xen: add the Xenbus sysfs and virtual device hotplug driver

This communicates with the machine control software via a registry
residing in a controlling virtual machine. This allows dynamic
creation, destruction and modification of virtual device
configurations (network devices, block devices and CPUS, to name some
examples).

[ Greg, would you mind giving this a review?  Thanks -J ]

Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Cc: Greg KH <greg@kroah.com>

1 files changed, 861 insertions, 0 deletions

diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
new file mode 100644
index 000000000000..9e943fbce81b
--- /dev/null
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -0,0 +1,861 @@
+/******************************************************************************
+ * 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;
+}
+
+/* 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_KERNEL, "%s", dir);
+        else
+                buffer = kasprintf(GFP_KERNEL, "%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_KERNEL);
+        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_KERNEL);
+        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_KERNEL);
+        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)) {
+                        kfree(msg);
+                        err = PTR_ERR(msg->u.watch.vec);
+                        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;
+}