
> +        goto err;
> +    }
> +
> +    r = libssh2_session_handshake(s->session, s->sock);
> +    if (r != 0) {
> +        ret = -EINVAL;
> +        session_error_report(s, "failed to establish SSH session");
> +        goto err;
> +    }
> +
> +    /* Check the remote host's key against known_hosts. */
> +    ret = check_host_key(s, host, port);
> +    if (ret < 0) {
> +        goto err;
> +    }
> +
> +    /* Authenticate. */
> +    ret = authenticate(s, user);
> +    if (ret < 0) {
> +        goto err;
> +    }
> +
> +    /* Start SFTP. */
> +    s->sftp = libssh2_sftp_init(s->session);
> +    if (!s->sftp) {
> +        session_error_report(s, "failed to initialize sftp handle");
> +        ret = -EINVAL;
> +        goto err;
> +    }
> +
> +    /* Open the remote file. */
> +    flags = LIBSSH2_FXF_READ;
> +    if (bdrv_flags & BDRV_O_RDWR) {
> +        flags |= LIBSSH2_FXF_WRITE;
> +    }
> +    s->sftp_handle = libssh2_sftp_open(s->sftp, path, flags, 0);
> +    if (!s->sftp_handle) {
> +        session_error_report(s, "failed to open remote file '%s'", path);
> +        ret = -EINVAL;
> +        goto err;
> +    }
> +
> +    r = libssh2_sftp_fstat(s->sftp_handle, &s->attrs);
> +    if (r < 0) {
> +        sftp_error_report(s, "failed to read file attributes of '%s'",
> +                          path);
> +        ret = -EINVAL;
> +        goto err;
> +    }
> +
> +    /* Delete the options we've used; any not deleted will cause the
> +     * block layer to give an error about unused options.
> +     */
> +    qdict_del(options, "host");
> +    qdict_del(options, "port");
> +    qdict_del(options, "user");
> +    qdict_del(options, "path");
> +
> +    g_free(hostport);
> +    return 0;
> +
> + err:
> +    if (s->sftp_handle) {
> +        libssh2_sftp_close(s->sftp_handle);
> +    }
> +    s->sftp_handle = NULL;
> +    if (s->sftp) {
> +        libssh2_sftp_shutdown(s->sftp);
> +    }
> +    s->sftp = NULL;
> +    if (s->session) {
> +        libssh2_session_disconnect(s->session,
> +                                   "from qemu ssh client: "
> +                                   "error opening connection");
> +        libssh2_session_free(s->session);
> +    }
> +    s->session = NULL;
> +    g_free(hostport);
> +
> +    return ret;
> +}
> +
> +static int ssh_file_open(BlockDriverState *bs, const char *filename,
> +                         QDict *options, int bdrv_flags)
> +{
> +    BDRVSSHState *s = bs->opaque;
> +    int ret;
> +
> +    s->sock = -1;
> +    qemu_co_mutex_init(&s->lock);
> +
> +    /* Start up SSH. */
> +    ret = connect_to_ssh(s, options, bdrv_flags);
> +    if (ret < 0) {
> +        goto err;
> +    }
> +
> +    /* Go non-blocking. */
> +    libssh2_session_set_blocking(s->session, 0);
> +
> +    return 0;
> +
> + err:
> +    if (s->sock >= 0) {
> +        close(s->sock);
> +    }
> +    s->sock = -1;
> +
> +    return ret;
> +}
> +
> +static void ssh_close(BlockDriverState *bs)
> +{
> +    BDRVSSHState *s = bs->opaque;
> +
> +    if (s->sftp_handle) {
> +        libssh2_sftp_close(s->sftp_handle);
> +    }
> +    if (s->sftp) {
> +        libssh2_sftp_shutdown(s->sftp);
> +    }
> +    if (s->session) {
> +        libssh2_session_disconnect(s->session,
> +                                   "from qemu ssh client: "
> +                                   "user closed the connection");
> +        libssh2_session_free(s->session);
> +    }
> +    if (s->sock >= 0) {
> +        close(s->sock);
> +    }
> +}
> +
> +static void restart_coroutine(void *opaque)
> +{
> +    Coroutine *co = opaque;
> +
> +    qemu_coroutine_enter(co, NULL);
> +}
> +
> +/* Always true because when we have called set_fd_handler there is
> + * always a request being processed.
> + */
> +static int return_true(void *opaque)
> +{
> +    return 1;
> +}
> +
> +static coroutine_fn void set_fd_handler(BDRVSSHState *s)
> +{
> +    int r;
> +    IOHandler *rd_handler = NULL, *wr_handler = NULL;
> +    Coroutine *co = qemu_coroutine_self();
> +
> +    r = libssh2_session_block_directions(s->session);
> +
> +    if (r & LIBSSH2_SESSION_BLOCK_INBOUND) {
> +        rd_handler = restart_coroutine;
> +    }
> +    if (r & LIBSSH2_SESSION_BLOCK_OUTBOUND) {
> +        wr_handler = restart_coroutine;
> +    }
> +
> +    DPRINTF("s->sock=%d rd_handler=%p wr_handler=%p", s->sock,
> +            rd_handler, wr_handler);
> +
> +    qemu_aio_set_fd_handler(s->sock, rd_handler, wr_handler, return_true, co);
> +}
> +
> +static coroutine_fn void clear_fd_handler(BDRVSSHState *s)
> +{
> +    qemu_aio_set_fd_handler(s->sock, NULL, NULL, NULL, NULL);
> +}
> +
> +/* A non-blocking call returned EAGAIN, so yield, ensuring the
> + * handlers are set up so that we'll be rescheduled when there is an
> + * interesting event on the socket.
> + */
> +static coroutine_fn void co_yield(BDRVSSHState *s)
> +{
> +    set_fd_handler(s);
> +    qemu_coroutine_yield();
> +    clear_fd_handler(s);
> +}
> +
> +static coroutine_fn int ssh_read(BDRVSSHState *s,
> +                                 int64_t offset, size_t size,
> +                                 QEMUIOVector *qiov)
> +{
> +    ssize_t r;
> +    size_t got;
> +    char *buf, *end_of_vec;
> +    struct iovec *i;
> +
> +    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
> +
> +    /* According to the docs, this just updates a field in the
> +     * sftp_handle structure, so there is no network traffic and it
> +     * cannot fail.
> +     */
> +    libssh2_sftp_seek64(s->sftp_handle, offset);
> +
> +    /* This keeps track of the current iovec element ('i'), where we
> +     * will write to next ('buf'), and the end of the current iovec
> +     * ('end_of_vec').
> +     */
> +    i = &qiov->iov[0];
> +    buf = i->iov_base;
> +    end_of_vec = i->iov_base + i->iov_len;
> +
> +    /* libssh2 has a hard-coded limit of 2000 bytes per request,
> +     * although it will also do readahead behind our backs.  Therefore
> +     * we may have to do repeated reads here until we have read 'size'
> +     * bytes.
> +     */
> +    for (got = 0; got < size; ) {
> +    again:
> +        DPRINTF("sftp_read buf=%p size=%zu", buf, end_of_vec - buf);
> +        r = libssh2_sftp_read(s->sftp_handle, buf, end_of_vec - buf);
> +
> +        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
> +            co_yield(s);
> +            goto again;
> +        }
> +        if (r < 0) {
> +            sftp_error_report(s, "read failed");
> +            return -EIO;
> +        }
> +        if (r == 0) {
> +            error_report("read failed (EOF)");
> +            return -EIO;
> +        }
> +
> +        got += r;
> +        buf += r;
> +        if (buf >= end_of_vec) {
> +            i++;
> +            buf = i->iov_base;
> +            end_of_vec = i->iov_base + i->iov_len;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +static coroutine_fn int ssh_co_readv(BlockDriverState *bs,
> +                                     int64_t sector_num,
> +                                     int nb_sectors, QEMUIOVector *qiov)
> +{
> +    BDRVSSHState *s = bs->opaque;
> +    int ret;
> +
> +    qemu_co_mutex_lock(&s->lock);
> +    ret = ssh_read(s, sector_num * BDRV_SECTOR_SIZE,
> +                   nb_sectors * BDRV_SECTOR_SIZE, qiov);
> +    qemu_co_mutex_unlock(&s->lock);
> +
> +    return ret;
> +}
> +
> +static int ssh_write(BDRVSSHState *s,
> +                     int64_t offset, size_t size,
> +                     QEMUIOVector *qiov)
> +{
> +    ssize_t r;
> +    size_t written;
> +    char *buf, *end_of_vec;
> +    struct iovec *i;
> +
> +    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
> +
> +    /* According to the docs, this just updates a field in the
> +     * sftp_handle structure, so there is no network traffic and it
> +     * cannot fail.
> +     */
> +    libssh2_sftp_seek64(s->sftp_handle, offset);
> +
> +    /* This keeps track of the current iovec element ('i'), where we
> +     * will read from next ('buf'), and the end of the current iovec
> +     * ('end_of_vec').
> +     */
> +    i = &qiov->iov[0];
> +    buf = i->iov_base;
> +    end_of_vec = i->iov_base + i->iov_len;
> +
> +    for (written = 0; written < size; ) {
> +    again:
> +        DPRINTF("sftp_write buf=%p size=%zu", buf, end_of_vec - buf);
> +        r = libssh2_sftp_write(s->sftp_handle, buf, end_of_vec - buf);
> +
> +        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
> +            co_yield(s);
> +            goto again;
> +        }
> +        if (r < 0) {
> +            sftp_error_report(s, "write failed");
> +            return -EIO;
> +        }
> +
> +        written += r;
> +        buf += r;
> +        if (buf >= end_of_vec) {
> +            i++;
> +            buf = i->iov_base;
> +            end_of_vec = i->iov_base + i->iov_len;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +static coroutine_fn int ssh_co_writev(BlockDriverState *bs,
> +                                      int64_t sector_num,
> +                                      int nb_sectors, QEMUIOVector *qiov)
> +{
> +    BDRVSSHState *s = bs->opaque;
> +    int ret;
> +
> +    qemu_co_mutex_lock(&s->lock);
> +    ret = ssh_write(s, sector_num * BDRV_SECTOR_SIZE,
> +                    nb_sectors * BDRV_SECTOR_SIZE, qiov);
> +    qemu_co_mutex_unlock(&s->lock);
> +
> +    return ret;
> +}
> +
> +static int64_t ssh_getlength(BlockDriverState *bs)
> +{
> +    BDRVSSHState *s = bs->opaque;
> +    return (int64_t) s->attrs.filesize;
> +}
> +
> +static BlockDriver bdrv_ssh = {
> +    .format_name                  = "ssh",
> +    .protocol_name                = "ssh",
> +    .instance_size                = sizeof(BDRVSSHState),
> +    .bdrv_parse_filename          = ssh_parse_filename,
> +    .bdrv_file_open               = ssh_file_open,
> +    .bdrv_close                   = ssh_close,
> +    .bdrv_co_readv                = ssh_co_readv,
> +    .bdrv_co_writev               = ssh_co_writev,
> +    .bdrv_getlength               = ssh_getlength,
> +};
> +
> +static void bdrv_ssh_init(void)
> +{
> +    int r;
> +
> +    r = libssh2_init(0);
> +    if (r != 0) {
> +        fprintf(stderr, "libssh2 initialization failed, %d\n", r);
> +        exit(EXIT_FAILURE);
> +    }
> +
> +    bdrv_register(&bdrv_ssh);
> +}
> +
> +block_init(bdrv_ssh_init);
> diff --git a/configure b/configure
> index f2af714..d666066 100755
> --- a/configure
> +++ b/configure
> @@ -229,6 +229,7 @@ virtio_blk_data_plane=""
>  gtk=""
>  gtkabi="2.0"
>  tpm="no"
> +libssh2=""
>  
>  # parse CC options first
>  for opt do
> @@ -908,6 +909,10 @@ for opt do
>    ;;
>    --enable-tpm) tpm="yes"
>    ;;
> +  --disable-libssh2) libssh2="no"
> +  ;;
> +  --enable-libssh2) libssh2="yes"
> +  ;;
>    *) echo "ERROR: unknown option $opt"; show_help="yes"
>    ;;
>    esac
> @@ -1166,6 +1171,7 @@ echo "  --disable-glusterfs      disable GlusterFS backend"
>  echo "  --enable-gcov            enable test coverage analysis with gcov"
>  echo "  --gcov=GCOV              use specified gcov [$gcov_tool]"
>  echo "  --enable-tpm             enable TPM support"
> +echo "  --enable-libssh2         enable ssh block device support"
>  echo ""
>  echo "NOTE: The object files are built at the place where configure is launched"
>  exit 1
> @@ -2330,6 +2336,42 @@ EOF
>  fi
>  
>  ##########################################
> +# libssh2 probe
> +if test "$libssh2" != "no" ; then
> +  cat > $TMPC <<EOF
> +#include <stdio.h>
> +#include <libssh2.h>
> +#include <libssh2_sftp.h>
> +int main(void) {
> +    LIBSSH2_SESSION *session;
> +    session = libssh2_session_init ();
> +    (void) libssh2_sftp_init (session);
> +    return 0;
> +}
> +EOF
> +
> +  if $pkg_config libssh2 --modversion >/dev/null 2>&1; then
> +    libssh2_cflags=`$pkg_config libssh2 --cflags`
> +    libssh2_libs=`$pkg_config libssh2 --libs`
> +  else
> +    libssh2_cflags=
> +    libssh2_libs="-lssh2"
> +  fi
> +
> +  if compile_prog "$libssh2_cflags" "$libssh2_libs" ; then
> +    libssh2=yes
> +    libs_tools="$libssh2_libs $libs_tools"
> +    libs_softmmu="$libssh2_libs $libs_softmmu"
> +    QEMU_CFLAGS="$QEMU_CFLAGS $libssh2_cflags"
> +  else
> +    if test "$libssh2" = "yes" ; then
> +      feature_not_found "libssh2"
> +    fi
> +    libssh2=no
> +  fi
> +fi
> +
> +##########################################
>  # linux-aio probe
>  
>  if test "$linux_aio" != "no" ; then
> @@ -3441,6 +3483,7 @@ echo "virtio-blk-data-plane $virtio_blk_data_plane"
>  echo "gcov              $gcov_tool"
>  echo "gcov enabled      $gcov"
>  echo "TPM support       $tpm"
> +echo "libssh2 support   $libssh2"
>  
>  if test "$sdl_too_old" = "yes"; then
>  echo "-> Your SDL version is too old - please upgrade to have SDL support"
> @@ -3806,6 +3849,10 @@ if test "$glusterfs" = "yes" ; then
>    echo "CONFIG_GLUSTERFS=y" >> $config_host_mak
>  fi
>  
> +if test "$libssh2" = "yes" ; then
> +  echo "CONFIG_LIBSSH2=y" >> $config_host_mak
> +fi
> +
>  if test "$virtio_blk_data_plane" = "yes" ; then
>    echo "CONFIG_VIRTIO_BLK_DATA_PLANE=y" >> $config_host_mak
>  fi
> diff --git a/qemu-doc.texi b/qemu-doc.texi
> index af84bef..7a0f373 100644
> --- a/qemu-doc.texi
> +++ b/qemu-doc.texi
> @@ -423,6 +423,7 @@ snapshots.
>  * disk_images_sheepdog::      Sheepdog disk images
>  * disk_images_iscsi::         iSCSI LUNs
>  * disk_images_gluster::       GlusterFS disk images
> +* disk_images_ssh::           Secure Shell (ssh) disk images
>  @end menu
>  
>  @node disk_images_quickstart
> @@ -1038,6 +1039,45 @@ qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glu
>  qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
>  @end example
>  
> +@node disk_images_ssh
> +@subsection Secure Shell (ssh) disk images
> +
> +You can access disk images located on a remote ssh server
> +by using the ssh protocol:
> +
> +@example
> +qemu-system-x86_64 -drive file=ssh://[@var{user}@@]@var{server}[:@var{port}]/@var{path}
> +@end example
> +
> +Alternative syntax using properties:
> +
> +@example
> +qemu-system-x86_64 -drive file.driver=ssh[,file.user=@var{user}],file.host=@var{server}[,file.port=@var{port}],file.path=@var{path}
> +@end example
> +
> +@var{ssh} is the protocol.
> +
> +@var{user} is the remote user.  If not specified, then the local
> +username is tried.
> +
> +@var{server} specifies the remote ssh server.  Any ssh server can be
> +used, but it must implement the sftp-server protocol.  Most Unix/Linux
> +systems should work without requiring any extra configuration.
> +
> +@var{port} is the port number on which sshd is listening.  By default
> +the standard ssh port (22) is used.
> +
> +@var{path} is the path to the disk image.
> +
> +Currently authentication must be done using ssh-agent.  Other
> +authentication methods may be supported in future.
> +
> +Note: The ssh driver does not obey disk flush requests (ie. to commit
> +data to the backing disk when the guest requests it).  This is because
> +the underlying protocol (SFTP) does not support this.  Thus there is a
> +risk of guest disk corruption if the remote server or network goes
> +down during writes.
> +
>  @node pcsys_network
>  @section Network emulation
>  
> diff --git a/qemu-options.hx b/qemu-options.hx
> index d7afeab..590a287 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -2106,6 +2106,18 @@ Example for Unix Domain Sockets
>  qemu-system-i386 --drive file=nbd:unix:/tmp/nbd-socket
>  @end example
>  
> +@item SSH
> +QEMU supports SSH (Secure Shell) access to remote disks.
> +
> +Examples:
> +@example
> +qemu-system-i386 -drive file=ssh://user@@host/path/to/disk.img
> +qemu-system-i386 -drive file.driver=ssh,file.user=user,file.host=host,file.port=22,file.path=/path/to/disk.img
> +@end example
> +
> +Currently authentication must be done using ssh-agent.  Other
> +authentication methods may be supported in future.
> +
>  @item Sheepdog
>  Sheepdog is a distributed storage system for QEMU.
>  QEMU supports using either local sheepdog devices or remote networked
> -- 
> 1.8.1.4
Richard W.M. Jones - March 27, 2013, 3:44 p.m.
On Wed, Mar 27, 2013 at 10:12:42AM -0500, Anthony Liguori wrote:
> "Richard W.M. Jones" <rjones@redhat.com> writes:
> 
> > From: "Richard W.M. Jones" <rjones@redhat.com>
> >
> >   qemu-system-x86_64 -drive file=ssh://hostname/some/image
> >
> > QEMU will ssh into 'hostname' and open '/some/image' which is made
> > available as a standard block device.
> >
> > You can specify a username (ssh://user@host/...) and/or a port number
> > (ssh://host:port/...).
> >
> > Current limitations:
> >
> > - Authentication must be done without passwords or passphrases, using
> >   ssh-agent.  Other authentication methods are not supported. (*)
> >
> > - New remote files cannot be created. (*)
> >
> > - Uses a single connection, instead of concurrent AIO with multiple
> >   SSH connections.
> >
> > (*) = potentially easy fix
> >
> > This is implemented using libssh2 on the client side.  The server just
> > requires a regular ssh daemon with sftp-server support.  Most ssh
> > daemons on Unix/Linux systems will work out of the box.
> >
> > Thanks: Stefan Hajnoczi, Kevin Wolf.
> 
> Curl actually supports sftp already.  In theory, we just need to add:
> 
> static BlockDriver bdrv_sftp = {
>     .format_name     = "sftp",
>     .protocol_name   = "sftp",
> 
>     .instance_size   = sizeof(BDRVCURLState),
>     .bdrv_file_open  = curl_open,
>     .bdrv_close      = curl_close,
>     .bdrv_getlength  = curl_getlength,
> 
>     .bdrv_aio_readv  = curl_aio_readv,
> };
> 
> To block/curl.c and it should Just Work.   Have you considered doing
> this through curl?

Yes, and it doesn't work.  See:

http://www.mail-archive.com/qemu-devel@nongnu.org/msg162605.html
(curl-based patch: http://www.mail-archive.com/qemu-devel@nongnu.org/msg162253.html )

Rich.
Anthony Liguori - March 27, 2013, 4:43 p.m.
"Richard W.M. Jones" <rjones@redhat.com> writes:

> On Wed, Mar 27, 2013 at 10:12:42AM -0500, Anthony Liguori wrote:
>> "Richard W.M. Jones" <rjones@redhat.com> writes:
>> 
>> > From: "Richard W.M. Jones" <rjones@redhat.com>
>> >
>> >   qemu-system-x86_64 -drive file=ssh://hostname/some/image
>> >
>> > QEMU will ssh into 'hostname' and open '/some/image' which is made
>> > available as a standard block device.
>> >
>> > You can specify a username (ssh://user@host/...) and/or a port number
>> > (ssh://host:port/...).
>> >
>> > Current limitations:
>> >
>> > - Authentication must be done without passwords or passphrases, using
>> >   ssh-agent.  Other authentication methods are not supported. (*)
>> >
>> > - New remote files cannot be created. (*)
>> >
>> > - Uses a single connection, instead of concurrent AIO with multiple
>> >   SSH connections.
>> >
>> > (*) = potentially easy fix
>> >
>> > This is implemented using libssh2 on the client side.  The server just
>> > requires a regular ssh daemon with sftp-server support.  Most ssh
>> > daemons on Unix/Linux systems will work out of the box.
>> >
>> > Thanks: Stefan Hajnoczi, Kevin Wolf.
>> 
>> Curl actually supports sftp already.  In theory, we just need to add:
>> 
>> static BlockDriver bdrv_sftp = {
>>     .format_name     = "sftp",
>>     .protocol_name   = "sftp",
>> 
>>     .instance_size   = sizeof(BDRVCURLState),
>>     .bdrv_file_open  = curl_open,
>>     .bdrv_close      = curl_close,
>>     .bdrv_getlength  = curl_getlength,
>> 
>>     .bdrv_aio_readv  = curl_aio_readv,
>> };
>> 
>> To block/curl.c and it should Just Work.   Have you considered doing
>> this through curl?
>
> Yes, and it doesn't work.  See:
>
> http://www.mail-archive.com/qemu-devel@nongnu.org/msg162605.html
> (curl-based patch:
> http://www.mail-archive.com/qemu-devel@nongnu.org/msg162253.html )

Ah, thanks.

Regards,

Anthony Liguori

>
> Rich.
>
> -- 
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> virt-df lists disk usage of guests without needing to install any
> software inside the virtual machine.  Supports Linux and Windows.
> http://people.redhat.com/~rjones/virt-df/

Patch

diff --git a/block/Makefile.objs b/block/Makefile.objs
index c067f38..6c4b5bc 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -13,6 +13,7 @@  block-obj-$(CONFIG_LIBISCSI) += iscsi.o
 block-obj-$(CONFIG_CURL) += curl.o
 block-obj-$(CONFIG_RBD) += rbd.o
 block-obj-$(CONFIG_GLUSTERFS) += gluster.o
+block-obj-$(CONFIG_LIBSSH2) += ssh.o
 endif
 
 common-obj-y += stream.o
diff --git a/block/ssh.c b/block/ssh.c
new file mode 100644
index 0000000..f7b8d32
--- /dev/null
+++ b/block/ssh.c
@@ -0,0 +1,750 @@ 
+/*
+ * Secure Shell (ssh) backend for QEMU.
+ *
+ * Copyright (C) 2013 Red Hat Inc., Richard W.M. Jones <rjones@redhat.com>
+ *
+ * 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 <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+
+#ifndef _WIN32
+#include <pwd.h>
+#endif
+
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+
+#include "block/block_int.h"
+#include "qemu/sockets.h"
+#include "qemu/uri.h"
+#include "qapi/qmp/qint.h"
+
+#define DEBUG_SSH 1
+
+#if DEBUG_SSH
+#define DPRINTF(fmt,...)                                                \
+    do {                                                                \
+        fprintf(stderr, "ssh: %-15s " fmt "\n", __func__, ##__VA_ARGS__); \
+    } while (0)
+#else
+#define DPRINTF(fmt,...) /* nothing */
+#endif
+
+typedef struct BDRVSSHState {
+    CoMutex lock;                     /* coroutine lock */
+
+    /* ssh connection */
+    int sock;                         /* socket */
+    LIBSSH2_SESSION *session;         /* ssh session */
+    LIBSSH2_SFTP *sftp;               /* sftp session */
+    LIBSSH2_SFTP_HANDLE *sftp_handle; /* sftp remote file handle */
+
+    /* file attributes (at open) */
+    LIBSSH2_SFTP_ATTRIBUTES attrs;
+} BDRVSSHState;
+
+/* Wrappers around error_report which make sure to dump as much
+ * information from libssh2 as possible.
+ */
+static void
+session_error_report(BDRVSSHState *s, const char *fs, ...)
+{
+    va_list args;
+
+    va_start(args, fs);
+
+    if ((s)->session) {
+        char *ssh_err;
+        int ssh_err_len = sizeof ssh_err;
+        int ssh_err_code;
+
+        libssh2_session_last_error((s)->session,
+                                   &ssh_err, &ssh_err_len, 0);
+        /* This is not an errno.  See <libssh2.h>. */
+        ssh_err_code = libssh2_session_last_errno((s)->session);
+
+        error_vprintf(fs, args);
+        error_printf(": %s (libssh2 error code: %d)", ssh_err, ssh_err_code);
+    }
+    else {
+        error_vprintf(fs, args);
+    }
+
+    va_end(args);
+    error_printf("\n");
+}
+
+static void
+sftp_error_report(BDRVSSHState *s, const char *fs, ...)
+{
+    va_list args;
+
+    va_start(args, fs);
+
+    if ((s)->sftp) {
+        char *ssh_err;
+        int ssh_err_len = sizeof ssh_err;
+        int ssh_err_code;
+        unsigned long sftp_err_code;
+
+        libssh2_session_last_error((s)->session,
+                                   &ssh_err, &ssh_err_len, 0);
+        /* This is not an errno.  See <libssh2.h>. */
+        ssh_err_code = libssh2_session_last_errno((s)->session);
+        /* See <libssh2_sftp.h>. */
+        sftp_err_code = libssh2_sftp_last_error((s)->sftp);
+
+        error_vprintf(fs, args);
+        error_printf(": %s (libssh2 error code: %d, sftp error code: %lu)",
+                     ssh_err, ssh_err_code, sftp_err_code);
+    }
+    else {
+        error_vprintf(fs, args);
+    }
+
+    va_end(args);
+    error_printf("\n");
+}
+
+#ifndef _WIN32
+static const char *get_username(void)
+{
+    struct passwd *pwd;
+
+    pwd = getpwuid(geteuid ());
+    if (!pwd) {
+        error_report("failed to get current user name");
+        return NULL;
+    }
+    return pwd->pw_name;
+}
+#else
+static const char *get_username(void)
+{
+    error_report("on Windows, specify a remote user name in the URI");
+    return NULL;
+}
+#endif
+
+static int parse_uri(const char *filename, QDict *options)
+{
+    URI *uri;
+    char *host = NULL;
+
+    uri = uri_parse(filename);
+    if (!uri) {
+        return -EINVAL;
+    }
+
+    if (strcmp(uri->scheme, "ssh") != 0) {
+        error_report("URI scheme must be 'ssh'");
+        goto err;
+    }
+
+    if (!uri->server || strcmp(uri->server, "") == 0) {
+        error_report("missing hostname in URI");
+        goto err;
+    }
+
+    if (!uri->path || strcmp(uri->path, "") == 0) {
+        error_report("missing remote path in URI");
+        goto err;
+    }
+
+    if(uri->user && strcmp(uri->user, "") != 0) {
+        qdict_put(options, "user", qstring_from_str(uri->user));
+    }
+
+    qdict_put(options, "host", qstring_from_str(uri->server));
+
+    if (uri->port) {
+        qdict_put(options, "port", qint_from_int(uri->port));
+    }
+
+    qdict_put(options, "path", qstring_from_str(uri->path));
+
+    uri_free(uri);
+    free(host);
+    return 0;
+
+ err:
+    uri_free(uri);
+    free(host);
+    return -EINVAL;
+}
+
+static void ssh_parse_filename(const char *filename, QDict *options,
+                               Error **errp)
+{
+    if (qdict_haskey(options, "user") ||
+        qdict_haskey(options, "host") ||
+        qdict_haskey(options, "port") ||
+        qdict_haskey(options, "path")) {
+        error_setg(errp, "user, host, port or path cannot be used at the same time as a file option");
+        return;
+    }
+
+    if (parse_uri(filename, options) < 0) {
+        error_setg(errp, "no valid URL specified");
+    }
+}
+
+static int check_host_key(BDRVSSHState *s, const char *host, int port)
+{
+    int ret;
+    const char *home;
+    char *knh_file = NULL;
+    LIBSSH2_KNOWNHOSTS *knh = NULL;
+    const char *fingerprint;
+    size_t len;
+    int type;
+
+    knh = libssh2_knownhost_init(s->session);
+    if (!knh) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to initialize known hosts support");
+        goto out;
+    }
+
+    home = getenv("HOME");
+    if (home) {
+        knh_file = g_strdup_printf("%s/.ssh/known_hosts", home);
+    } else {
+        knh_file = g_strdup_printf("/root/.ssh/known_hosts");
+    }
+
+    /* Read all known hosts from OpenSSH-style known_hosts file. */
+    libssh2_knownhost_readfile(knh, knh_file, LIBSSH2_KNOWNHOST_FILE_OPENSSH);
+
+    fingerprint = libssh2_session_hostkey(s->session, &len, &type);
+    if (fingerprint) {
+        struct libssh2_knownhost *found;
+        int r;
+
+        r = libssh2_knownhost_checkp(knh, host, port, fingerprint, len,
+                                     LIBSSH2_KNOWNHOST_TYPE_PLAIN|
+                                     LIBSSH2_KNOWNHOST_KEYENC_RAW,
+                                     &found);
+        switch (r) {
+        case LIBSSH2_KNOWNHOST_CHECK_MATCH:
+            /* OK */
+            DPRINTF("host key OK: %s", found->key);
+            break;
+        case LIBSSH2_KNOWNHOST_CHECK_MISMATCH:
+            ret = -EINVAL;
+            session_error_report(s, "host key does not match the one in known_hosts (found key %s)",
+                                 found->key);
+            goto out;
+        case LIBSSH2_KNOWNHOST_CHECK_NOTFOUND:
+            ret = -EINVAL;
+            session_error_report(s, "no host key was found in known_hosts");
+            goto out;
+        case LIBSSH2_KNOWNHOST_CHECK_FAILURE:
+            ret = -EINVAL;
+            session_error_report(s, "failure matching the host key with known_hosts");
+            goto out;
+        default:
+            ret = -EINVAL;
+            session_error_report(s, "unknown error matching the host key with known_hosts (%d)",
+                                 r);
+            goto out;
+        }
+    }
+
+    ret = 0;
+
+ out:
+    if (knh != NULL) {
+        libssh2_knownhost_free(knh);
+    }
+    g_free(knh_file);
+    return ret;
+}
+
+static int authenticate(BDRVSSHState *s, const char *user)
+{
+    int r, ret;
+    const char *userauthlist;
+    LIBSSH2_AGENT *agent = NULL;
+    struct libssh2_agent_publickey *identity;
+    struct libssh2_agent_publickey *prev_identity = NULL;
+
+    userauthlist = libssh2_userauth_list(s->session, user, strlen(user));
+    if (strstr(userauthlist, "publickey") == NULL) {
+        ret = -EPERM;
+        error_report("remote server does not support \"publickey\" authentication");
+        goto out;
+    }
+
+    /* Connect to ssh-agent and try each identity in turn. */
+    agent = libssh2_agent_init(s->session);
+    if (!agent) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to initialize ssh-agent support");
+        goto out;
+    }
+    if (libssh2_agent_connect(agent)) {
+        ret = -ECONNREFUSED;
+        session_error_report(s, "failed to connect to ssh-agent");
+        goto out;
+    }
+    if (libssh2_agent_list_identities(agent)) {
+        ret = -EINVAL;
+        session_error_report(s, "failed requesting identities from ssh-agent");
+        goto out;
+    }
+
+    for(;;) {
+        r = libssh2_agent_get_identity(agent, &identity, prev_identity);
+        if (r == 1) {           /* end of list */
+            break;
+        }
+        if (r < 0) {
+            ret = -EINVAL;
+            session_error_report(s, "failed to obtain identity from ssh-agent");
+            goto out;
+        }
+        r = libssh2_agent_userauth(agent, user, identity);
+        if (r == 0) {
+            /* Authenticated! */
+            ret = 0;
+            goto out;
+        }
+        /* Failed to authenticate with this identity, try the next one. */
+        prev_identity = identity;
+    }
+
+    ret = -EPERM;
+    error_report("failed to authenticate using publickey authentication "
+                 "and the identities held by your ssh-agent");
+
+ out:
+    if (agent != NULL) {
+        /* Note: libssh2 implementation implicitly calls
+         * libssh2_agent_disconnect if necessary.
+         */
+        libssh2_agent_free(agent);
+    }
+
+    return ret;
+}
+
+static int connect_to_ssh(BDRVSSHState *s, QDict *options, int bdrv_flags)
+{
+    int r, ret, flags;
+    Error *err = NULL;
+    const char *host, *user, *path;
+    int port;
+    char *hostport = NULL;
+
+    host = qdict_get_str(options, "host");
+
+    if (qdict_haskey(options, "port")) {
+        port = qdict_get_int(options, "port");
+    } else {
+        port = 22;
+    }
+
+    path = qdict_get_str(options, "path");
+
+    if (qdict_haskey(options, "user")) {
+        user = qdict_get_str(options, "user");
+    } else {
+        user = get_username();
+        if (!user) {
+            ret = -EINVAL;
+            goto err;
+        }
+    }
+
+    /* Open the socket and connect. */
+    hostport = g_strdup_printf("%s:%d", host, port);
+    s->sock = inet_connect(hostport, &err);
+    if (err != NULL) {
+        ret = -errno;
+        qerror_report_err(err);
+        error_free(err);
+        goto err;
+    }
+
+    /* Create SSH session. */
+    s->session = libssh2_session_init();
+    if (!s->session) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to initialize libssh2 session");
+        goto err;
+    }
+
+    r = libssh2_session_handshake(s->session, s->sock);
+    if (r != 0) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to establish SSH session");
+        goto err;
+    }
+
+    /* Check the remote host's key against known_hosts. */
+    ret = check_host_key(s, host, port);
+    if (ret < 0) {
+        goto err;
+    }
+
+    /* Authenticate. */
+    ret = authenticate(s, user);
+    if (ret < 0) {
+        goto err;
+    }
+
+    /* Start SFTP. */
+    s->sftp = libssh2_sftp_init(s->session);
+    if (!s->sftp) {
+        session_error_report(s, "failed to initialize sftp handle");
+        ret = -EINVAL;
+        goto err;
+    }
+
+    /* Open the remote file. */
+    flags = LIBSSH2_FXF_READ;
+    if (bdrv_flags & BDRV_O_RDWR) {
+        flags |= LIBSSH2_FXF_WRITE;
+    }
+    s->sftp_handle = libssh2_sftp_open(s->sftp, path, flags, 0);
+    if (!s->sftp_handle) {
+        session_error_report(s, "failed to open remote file '%s'", path);
+        ret = -EINVAL;
+        goto err;
+    }
+
+    r = libssh2_sftp_fstat(s->sftp_handle, &s->attrs);
+    if (r < 0) {
+        sftp_error_report(s, "failed to read file attributes of '%s'",
+                          path);
+        ret = -EINVAL;
+        goto err;
+    }
+
+    /* Delete the options we've used; any not deleted will cause the
+     * block layer to give an error about unused options.
+     */
+    qdict_del(options, "host");
+    qdict_del(options, "port");
+    qdict_del(options, "user");
+    qdict_del(options, "path");
+
+    g_free(hostport);
+    return 0;
+
+ err:
+    if (s->sftp_handle) {
+        libssh2_sftp_close(s->sftp_handle);
+    }
+    s->sftp_handle = NULL;
+    if (s->sftp) {
+        libssh2_sftp_shutdown(s->sftp);
+    }
+    s->sftp = NULL;
+    if (s->session) {
+        libssh2_session_disconnect(s->session,
+                                   "from qemu ssh client: "
+                                   "error opening connection");
+        libssh2_session_free(s->session);
+    }
+    s->session = NULL;
+    g_free(hostport);
+
+    return ret;
+}
+
+static int ssh_file_open(BlockDriverState *bs, const char *filename,
+                         QDict *options, int bdrv_flags)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    s->sock = -1;
+    qemu_co_mutex_init(&s->lock);
+
+    /* Start up SSH. */
+    ret = connect_to_ssh(s, options, bdrv_flags);
+    if (ret < 0) {
+        goto err;
+    }
+
+    /* Go non-blocking. */
+    libssh2_session_set_blocking(s->session, 0);
+
+    return 0;
+
+ err:
+    if (s->sock >= 0) {
+        close(s->sock);
+    }
+    s->sock = -1;
+
+    return ret;
+}
+
+static void ssh_close(BlockDriverState *bs)
+{
+    BDRVSSHState *s = bs->opaque;
+
+    if (s->sftp_handle) {
+        libssh2_sftp_close(s->sftp_handle);
+    }
+    if (s->sftp) {
+        libssh2_sftp_shutdown(s->sftp);
+    }
+    if (s->session) {
+        libssh2_session_disconnect(s->session,
+                                   "from qemu ssh client: "
+                                   "user closed the connection");
+        libssh2_session_free(s->session);
+    }
+    if (s->sock >= 0) {
+        close(s->sock);
+    }
+}
+
+static void restart_coroutine(void *opaque)
+{
+    Coroutine *co = opaque;
+
+    qemu_coroutine_enter(co, NULL);
+}
+
+/* Always true because when we have called set_fd_handler there is
+ * always a request being processed.
+ */
+static int return_true(void *opaque)
+{
+    return 1;
+}
+
+static coroutine_fn void set_fd_handler(BDRVSSHState *s)
+{
+    int r;
+    IOHandler *rd_handler = NULL, *wr_handler = NULL;
+    Coroutine *co = qemu_coroutine_self();
+
+    r = libssh2_session_block_directions(s->session);
+
+    if (r & LIBSSH2_SESSION_BLOCK_INBOUND) {
+        rd_handler = restart_coroutine;
+    }
+    if (r & LIBSSH2_SESSION_BLOCK_OUTBOUND) {
+        wr_handler = restart_coroutine;
+    }
+
+    DPRINTF("s->sock=%d rd_handler=%p wr_handler=%p", s->sock,
+            rd_handler, wr_handler);
+
+    qemu_aio_set_fd_handler(s->sock, rd_handler, wr_handler, return_true, co);
+}
+
+static coroutine_fn void clear_fd_handler(BDRVSSHState *s)
+{
+    qemu_aio_set_fd_handler(s->sock, NULL, NULL, NULL, NULL);
+}
+
+/* A non-blocking call returned EAGAIN, so yield, ensuring the
+ * handlers are set up so that we'll be rescheduled when there is an
+ * interesting event on the socket.
+ */
+static coroutine_fn void co_yield(BDRVSSHState *s)
+{
+    set_fd_handler(s);
+    qemu_coroutine_yield();
+    clear_fd_handler(s);
+}
+
+static coroutine_fn int ssh_read(BDRVSSHState *s,
+                                 int64_t offset, size_t size,
+                                 QEMUIOVector *qiov)
+{
+    ssize_t r;
+    size_t got;
+    char *buf, *end_of_vec;
+    struct iovec *i;
+
+    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
+
+    /* According to the docs, this just updates a field in the
+     * sftp_handle structure, so there is no network traffic and it
+     * cannot fail.
+     */
+    libssh2_sftp_seek64(s->sftp_handle, offset);
+
+    /* This keeps track of the current iovec element ('i'), where we
+     * will write to next ('buf'), and the end of the current iovec
+     * ('end_of_vec').
+     */
+    i = &qiov->iov[0];
+    buf = i->iov_base;
+    end_of_vec = i->iov_base + i->iov_len;
+
+    /* libssh2 has a hard-coded limit of 2000 bytes per request,
+     * although it will also do readahead behind our backs.  Therefore
+     * we may have to do repeated reads here until we have read 'size'
+     * bytes.
+     */
+    for (got = 0; got < size; ) {
+    again:
+        DPRINTF("sftp_read buf=%p size=%zu", buf, end_of_vec - buf);
+        r = libssh2_sftp_read(s->sftp_handle, buf, end_of_vec - buf);
+
+        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
+            co_yield(s);
+            goto again;
+        }
+        if (r < 0) {
+            sftp_error_report(s, "read failed");
+            return -EIO;
+        }
+        if (r == 0) {
+            error_report("read failed (EOF)");
+            return -EIO;
+        }
+
+        got += r;
+        buf += r;
+        if (buf >= end_of_vec) {
+            i++;
+            buf = i->iov_base;
+            end_of_vec = i->iov_base + i->iov_len;
+        }
+    }
+
+    return 0;
+}
+
+static coroutine_fn int ssh_co_readv(BlockDriverState *bs,
+                                     int64_t sector_num,
+                                     int nb_sectors, QEMUIOVector *qiov)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    qemu_co_mutex_lock(&s->lock);
+    ret = ssh_read(s, sector_num * BDRV_SECTOR_SIZE,
+                   nb_sectors * BDRV_SECTOR_SIZE, qiov);
+    qemu_co_mutex_unlock(&s->lock);
+
+    return ret;
+}
+
+static int ssh_write(BDRVSSHState *s,
+                     int64_t offset, size_t size,
+                     QEMUIOVector *qiov)
+{
+    ssize_t r;
+    size_t written;
+    char *buf, *end_of_vec;
+    struct iovec *i;
+
+    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
+
+    /* According to the docs, this just updates a field in the
+     * sftp_handle structure, so there is no network traffic and it
+     * cannot fail.
+     */
+    libssh2_sftp_seek64(s->sftp_handle, offset);
+
+    /* This keeps track of the current iovec element ('i'), where we
+     * will read from next ('buf'), and the end of the current iovec
+     * ('end_of_vec').
+     */
+    i = &qiov->iov[0];
+    buf = i->iov_base;
+    end_of_vec = i->iov_base + i->iov_len;
+
+    for (written = 0; written < size; ) {
+    again:
+        DPRINTF("sftp_write buf=%p size=%zu", buf, end_of_vec - buf);
+        r = libssh2_sftp_write(s->sftp_handle, buf, end_of_vec - buf);
+
+        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
+            co_yield(s);
+            goto again;
+        }
+        if (r < 0) {
+            sftp_error_report(s, "write failed");
+            return -EIO;
+        }
+
+        written += r;
+        buf += r;
+        if (buf >= end_of_vec) {
+            i++;
+            buf = i->iov_base;
+            end_of_vec = i->iov_base + i->iov_len;
+        }
+    }
+
+    return 0;
+}
+
+static coroutine_fn int ssh_co_writev(BlockDriverState *bs,
+                                      int64_t sector_num,
+                                      int nb_sectors, QEMUIOVector *qiov)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    qemu_co_mutex_lock(&s->lock);
+    ret = ssh_write(s, sector_num * BDRV_SECTOR_SIZE,
+                    nb_sectors * BDRV_SECTOR_SIZE, qiov);
+    qemu_co_mutex_unlock(&s->lock);
+
+    return ret;
+}
+
+static int64_t ssh_getlength(BlockDriverState *bs)
+{
+    BDRVSSHState *s = bs->opaque;
+    return (int64_t) s->attrs.filesize;
+}
+
+static BlockDriver bdrv_ssh = {
+    .format_name                  = "ssh",
+    .protocol_name                = "ssh",
+    .instance_size                = sizeof(BDRVSSHState),
+    .bdrv_parse_filename          = ssh_parse_filename,
+    .bdrv_file_open               = ssh_file_open,
+    .bdrv_close                   = ssh_close,
+    .bdrv_co_readv                = ssh_co_readv,
+    .bdrv_co_writev               = ssh_co_writev,
+    .bdrv_getlength               = ssh_getlength,
+};
+
+static void bdrv_ssh_init(void)
+{
+    int r;
+
+    r = libssh2_init(0);
+    if (r != 0) {
+        fprintf(stderr, "libssh2 initialization failed, %d\n", r);
+        exit(EXIT_FAILURE);
+    }
+
+    bdrv_register(&bdrv_ssh);
+}
+
+block_init(bdrv_ssh_init);
diff --git a/configure b/configure
index f2af714..d666066 100755
--- a/configure
+++ b/configure
@@ -229,6 +229,7 @@  virtio_blk_data_plane=""
 gtk=""
 gtkabi="2.0"
 tpm="no"
+libssh2=""
 
 # parse CC options first
 for opt do
@@ -908,6 +909,10 @@  for opt do
   ;;
   --enable-tpm) tpm="yes"
   ;;
+  --disable-libssh2) libssh2="no"
+  ;;
+  --enable-libssh2) libssh2="yes"
+  ;;
   *) echo "ERROR: unknown option $opt"; show_help="yes"
   ;;
   esac
@@ -1166,6 +1171,7 @@  echo "  --disable-glusterfs      disable GlusterFS backend"
 echo "  --enable-gcov            enable test coverage analysis with gcov"
 echo "  --gcov=GCOV              use specified gcov [$gcov_tool]"
 echo "  --enable-tpm             enable TPM support"
+echo "  --enable-libssh2         enable ssh block device support"
 echo ""
 echo "NOTE: The object files are built at the place where configure is launched"
 exit 1
@@ -2330,6 +2336,42 @@  EOF
 fi
 
 ##########################################
+# libssh2 probe
+if test "$libssh2" != "no" ; then
+  cat > $TMPC <<EOF
+#include <stdio.h>
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+int main(void) {
+    LIBSSH2_SESSION *session;
+    session = libssh2_session_init ();
+    (void) libssh2_sftp_init (session);
+    return 0;
+}
+EOF
+
+  if $pkg_config libssh2 --modversion >/dev/null 2>&1; then
+    libssh2_cflags=`$pkg_config libssh2 --cflags`
+    libssh2_libs=`$pkg_config libssh2 --libs`
+  else
+    libssh2_cflags=
+    libssh2_libs="-lssh2"
+  fi
+
+  if compile_prog "$libssh2_cflags" "$libssh2_libs" ; then
+    libssh2=yes
+    libs_tools="$libssh2_libs $libs_tools"
+    libs_softmmu="$libssh2_libs $libs_softmmu"
+    QEMU_CFLAGS="$QEMU_CFLAGS $libssh2_cflags"
+  else
+    if test "$libssh2" = "yes" ; then
+      feature_not_found "libssh2"
+    fi
+    libssh2=no
+  fi
+fi
+
+##########################################
 # linux-aio probe
 
 if test "$linux_aio" != "no" ; then
@@ -3441,6 +3483,7 @@  echo "virtio-blk-data-plane $virtio_blk_data_plane"
 echo "gcov              $gcov_tool"
 echo "gcov enabled      $gcov"
 echo "TPM support       $tpm"
+echo "libssh2 support   $libssh2"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -3806,6 +3849,10 @@  if test "$glusterfs" = "yes" ; then
   echo "CONFIG_GLUSTERFS=y" >> $config_host_mak
 fi
 
+if test "$libssh2" = "yes" ; then
+  echo "CONFIG_LIBSSH2=y" >> $config_host_mak
+fi
+
 if test "$virtio_blk_data_plane" = "yes" ; then
   echo "CONFIG_VIRTIO_BLK_DATA_PLANE=y" >> $config_host_mak
 fi
diff --git a/qemu-doc.texi b/qemu-doc.texi
index af84bef..7a0f373 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -423,6 +423,7 @@  snapshots.
 * disk_images_sheepdog::      Sheepdog disk images
 * disk_images_iscsi::         iSCSI LUNs
 * disk_images_gluster::       GlusterFS disk images
+* disk_images_ssh::           Secure Shell (ssh) disk images
 @end menu
 
 @node disk_images_quickstart
@@ -1038,6 +1039,45 @@  qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glu
 qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
 @end example
 
+@node disk_images_ssh
+@subsection Secure Shell (ssh) disk images
+
+You can access disk images located on a remote ssh server
+by using the ssh protocol:
+
+@example
+qemu-system-x86_64 -drive file=ssh://[@var{user}@@]@var{server}[:@var{port}]/@var{path}
+@end example
+
+Alternative syntax using properties:
+
+@example
+qemu-system-x86_64 -drive file.driver=ssh[,file.user=@var{user}],file.host=@var{server}[,file.port=@var{port}],file.path=@var{path}
+@end example
+
+@var{ssh} is the protocol.
+
+@var{user} is the remote user.  If not specified, then the local
+username is tried.
+
+@var{server} specifies the remote ssh server.  Any ssh server can be
+used, but it must implement the sftp-server protocol.  Most Unix/Linux
+systems should work without requiring any extra configuration.
+
+@var{port} is the port number on which sshd is listening.  By default
+the standard ssh port (22) is used.
+
+@var{path} is the path to the disk image.
+
+Currently authentication must be done using ssh-agent.  Other
+authentication methods may be supported in future.
+
+Note: The ssh driver does not obey disk flush requests (ie. to commit
+data to the backing disk when the guest requests it).  This is because
+the underlying protocol (SFTP) does not support this.  Thus there is a
+risk of guest disk corruption if the remote server or network goes
+down during writes.
+
 @node pcsys_network
 @section Network emulation
 
diff --git a/qemu-options.hx b/qemu-options.hx
index d7afeab..590a287 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -2106,6 +2106,18 @@  Example for Unix Domain Sockets
 qemu-system-i386 --drive file=nbd:unix:/tmp/nbd-socket
 @end example
 
+@item SSH
+QEMU supports SSH (Secure Shell) access to remote disks.
+
+Examples:
+@example
+qemu-system-i386 -drive file=ssh://user@@host/path/to/disk.img
+qemu-system-i386 -drive file.driver=ssh,file.user=user,file.host=host,file.port=22,file.path=/path/to/disk.img
+@end example
+
+Currently authentication must be done using ssh-agent.  Other
+authentication methods may be supported in future.
+
 @item Sheepdog
 Sheepdog is a distributed storage system for QEMU.
 QEMU supports using either local sheepdog devices or remote networked