xen: add xenfs to allow usermode <-> Xen interaction

[linux-2.6/kvm.git] / drivers / xen / xenbus / xenbus_xs.c

/******************************************************************************
 * xenbus_xs.c
 *
 * This is the kernel equivalent of the "xs" library.  We don't need everything
 * and we use xenbus_comms for communication.
 *
 * Copyright (C) 2005 Rusty Russell, IBM Corporation
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version 2
 * as published by the Free Software Foundation; or, when distributed
 * separately from the Linux kernel or incorporated into other
 * software packages, subject to the following license:
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this source file (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 <linux/unistd.h>
#include <linux/errno.h>
#include <linux/types.h>
#include <linux/uio.h>
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/err.h>
#include <linux/slab.h>
#include <linux/fcntl.h>
#include <linux/kthread.h>
#include <linux/rwsem.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <xen/xenbus.h>
#include "xenbus_comms.h"

struct xs_stored_msg {
        struct list_head list;

        struct xsd_sockmsg hdr;

        union {
                /* Queued replies. */
                struct {
                        char *body;
                } reply;

                /* Queued watch events. */
                struct {
                        struct xenbus_watch *handle;
                        char **vec;
                        unsigned int vec_size;
                } watch;
        } u;
};

struct xs_handle {
        /* A list of replies. Currently only one will ever be outstanding. */
        struct list_head reply_list;
        spinlock_t reply_lock;
        wait_queue_head_t reply_waitq;

        /*
         * Mutex ordering: transaction_mutex -> watch_mutex -> request_mutex.
         * response_mutex is never taken simultaneously with the other three.
         */

        /* One request at a time. */
        struct mutex request_mutex;

        /* Protect xenbus reader thread against save/restore. */
        struct mutex response_mutex;

        /* Protect transactions against save/restore. */
        struct rw_semaphore transaction_mutex;

        /* Protect watch (de)register against save/restore. */
        struct rw_semaphore watch_mutex;
};

static struct xs_handle xs_state;

/* List of registered watches, and a lock to protect it. */
static LIST_HEAD(watches);
static DEFINE_SPINLOCK(watches_lock);

/* List of pending watch callback events, and a lock to protect it. */
static LIST_HEAD(watch_events);
static DEFINE_SPINLOCK(watch_events_lock);

/*
 * Details of the xenwatch callback kernel thread. The thread waits on the
 * watch_events_waitq for work to do (queued on watch_events list). When it
 * wakes up it acquires the xenwatch_mutex before reading the list and
 * carrying out work.
 */
static pid_t xenwatch_pid;
static DEFINE_MUTEX(xenwatch_mutex);
static DECLARE_WAIT_QUEUE_HEAD(watch_events_waitq);

static int get_error(const char *errorstring)
{
        unsigned int i;

        for (i = 0; strcmp(errorstring, xsd_errors[i].errstring) != 0; i++) {
                if (i == ARRAY_SIZE(xsd_errors) - 1) {
                        printk(KERN_WARNING
                               "XENBUS xen store gave: unknown error %s",
                               errorstring);
                        return EINVAL;
                }
        }
        return xsd_errors[i].errnum;
}

static void *read_reply(enum xsd_sockmsg_type *type, unsigned int *len)
{
        struct xs_stored_msg *msg;
        char *body;

        spin_lock(&xs_state.reply_lock);

        while (list_empty(&xs_state.reply_list)) {
                spin_unlock(&xs_state.reply_lock);
                /* XXX FIXME: Avoid synchronous wait for response here. */
                wait_event(xs_state.reply_waitq,
                           !list_empty(&xs_state.reply_list));
                spin_lock(&xs_state.reply_lock);
        }

        msg = list_entry(xs_state.reply_list.next,
                         struct xs_stored_msg, list);
        list_del(&msg->list);

        spin_unlock(&xs_state.reply_lock);

        *type = msg->hdr.type;
        if (len)
                *len = msg->hdr.len;
        body = msg->u.reply.body;

        kfree(msg);

        return body;
}

void *xenbus_dev_request_and_reply(struct xsd_sockmsg *msg)
{
        void *ret;
        struct xsd_sockmsg req_msg = *msg;
        int err;

        if (req_msg.type == XS_TRANSACTION_START)
                down_read(&xs_state.transaction_mutex);

        mutex_lock(&xs_state.request_mutex);

        err = xb_write(msg, sizeof(*msg) + msg->len);
        if (err) {
                msg->type = XS_ERROR;
                ret = ERR_PTR(err);
        } else
                ret = read_reply(&msg->type, &msg->len);

        mutex_unlock(&xs_state.request_mutex);

        if ((msg->type == XS_TRANSACTION_END) ||
            ((req_msg.type == XS_TRANSACTION_START) &&
             (msg->type == XS_ERROR)))
                up_read(&xs_state.transaction_mutex);

        return ret;
}
EXPORT_SYMBOL(xenbus_dev_request_and_reply);

/* Send message to xs, get kmalloc'ed reply.  ERR_PTR() on error. */
static void *xs_talkv(struct xenbus_transaction t,
                      enum xsd_sockmsg_type type,
                      const struct kvec *iovec,
                      unsigned int num_vecs,
                      unsigned int *len)
{
        struct xsd_sockmsg msg;
        void *ret = NULL;
        unsigned int i;
        int err;

        msg.tx_id = t.id;
        msg.req_id = 0;
        msg.type = type;
        msg.len = 0;
        for (i = 0; i < num_vecs; i++)
                msg.len += iovec[i].iov_len;

        mutex_lock(&xs_state.request_mutex);

        err = xb_write(&msg, sizeof(msg));
        if (err) {
                mutex_unlock(&xs_state.request_mutex);
                return ERR_PTR(err);
        }

        for (i = 0; i < num_vecs; i++) {
                err = xb_write(iovec[i].iov_base, iovec[i].iov_len);
                if (err) {
                        mutex_unlock(&xs_state.request_mutex);
                        return ERR_PTR(err);
                }
        }

        ret = read_reply(&msg.type, len);

        mutex_unlock(&xs_state.request_mutex);

        if (IS_ERR(ret))
                return ret;

        if (msg.type == XS_ERROR) {
                err = get_error(ret);
                kfree(ret);
                return ERR_PTR(-err);
        }

        if (msg.type != type) {
                if (printk_ratelimit())
                        printk(KERN_WARNING
                               "XENBUS unexpected type [%d], expected [%d]\n",
                               msg.type, type);
                kfree(ret);
                return ERR_PTR(-EINVAL);
        }
        return ret;
}

/* Simplified version of xs_talkv: single message. */
static void *xs_single(struct xenbus_transaction t,
                       enum xsd_sockmsg_type type,
                       const char *string,
                       unsigned int *len)
{
        struct kvec iovec;

        iovec.iov_base = (void *)string;
        iovec.iov_len = strlen(string) + 1;
        return xs_talkv(t, type, &iovec, 1, len);
}

/* Many commands only need an ack, don't care what it says. */
static int xs_error(char *reply)
{
        if (IS_ERR(reply))
                return PTR_ERR(reply);
        kfree(reply);
        return 0;
}

static unsigned int count_strings(const char *strings, unsigned int len)
{
        unsigned int num;
        const char *p;

        for (p = strings, num = 0; p < strings + len; p += strlen(p) + 1)
                num++;

        return num;
}

/* Return the path to dir with /name appended. Buffer must be kfree()'ed. */
static char *join(const char *dir, const char *name)
{
        char *buffer;

        if (strlen(name) == 0)
                buffer = kasprintf(GFP_NOIO | __GFP_HIGH, "%s", dir);
        else
                buffer = kasprintf(GFP_NOIO | __GFP_HIGH, "%s/%s", dir, name);
        return (!buffer) ? ERR_PTR(-ENOMEM) : buffer;
}

static char **split(char *strings, unsigned int len, unsigned int *num)
{
        char *p, **ret;

        /* Count the strings. */
        *num = count_strings(strings, len);

        /* Transfer to one big alloc for easy freeing. */
        ret = kmalloc(*num * sizeof(char *) + len, GFP_NOIO | __GFP_HIGH);
        if (!ret) {
                kfree(strings);
                return ERR_PTR(-ENOMEM);
        }
        memcpy(&ret[*num], strings, len);
        kfree(strings);

        strings = (char *)&ret[*num];
        for (p = strings, *num = 0; p < strings + len; p += strlen(p) + 1)
                ret[(*num)++] = p;

        return ret;
}

char **xenbus_directory(struct xenbus_transaction t,
                        const char *dir, const char *node, unsigned int *num)
{
        char *strings, *path;
        unsigned int len;

        path = join(dir, node);
        if (IS_ERR(path))
                return (char **)path;

        strings = xs_single(t, XS_DIRECTORY, path, &len);
        kfree(path);
        if (IS_ERR(strings))
                return (char **)strings;

        return split(strings, len, num);
}
EXPORT_SYMBOL_GPL(xenbus_directory);

/* Check if a path exists. Return 1 if it does. */
int xenbus_exists(struct xenbus_transaction t,
                  const char *dir, const char *node)
{
        char **d;
        int dir_n;

        d = xenbus_directory(t, dir, node, &dir_n);
        if (IS_ERR(d))
                return 0;
        kfree(d);
        return 1;
}
EXPORT_SYMBOL_GPL(xenbus_exists);

/* Get the value of a single file.
 * Returns a kmalloced value: call free() on it after use.
 * len indicates length in bytes.
 */
void *xenbus_read(struct xenbus_transaction t,
                  const char *dir, const char *node, unsigned int *len)
{
        char *path;
        void *ret;

        path = join(dir, node);
        if (IS_ERR(path))
                return (void *)path;

        ret = xs_single(t, XS_READ, path, len);
        kfree(path);
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_read);

/* Write the value of a single file.
 * Returns -err on failure.
 */
int xenbus_write(struct xenbus_transaction t,
                 const char *dir, const char *node, const char *string)
{
        const char *path;
        struct kvec iovec[2];
        int ret;

        path = join(dir, node);
        if (IS_ERR(path))
                return PTR_ERR(path);

        iovec[0].iov_base = (void *)path;
        iovec[0].iov_len = strlen(path) + 1;
        iovec[1].iov_base = (void *)string;
        iovec[1].iov_len = strlen(string);

        ret = xs_error(xs_talkv(t, XS_WRITE, iovec, ARRAY_SIZE(iovec), NULL));
        kfree(path);
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_write);

/* Create a new directory. */
int xenbus_mkdir(struct xenbus_transaction t,
                 const char *dir, const char *node)
{
        char *path;
        int ret;

        path = join(dir, node);
        if (IS_ERR(path))
                return PTR_ERR(path);

        ret = xs_error(xs_single(t, XS_MKDIR, path, NULL));
        kfree(path);
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_mkdir);

/* Destroy a file or directory (directories must be empty). */
int xenbus_rm(struct xenbus_transaction t, const char *dir, const char *node)
{
        char *path;
        int ret;

        path = join(dir, node);
        if (IS_ERR(path))
                return PTR_ERR(path);

        ret = xs_error(xs_single(t, XS_RM, path, NULL));
        kfree(path);
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_rm);

/* Start a transaction: changes by others will not be seen during this
 * transaction, and changes will not be visible to others until end.
 */
int xenbus_transaction_start(struct xenbus_transaction *t)
{
        char *id_str;

        down_read(&xs_state.transaction_mutex);

        id_str = xs_single(XBT_NIL, XS_TRANSACTION_START, "", NULL);
        if (IS_ERR(id_str)) {
                up_read(&xs_state.transaction_mutex);
                return PTR_ERR(id_str);
        }

        t->id = simple_strtoul(id_str, NULL, 0);
        kfree(id_str);
        return 0;
}
EXPORT_SYMBOL_GPL(xenbus_transaction_start);

/* End a transaction.
 * If abandon is true, transaction is discarded instead of committed.
 */
int xenbus_transaction_end(struct xenbus_transaction t, int abort)
{
        char abortstr[2];
        int err;

        if (abort)
                strcpy(abortstr, "F");
        else
                strcpy(abortstr, "T");

        err = xs_error(xs_single(t, XS_TRANSACTION_END, abortstr, NULL));

        up_read(&xs_state.transaction_mutex);

        return err;
}
EXPORT_SYMBOL_GPL(xenbus_transaction_end);

/* Single read and scanf: returns -errno or num scanned. */
int xenbus_scanf(struct xenbus_transaction t,
                 const char *dir, const char *node, const char *fmt, ...)
{
        va_list ap;
        int ret;
        char *val;

        val = xenbus_read(t, dir, node, NULL);
        if (IS_ERR(val))
                return PTR_ERR(val);

        va_start(ap, fmt);
        ret = vsscanf(val, fmt, ap);
        va_end(ap);
        kfree(val);
        /* Distinctive errno. */
        if (ret == 0)
                return -ERANGE;
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_scanf);

/* Single printf and write: returns -errno or 0. */
int xenbus_printf(struct xenbus_transaction t,
                  const char *dir, const char *node, const char *fmt, ...)
{
        va_list ap;
        int ret;
#define PRINTF_BUFFER_SIZE 4096
        char *printf_buffer;

        printf_buffer = kmalloc(PRINTF_BUFFER_SIZE, GFP_KERNEL);
        if (printf_buffer == NULL)
                return -ENOMEM;

        va_start(ap, fmt);
        ret = vsnprintf(printf_buffer, PRINTF_BUFFER_SIZE, fmt, ap);
        va_end(ap);

        BUG_ON(ret > PRINTF_BUFFER_SIZE-1);
        ret = xenbus_write(t, dir, node, printf_buffer);

        kfree(printf_buffer);

        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_printf);

/* Takes tuples of names, scanf-style args, and void **, NULL terminated. */
int xenbus_gather(struct xenbus_transaction t, const char *dir, ...)
{
        va_list ap;
        const char *name;
        int ret = 0;

        va_start(ap, dir);
        while (ret == 0 && (name = va_arg(ap, char *)) != NULL) {
                const char *fmt = va_arg(ap, char *);
                void *result = va_arg(ap, void *);
                char *p;

                p = xenbus_read(t, dir, name, NULL);
                if (IS_ERR(p)) {
                        ret = PTR_ERR(p);
                        break;
                }
                if (fmt) {
                        if (sscanf(p, fmt, result) == 0)
                                ret = -EINVAL;
                        kfree(p);
                } else
                        *(char **)result = p;
        }
        va_end(ap);
        return ret;
}
EXPORT_SYMBOL_GPL(xenbus_gather);

static int xs_watch(const char *path, const char *token)
{
        struct kvec iov[2];

        iov[0].iov_base = (void *)path;
        iov[0].iov_len = strlen(path) + 1;
        iov[1].iov_base = (void *)token;
        iov[1].iov_len = strlen(token) + 1;

        return xs_error(xs_talkv(XBT_NIL, XS_WATCH, iov,
                                 ARRAY_SIZE(iov), NULL));
}

static int xs_unwatch(const char *path, const char *token)
{
        struct kvec iov[2];

        iov[0].iov_base = (char *)path;
        iov[0].iov_len = strlen(path) + 1;
        iov[1].iov_base = (char *)token;
        iov[1].iov_len = strlen(token) + 1;

        return xs_error(xs_talkv(XBT_NIL, XS_UNWATCH, iov,
                                 ARRAY_SIZE(iov), NULL));
}

static struct xenbus_watch *find_watch(const char *token)
{
        struct xenbus_watch *i, *cmp;

        cmp = (void *)simple_strtoul(token, NULL, 16);

        list_for_each_entry(i, &watches, list)
                if (i == cmp)
                        return i;

        return NULL;
}

/* Register callback to watch this node. */
int register_xenbus_watch(struct xenbus_watch *watch)
{
        /* Pointer in ascii is the token. */
        char token[sizeof(watch) * 2 + 1];
        int err;

        sprintf(token, "%lX", (long)watch);

        down_read(&xs_state.watch_mutex);

        spin_lock(&watches_lock);
        BUG_ON(find_watch(token));
        list_add(&watch->list, &watches);
        spin_unlock(&watches_lock);

        err = xs_watch(watch->node, token);

        /* Ignore errors due to multiple registration. */
        if ((err != 0) && (err != -EEXIST)) {
                spin_lock(&watches_lock);
                list_del(&watch->list);
                spin_unlock(&watches_lock);
        }

        up_read(&xs_state.watch_mutex);

        return err;
}
EXPORT_SYMBOL_GPL(register_xenbus_watch);

void unregister_xenbus_watch(struct xenbus_watch *watch)
{
        struct xs_stored_msg *msg, *tmp;
        char token[sizeof(watch) * 2 + 1];
        int err;

        sprintf(token, "%lX", (long)watch);

        down_read(&xs_state.watch_mutex);

        spin_lock(&watches_lock);
        BUG_ON(!find_watch(token));
        list_del(&watch->list);
        spin_unlock(&watches_lock);

        err = xs_unwatch(watch->node, token);
        if (err)
                printk(KERN_WARNING
                       "XENBUS Failed to release watch %s: %i\n",
                       watch->node, err);

        up_read(&xs_state.watch_mutex);

        /* Make sure there are no callbacks running currently (unless
           its us) */
        if (current->pid != xenwatch_pid)
                mutex_lock(&xenwatch_mutex);

        /* Cancel pending watch events. */
        spin_lock(&watch_events_lock);
        list_for_each_entry_safe(msg, tmp, &watch_events, list) {
                if (msg->u.watch.handle != watch)
                        continue;
                list_del(&msg->list);
                kfree(msg->u.watch.vec);
                kfree(msg);
        }
        spin_unlock(&watch_events_lock);

        if (current->pid != xenwatch_pid)
                mutex_unlock(&xenwatch_mutex);
}
EXPORT_SYMBOL_GPL(unregister_xenbus_watch);

void xs_suspend(void)
{
        down_write(&xs_state.transaction_mutex);
        down_write(&xs_state.watch_mutex);
        mutex_lock(&xs_state.request_mutex);
        mutex_lock(&xs_state.response_mutex);
}

void xs_resume(void)
{
        struct xenbus_watch *watch;
        char token[sizeof(watch) * 2 + 1];

        mutex_unlock(&xs_state.response_mutex);
        mutex_unlock(&xs_state.request_mutex);
        up_write(&xs_state.transaction_mutex);

        /* No need for watches_lock: the watch_mutex is sufficient. */
        list_for_each_entry(watch, &watches, list) {
                sprintf(token, "%lX", (long)watch);
                xs_watch(watch->node, token);
        }

        up_write(&xs_state.watch_mutex);
}

void xs_suspend_cancel(void)
{
        mutex_unlock(&xs_state.response_mutex);
        mutex_unlock(&xs_state.request_mutex);
        up_write(&xs_state.watch_mutex);
        up_write(&xs_state.transaction_mutex);
}

static int xenwatch_thread(void *unused)
{
        struct list_head *ent;
        struct xs_stored_msg *msg;

        for (;;) {
                wait_event_interruptible(watch_events_waitq,
                                         !list_empty(&watch_events));

                if (kthread_should_stop())
                        break;

                mutex_lock(&xenwatch_mutex);

                spin_lock(&watch_events_lock);
                ent = watch_events.next;
                if (ent != &watch_events)
                        list_del(ent);
                spin_unlock(&watch_events_lock);

                if (ent != &watch_events) {
                        msg = list_entry(ent, struct xs_stored_msg, list);
                        msg->u.watch.handle->callback(
                                msg->u.watch.handle,
                                (const char **)msg->u.watch.vec,
                                msg->u.watch.vec_size);
                        kfree(msg->u.watch.vec);
                        kfree(msg);
                }

                mutex_unlock(&xenwatch_mutex);
        }

        return 0;
}

static int process_msg(void)
{
        struct xs_stored_msg *msg;
        char *body;
        int err;

        /*
         * We must disallow save/restore while reading a xenstore message.
         * A partial read across s/r leaves us out of sync with xenstored.
         */
        for (;;) {
                err = xb_wait_for_data_to_read();
                if (err)
                        return err;
                mutex_lock(&xs_state.response_mutex);
                if (xb_data_to_read())
                        break;
                /* We raced with save/restore: pending data 'disappeared'. */
                mutex_unlock(&xs_state.response_mutex);
        }


        msg = kmalloc(sizeof(*msg), GFP_NOIO | __GFP_HIGH);
        if (msg == NULL) {
                err = -ENOMEM;
                goto out;
        }

        err = xb_read(&msg->hdr, sizeof(msg->hdr));
        if (err) {
                kfree(msg);
                goto out;
        }

        body = kmalloc(msg->hdr.len + 1, GFP_NOIO | __GFP_HIGH);
        if (body == NULL) {
                kfree(msg);
                err = -ENOMEM;
                goto out;
        }

        err = xb_read(body, msg->hdr.len);
        if (err) {
                kfree(body);
                kfree(msg);
                goto out;
        }
        body[msg->hdr.len] = '\0';

        if (msg->hdr.type == XS_WATCH_EVENT) {
                msg->u.watch.vec = split(body, msg->hdr.len,
                                         &msg->u.watch.vec_size);
                if (IS_ERR(msg->u.watch.vec)) {
                        err = PTR_ERR(msg->u.watch.vec);
                        kfree(msg);
                        goto out;
                }

                spin_lock(&watches_lock);
                msg->u.watch.handle = find_watch(
                        msg->u.watch.vec[XS_WATCH_TOKEN]);
                if (msg->u.watch.handle != NULL) {
                        spin_lock(&watch_events_lock);
                        list_add_tail(&msg->list, &watch_events);
                        wake_up(&watch_events_waitq);
                        spin_unlock(&watch_events_lock);
                } else {
                        kfree(msg->u.watch.vec);
                        kfree(msg);
                }
                spin_unlock(&watches_lock);
        } else {
                msg->u.reply.body = body;
                spin_lock(&xs_state.reply_lock);
                list_add_tail(&msg->list, &xs_state.reply_list);
                spin_unlock(&xs_state.reply_lock);
                wake_up(&xs_state.reply_waitq);
        }

 out:
        mutex_unlock(&xs_state.response_mutex);
        return err;
}

static int xenbus_thread(void *unused)
{
        int err;

        for (;;) {
                err = process_msg();
                if (err)
                        printk(KERN_WARNING "XENBUS error %d while reading "
                               "message\n", err);
                if (kthread_should_stop())
                        break;
        }

        return 0;
}

int xs_init(void)
{
        int err;
        struct task_struct *task;

        INIT_LIST_HEAD(&xs_state.reply_list);
        spin_lock_init(&xs_state.reply_lock);
        init_waitqueue_head(&xs_state.reply_waitq);

        mutex_init(&xs_state.request_mutex);
        mutex_init(&xs_state.response_mutex);
        init_rwsem(&xs_state.transaction_mutex);
        init_rwsem(&xs_state.watch_mutex);

        /* Initialize the shared memory rings to talk to xenstored */
        err = xb_init_comms();
        if (err)
                return err;

        task = kthread_run(xenwatch_thread, NULL, "xenwatch");
        if (IS_ERR(task))
                return PTR_ERR(task);
        xenwatch_pid = task->pid;

        task = kthread_run(xenbus_thread, NULL, "xenbus");
        if (IS_ERR(task))
                return PTR_ERR(task);

        return 0;
}