xen: add balloon driver

The balloon driver allows memory to be dynamically added or removed from the domain,
in order to allow host memory to be balanced between multiple domains.

This patch introduces the Xen balloon driver, though it currently only
allows a domain to be shrunk from its initial size (and re-grown back to
that size).  A later patch will add the ability to grow a domain beyond
its initial size.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

4 files changed, 734 insertions, 0 deletions

diff --git a/drivers/Kconfig b/drivers/Kconfig
index 3a0e3549739f..80f0ec91e2cf 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -97,4 +97,6 @@ source "drivers/dca/Kconfig"
 source "drivers/auxdisplay/Kconfig"
 
 source "drivers/uio/Kconfig"
+
+source "drivers/xen/Kconfig"
 endmenu
diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
new file mode 100644
index 000000000000..4b75a16de009
--- /dev/null
+++ b/drivers/xen/Kconfig
@@ -0,0 +1,19 @@
+config XEN_BALLOON
+        bool "Xen memory balloon driver"
+        depends on XEN
+        default y
+        help
+          The balloon driver allows the Xen domain to request more memory from
+          the system to expand the domain's memory allocation, or alternatively
+          return unneeded memory to the system.
+
+config XEN_SCRUB_PAGES
+        bool "Scrub pages before returning them to system"
+        depends on XEN_BALLOON
+        default y
+        help
+          Scrub pages before returning them to the system for reuse by
+          other domains.  This makes sure that any confidential data
+          is not accidentally visible to other domains.  Is it more
+          secure, but slightly less efficient.
+          If in doubt, say yes.
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 43f014cfb458..37af04f1ffd9 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -1,3 +1,4 @@
 obj-y   += grant-table.o features.o events.o
 obj-y   += xenbus/
 obj-$(CONFIG_XEN_XENCOMM)       += xencomm.o
+obj-$(CONFIG_XEN_BALLOON)       += balloon.o
diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
new file mode 100644
index 000000000000..ab25ba6cbbb9
--- /dev/null
+++ b/drivers/xen/balloon.c
@@ -0,0 +1,712 @@
+/******************************************************************************
+ * balloon.c
+ *
+ * Xen balloon driver - enables returning/claiming memory to/from Xen.
+ *
+ * Copyright (c) 2003, B Dragovic
+ * Copyright (c) 2003-2004, M Williamson, K Fraser
+ * Copyright (c) 2005 Dan M. Smith, 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/kernel.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/errno.h>
+#include <linux/mm.h>
+#include <linux/bootmem.h>
+#include <linux/pagemap.h>
+#include <linux/highmem.h>
+#include <linux/mutex.h>
+#include <linux/highmem.h>
+#include <linux/list.h>
+#include <linux/sysdev.h>
+
+#include <asm/xen/hypervisor.h>
+#include <asm/page.h>
+#include <asm/pgalloc.h>
+#include <asm/pgtable.h>
+#include <asm/uaccess.h>
+#include <asm/tlb.h>
+
+#include <xen/interface/memory.h>
+#include <xen/balloon.h>
+#include <xen/xenbus.h>
+#include <xen/features.h>
+#include <xen/page.h>
+
+#define PAGES2KB(_p) ((_p)<<(PAGE_SHIFT-10))
+
+#define BALLOON_CLASS_NAME "memory"
+
+struct balloon_stats {
+        /* We aim for 'current allocation' == 'target allocation'. */
+        unsigned long current_pages;
+        unsigned long target_pages;
+        /* We may hit the hard limit in Xen. If we do then we remember it. */
+        unsigned long hard_limit;
+        /*
+         * Drivers may alter the memory reservation independently, but they
+         * must inform the balloon driver so we avoid hitting the hard limit.
+         */
+        unsigned long driver_pages;
+        /* Number of pages in high- and low-memory balloons. */
+        unsigned long balloon_low;
+        unsigned long balloon_high;
+};
+
+static DEFINE_MUTEX(balloon_mutex);
+
+static struct sys_device balloon_sysdev;
+
+static int register_balloon(struct sys_device *sysdev);
+
+/*
+ * Protects atomic reservation decrease/increase against concurrent increases.
+ * Also protects non-atomic updates of current_pages and driver_pages, and
+ * balloon lists.
+ */
+static DEFINE_SPINLOCK(balloon_lock);
+
+static struct balloon_stats balloon_stats;
+
+/* We increase/decrease in batches which fit in a page */
+static unsigned long frame_list[PAGE_SIZE / sizeof(unsigned long)];
+
+/* VM /proc information for memory */
+extern unsigned long totalram_pages;
+
+#ifdef CONFIG_HIGHMEM
+extern unsigned long totalhigh_pages;
+#define inc_totalhigh_pages() (totalhigh_pages++)
+#define dec_totalhigh_pages() (totalhigh_pages--)
+#else
+#define inc_totalhigh_pages() do {} while(0)
+#define dec_totalhigh_pages() do {} while(0)
+#endif
+
+/* List of ballooned pages, threaded through the mem_map array. */
+static LIST_HEAD(ballooned_pages);
+
+/* Main work function, always executed in process context. */
+static void balloon_process(struct work_struct *work);
+static DECLARE_WORK(balloon_worker, balloon_process);
+static struct timer_list balloon_timer;
+
+/* When ballooning out (allocating memory to return to Xen) we don't really
+   want the kernel to try too hard since that can trigger the oom killer. */
+#define GFP_BALLOON \
+        (GFP_HIGHUSER | __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC)
+
+static void scrub_page(struct page *page)
+{
+#ifdef CONFIG_XEN_SCRUB_PAGES
+        if (PageHighMem(page)) {
+                void *v = kmap(page);
+                clear_page(v);
+                kunmap(v);
+        } else {
+                void *v = page_address(page);
+                clear_page(v);
+        }
+#endif
+}
+
+/* balloon_append: add the given page to the balloon. */
+static void balloon_append(struct page *page)
+{
+        /* Lowmem is re-populated first, so highmem pages go at list tail. */
+        if (PageHighMem(page)) {
+                list_add_tail(&page->lru, &ballooned_pages);
+                balloon_stats.balloon_high++;
+                dec_totalhigh_pages();
+        } else {
+                list_add(&page->lru, &ballooned_pages);
+                balloon_stats.balloon_low++;
+        }
+}
+
+/* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
+static struct page *balloon_retrieve(void)
+{
+        struct page *page;
+
+        if (list_empty(&ballooned_pages))
+                return NULL;
+
+        page = list_entry(ballooned_pages.next, struct page, lru);
+        list_del(&page->lru);
+
+        if (PageHighMem(page)) {
+                balloon_stats.balloon_high--;
+                inc_totalhigh_pages();
+        }
+        else
+                balloon_stats.balloon_low--;
+
+        return page;
+}
+
+static struct page *balloon_first_page(void)
+{
+        if (list_empty(&ballooned_pages))
+                return NULL;
+        return list_entry(ballooned_pages.next, struct page, lru);
+}
+
+static struct page *balloon_next_page(struct page *page)
+{
+        struct list_head *next = page->lru.next;
+        if (next == &ballooned_pages)
+                return NULL;
+        return list_entry(next, struct page, lru);
+}
+
+static void balloon_alarm(unsigned long unused)
+{
+        schedule_work(&balloon_worker);
+}
+
+static unsigned long current_target(void)
+{
+        unsigned long target = min(balloon_stats.target_pages, balloon_stats.hard_limit);
+
+        target = min(target,
+                     balloon_stats.current_pages +
+                     balloon_stats.balloon_low +
+                     balloon_stats.balloon_high);
+
+        return target;
+}
+
+static int increase_reservation(unsigned long nr_pages)
+{
+        unsigned long  pfn, i, flags;
+        struct page   *page;
+        long           rc;
+        struct xen_memory_reservation reservation = {
+                .address_bits = 0,
+                .extent_order = 0,
+                .domid        = DOMID_SELF
+        };
+
+        if (nr_pages > ARRAY_SIZE(frame_list))
+                nr_pages = ARRAY_SIZE(frame_list);
+
+        spin_lock_irqsave(&balloon_lock, flags);
+
+        page = balloon_first_page();
+        for (i = 0; i < nr_pages; i++) {
+                BUG_ON(page == NULL);
+                frame_list[i] = page_to_pfn(page);;
+                page = balloon_next_page(page);
+        }
+
+        reservation.extent_start = (unsigned long)frame_list;
+        reservation.nr_extents   = nr_pages;
+        rc = HYPERVISOR_memory_op(
+                XENMEM_populate_physmap, &reservation);
+        if (rc < nr_pages) {
+                if (rc > 0) {
+                        int ret;
+
+                        /* We hit the Xen hard limit: reprobe. */
+                        reservation.nr_extents = rc;
+                        ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation,
+                                        &reservation);
+                        BUG_ON(ret != rc);
+                }
+                if (rc >= 0)
+                        balloon_stats.hard_limit = (balloon_stats.current_pages + rc -
+                                                    balloon_stats.driver_pages);
+                goto out;
+        }
+
+        for (i = 0; i < nr_pages; i++) {
+                page = balloon_retrieve();
+                BUG_ON(page == NULL);
+
+                pfn = page_to_pfn(page);
+                BUG_ON(!xen_feature(XENFEAT_auto_translated_physmap) &&
+                       phys_to_machine_mapping_valid(pfn));
+
+                set_phys_to_machine(pfn, frame_list[i]);
+
+                /* Link back into the page tables if not highmem. */
+                if (pfn < max_low_pfn) {
+                        int ret;
+                        ret = HYPERVISOR_update_va_mapping(
+                                (unsigned long)__va(pfn << PAGE_SHIFT),
+                                mfn_pte(frame_list[i], PAGE_KERNEL),
+                                0);
+                        BUG_ON(ret);
+                }
+
+                /* Relinquish the page back to the allocator. */
+                ClearPageReserved(page);
+                init_page_count(page);
+                __free_page(page);
+        }
+
+        balloon_stats.current_pages += nr_pages;
+        totalram_pages = balloon_stats.current_pages;
+
+ out:
+        spin_unlock_irqrestore(&balloon_lock, flags);
+
+        return 0;
+}
+
+static int decrease_reservation(unsigned long nr_pages)
+{
+        unsigned long  pfn, i, flags;
+        struct page   *page;
+        int            need_sleep = 0;
+        int ret;
+        struct xen_memory_reservation reservation = {
+                .address_bits = 0,
+                .extent_order = 0,
+                .domid        = DOMID_SELF
+        };
+
+        if (nr_pages > ARRAY_SIZE(frame_list))
+                nr_pages = ARRAY_SIZE(frame_list);
+
+        for (i = 0; i < nr_pages; i++) {
+                if ((page = alloc_page(GFP_BALLOON)) == NULL) {
+                        nr_pages = i;
+                        need_sleep = 1;
+                        break;
+                }
+
+                pfn = page_to_pfn(page);
+                frame_list[i] = pfn_to_mfn(pfn);
+
+                scrub_page(page);
+        }
+
+        /* Ensure that ballooned highmem pages don't have kmaps. */
+        kmap_flush_unused();
+        flush_tlb_all();
+
+        spin_lock_irqsave(&balloon_lock, flags);
+
+        /* No more mappings: invalidate P2M and add to balloon. */
+        for (i = 0; i < nr_pages; i++) {
+                pfn = mfn_to_pfn(frame_list[i]);
+                set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
+                balloon_append(pfn_to_page(pfn));
+        }
+
+        reservation.extent_start = (unsigned long)frame_list;
+        reservation.nr_extents   = nr_pages;
+        ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
+        BUG_ON(ret != nr_pages);
+
+        balloon_stats.current_pages -= nr_pages;
+        totalram_pages = balloon_stats.current_pages;
+
+        spin_unlock_irqrestore(&balloon_lock, flags);
+
+        return need_sleep;
+}
+
+/*
+ * We avoid multiple worker processes conflicting via the balloon mutex.
+ * We may of course race updates of the target counts (which are protected
+ * by the balloon lock), or with changes to the Xen hard limit, but we will
+ * recover from these in time.
+ */
+static void balloon_process(struct work_struct *work)
+{
+        int need_sleep = 0;
+        long credit;
+
+        mutex_lock(&balloon_mutex);
+
+        do {
+                credit = current_target() - balloon_stats.current_pages;
+                if (credit > 0)
+                        need_sleep = (increase_reservation(credit) != 0);
+                if (credit < 0)
+                        need_sleep = (decrease_reservation(-credit) != 0);
+
+#ifndef CONFIG_PREEMPT
+                if (need_resched())
+                        schedule();
+#endif
+        } while ((credit != 0) && !need_sleep);
+
+        /* Schedule more work if there is some still to be done. */
+        if (current_target() != balloon_stats.current_pages)
+                mod_timer(&balloon_timer, jiffies + HZ);
+
+        mutex_unlock(&balloon_mutex);
+}
+
+/* Resets the Xen limit, sets new target, and kicks off processing. */
+void balloon_set_new_target(unsigned long target)
+{
+        /* No need for lock. Not read-modify-write updates. */
+        balloon_stats.hard_limit   = ~0UL;
+        balloon_stats.target_pages = target;
+        schedule_work(&balloon_worker);
+}
+
+static struct xenbus_watch target_watch =
+{
+        .node = "memory/target"
+};
+
+/* React to a change in the target key */
+static void watch_target(struct xenbus_watch *watch,
+                         const char **vec, unsigned int len)
+{
+        unsigned long long new_target;
+        int err;
+
+        err = xenbus_scanf(XBT_NIL, "memory", "target", "%llu", &new_target);
+        if (err != 1) {
+                /* This is ok (for domain0 at least) - so just return */
+                return;
+        }
+
+        /* The given memory/target value is in KiB, so it needs converting to
+         * pages. PAGE_SHIFT converts bytes to pages, hence PAGE_SHIFT - 10.
+         */
+        balloon_set_new_target(new_target >> (PAGE_SHIFT - 10));
+}
+
+static int balloon_init_watcher(struct notifier_block *notifier,
+                                unsigned long event,
+                                void *data)
+{
+        int err;
+
+        err = register_xenbus_watch(&target_watch);
+        if (err)
+                printk(KERN_ERR "Failed to set balloon watcher\n");
+
+        return NOTIFY_DONE;
+}
+
+static struct notifier_block xenstore_notifier;
+
+static int __init balloon_init(void)
+{
+        unsigned long pfn;
+        struct page *page;
+
+        if (!is_running_on_xen())
+                return -ENODEV;
+
+        pr_info("xen_balloon: Initialising balloon driver.\n");
+
+        balloon_stats.current_pages = min(xen_start_info->nr_pages, max_pfn);
+        totalram_pages   = balloon_stats.current_pages;
+        balloon_stats.target_pages  = balloon_stats.current_pages;
+        balloon_stats.balloon_low   = 0;
+        balloon_stats.balloon_high  = 0;
+        balloon_stats.driver_pages  = 0UL;
+        balloon_stats.hard_limit    = ~0UL;
+
+        init_timer(&balloon_timer);
+        balloon_timer.data = 0;
+        balloon_timer.function = balloon_alarm;
+
+        register_balloon(&balloon_sysdev);
+
+        /* Initialise the balloon with excess memory space. */
+        for (pfn = xen_start_info->nr_pages; pfn < max_pfn; pfn++) {
+                page = pfn_to_page(pfn);
+                if (!PageReserved(page))
+                        balloon_append(page);
+        }
+
+        target_watch.callback = watch_target;
+        xenstore_notifier.notifier_call = balloon_init_watcher;
+
+        register_xenstore_notifier(&xenstore_notifier);
+
+        return 0;
+}
+
+subsys_initcall(balloon_init);
+
+static void balloon_exit(void)
+{
+    /* XXX - release balloon here */
+    return;
+}
+
+module_exit(balloon_exit);
+
+static void balloon_update_driver_allowance(long delta)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&balloon_lock, flags);
+        balloon_stats.driver_pages += delta;
+        spin_unlock_irqrestore(&balloon_lock, flags);
+}
+
+static int dealloc_pte_fn(
+        pte_t *pte, struct page *pmd_page, unsigned long addr, void *data)
+{
+        unsigned long mfn = pte_mfn(*pte);
+        int ret;
+        struct xen_memory_reservation reservation = {
+                .nr_extents   = 1,
+                .extent_order = 0,
+                .domid        = DOMID_SELF
+        };
+        reservation.extent_start = (unsigned long)&mfn;
+        set_pte_at(&init_mm, addr, pte, __pte_ma(0ull));
+        set_phys_to_machine(__pa(addr) >> PAGE_SHIFT, INVALID_P2M_ENTRY);
+        ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation, &reservation);
+        BUG_ON(ret != 1);
+        return 0;
+}
+
+static struct page **alloc_empty_pages_and_pagevec(int nr_pages)
+{
+        unsigned long vaddr, flags;
+        struct page *page, **pagevec;
+        int i, ret;
+
+        pagevec = kmalloc(sizeof(page) * nr_pages, GFP_KERNEL);
+        if (pagevec == NULL)
+                return NULL;
+
+        for (i = 0; i < nr_pages; i++) {
+                page = pagevec[i] = alloc_page(GFP_KERNEL);
+                if (page == NULL)
+                        goto err;
+
+                vaddr = (unsigned long)page_address(page);
+
+                scrub_page(page);
+
+                spin_lock_irqsave(&balloon_lock, flags);
+
+                if (xen_feature(XENFEAT_auto_translated_physmap)) {
+                        unsigned long gmfn = page_to_pfn(page);
+                        struct xen_memory_reservation reservation = {
+                                .nr_extents   = 1,
+                                .extent_order = 0,
+                                .domid        = DOMID_SELF
+                        };
+                        reservation.extent_start = (unsigned long)&gmfn;
+                        ret = HYPERVISOR_memory_op(XENMEM_decrease_reservation,
+                                                   &reservation);
+                        if (ret == 1)
+                                ret = 0; /* success */
+                } else {
+                        ret = apply_to_page_range(&init_mm, vaddr, PAGE_SIZE,
+                                                  dealloc_pte_fn, NULL);
+                }
+
+                if (ret != 0) {
+                        spin_unlock_irqrestore(&balloon_lock, flags);
+                        __free_page(page);
+                        goto err;
+                }
+
+                totalram_pages = --balloon_stats.current_pages;
+
+                spin_unlock_irqrestore(&balloon_lock, flags);
+        }
+
+ out:
+        schedule_work(&balloon_worker);
+        flush_tlb_all();
+        return pagevec;
+
+ err:
+        spin_lock_irqsave(&balloon_lock, flags);
+        while (--i >= 0)
+                balloon_append(pagevec[i]);
+        spin_unlock_irqrestore(&balloon_lock, flags);
+        kfree(pagevec);
+        pagevec = NULL;
+        goto out;
+}
+
+static void free_empty_pages_and_pagevec(struct page **pagevec, int nr_pages)
+{
+        unsigned long flags;
+        int i;
+
+        if (pagevec == NULL)
+                return;
+
+        spin_lock_irqsave(&balloon_lock, flags);
+        for (i = 0; i < nr_pages; i++) {
+                BUG_ON(page_count(pagevec[i]) != 1);
+                balloon_append(pagevec[i]);
+        }
+        spin_unlock_irqrestore(&balloon_lock, flags);
+
+        kfree(pagevec);
+
+        schedule_work(&balloon_worker);
+}
+
+static void balloon_release_driver_page(struct page *page)
+{
+        unsigned long flags;
+
+        spin_lock_irqsave(&balloon_lock, flags);
+        balloon_append(page);
+        balloon_stats.driver_pages--;
+        spin_unlock_irqrestore(&balloon_lock, flags);
+
+        schedule_work(&balloon_worker);
+}
+
+
+#define BALLOON_SHOW(name, format, args...)                     \
+        static ssize_t show_##name(struct sys_device *dev,      \
+                                   char *buf)                   \
+        {                                                       \
+                return sprintf(buf, format, ##args);            \
+        }                                                       \
+        static SYSDEV_ATTR(name, S_IRUGO, show_##name, NULL)
+
+BALLOON_SHOW(current_kb, "%lu\n", PAGES2KB(balloon_stats.current_pages));
+BALLOON_SHOW(low_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_low));
+BALLOON_SHOW(high_kb, "%lu\n", PAGES2KB(balloon_stats.balloon_high));
+BALLOON_SHOW(hard_limit_kb,
+             (balloon_stats.hard_limit!=~0UL) ? "%lu\n" : "???\n",
+             (balloon_stats.hard_limit!=~0UL) ? PAGES2KB(balloon_stats.hard_limit) : 0);
+BALLOON_SHOW(driver_kb, "%lu\n", PAGES2KB(balloon_stats.driver_pages));
+
+static ssize_t show_target_kb(struct sys_device *dev, char *buf)
+{
+        return sprintf(buf, "%lu\n", PAGES2KB(balloon_stats.target_pages));
+}
+
+static ssize_t store_target_kb(struct sys_device *dev,
+                               const char *buf,
+                               size_t count)
+{
+        char memstring[64], *endchar;
+        unsigned long long target_bytes;
+
+        if (!capable(CAP_SYS_ADMIN))
+                return -EPERM;
+
+        if (count <= 1)
+                return -EBADMSG; /* runt */
+        if (count > sizeof(memstring))
+                return -EFBIG;   /* too long */
+        strcpy(memstring, buf);
+
+        target_bytes = memparse(memstring, &endchar);
+        balloon_set_new_target(target_bytes >> PAGE_SHIFT);
+
+        return count;
+}
+
+static SYSDEV_ATTR(target_kb, S_IRUGO | S_IWUSR,
+                   show_target_kb, store_target_kb);
+
+static struct sysdev_attribute *balloon_attrs[] = {
+        &attr_target_kb,
+};
+
+static struct attribute *balloon_info_attrs[] = {
+        &attr_current_kb.attr,
+        &attr_low_kb.attr,
+        &attr_high_kb.attr,
+        &attr_hard_limit_kb.attr,
+        &attr_driver_kb.attr,
+        NULL
+};
+
+static struct attribute_group balloon_info_group = {
+        .name = "info",
+        .attrs = balloon_info_attrs,
+};
+
+static struct sysdev_class balloon_sysdev_class = {
+        .name = BALLOON_CLASS_NAME,
+};
+
+static int register_balloon(struct sys_device *sysdev)
+{
+        int i, error;
+
+        error = sysdev_class_register(&balloon_sysdev_class);
+        if (error)
+                return error;
+
+        sysdev->id = 0;
+        sysdev->cls = &balloon_sysdev_class;
+
+        error = sysdev_register(sysdev);
+        if (error) {
+                sysdev_class_unregister(&balloon_sysdev_class);
+                return error;
+        }
+
+        for (i = 0; i < ARRAY_SIZE(balloon_attrs); i++) {
+                error = sysdev_create_file(sysdev, balloon_attrs[i]);
+                if (error)
+                        goto fail;
+        }
+
+        error = sysfs_create_group(&sysdev->kobj, &balloon_info_group);
+        if (error)
+                goto fail;
+
+        return 0;
+
+ fail:
+        while (--i >= 0)
+                sysdev_remove_file(sysdev, balloon_attrs[i]);
+        sysdev_unregister(sysdev);
+        sysdev_class_unregister(&balloon_sysdev_class);
+        return error;
+}
+
+static void unregister_balloon(struct sys_device *sysdev)
+{
+        int i;
+
+        sysfs_remove_group(&sysdev->kobj, &balloon_info_group);
+        for (i = 0; i < ARRAY_SIZE(balloon_attrs); i++)
+                sysdev_remove_file(sysdev, balloon_attrs[i]);
+        sysdev_unregister(sysdev);
+        sysdev_class_unregister(&balloon_sysdev_class);
+}
+
+static void balloon_sysfs_exit(void)
+{
+        unregister_balloon(&balloon_sysdev);
+}
+
+MODULE_LICENSE("GPL");