x86/speculation/mds: Add mitigation control for MDS

[mirror_ubuntu-bionic-kernel.git] / drivers / xen / 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
 * Copyright (c) 2010 Daniel Kiper
 *
 * Memory hotplug support was written by Daniel Kiper. Work on
 * it was sponsored by Google under Google Summer of Code 2010
 * program. Jeremy Fitzhardinge from Citrix was the mentor for
 * this project.
 *
 * 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.
 */

#define pr_fmt(fmt) "xen:" KBUILD_MODNAME ": " fmt

#include <linux/cpu.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/cred.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/list.h>
#include <linux/gfp.h>
#include <linux/notifier.h>
#include <linux/memory.h>
#include <linux/memory_hotplug.h>
#include <linux/percpu-defs.h>
#include <linux/slab.h>
#include <linux/sysctl.h>

#include <asm/page.h>
#include <asm/pgalloc.h>
#include <asm/pgtable.h>
#include <asm/tlb.h>

#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>

#include <xen/xen.h>
#include <xen/interface/xen.h>
#include <xen/interface/memory.h>
#include <xen/balloon.h>
#include <xen/features.h>
#include <xen/page.h>

static int xen_hotplug_unpopulated;

#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG

static int zero;
static int one = 1;

static struct ctl_table balloon_table[] = {
        {
                .procname       = "hotplug_unpopulated",
                .data           = &xen_hotplug_unpopulated,
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .proc_handler   = proc_dointvec_minmax,
                .extra1         = &zero,
                .extra2         = &one,
        },
        { }
};

static struct ctl_table balloon_root[] = {
        {
                .procname       = "balloon",
                .mode           = 0555,
                .child          = balloon_table,
        },
        { }
};

static struct ctl_table xen_root[] = {
        {
                .procname       = "xen",
                .mode           = 0555,
                .child          = balloon_root,
        },
        { }
};

#endif

/*
 * Use one extent per PAGE_SIZE to avoid to break down the page into
 * multiple frame.
 */
#define EXTENT_ORDER (fls(XEN_PFN_PER_PAGE) - 1)

/*
 * balloon_process() state:
 *
 * BP_DONE: done or nothing to do,
 * BP_WAIT: wait to be rescheduled,
 * BP_EAGAIN: error, go to sleep,
 * BP_ECANCELED: error, balloon operation canceled.
 */

enum bp_state {
        BP_DONE,
        BP_WAIT,
        BP_EAGAIN,
        BP_ECANCELED
};


static DEFINE_MUTEX(balloon_mutex);

struct balloon_stats balloon_stats;
EXPORT_SYMBOL_GPL(balloon_stats);

/* We increase/decrease in batches which fit in a page */
static xen_pfn_t frame_list[PAGE_SIZE / sizeof(xen_pfn_t)];


/* List of ballooned pages, threaded through the mem_map array. */
static LIST_HEAD(ballooned_pages);
static DECLARE_WAIT_QUEUE_HEAD(balloon_wq);

/* Main work function, always executed in process context. */
static void balloon_process(struct work_struct *work);
static DECLARE_DELAYED_WORK(balloon_worker, balloon_process);

/* 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
        clear_highpage(page);
#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++;
        } else {
                list_add(&page->lru, &ballooned_pages);
                balloon_stats.balloon_low++;
        }
        wake_up(&balloon_wq);
}

static void balloon_append(struct page *page)
{
        __balloon_append(page);
}

/* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
static struct page *balloon_retrieve(bool require_lowmem)
{
        struct page *page;

        if (list_empty(&ballooned_pages))
                return NULL;

        page = list_entry(ballooned_pages.next, struct page, lru);
        if (require_lowmem && PageHighMem(page))
                return NULL;
        list_del(&page->lru);

        if (PageHighMem(page))
                balloon_stats.balloon_high--;
        else
                balloon_stats.balloon_low--;

        return page;
}

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 enum bp_state update_schedule(enum bp_state state)
{
        if (state == BP_WAIT)
                return BP_WAIT;

        if (state == BP_ECANCELED)
                return BP_ECANCELED;

        if (state == BP_DONE) {
                balloon_stats.schedule_delay = 1;
                balloon_stats.retry_count = 1;
                return BP_DONE;
        }

        ++balloon_stats.retry_count;

        if (balloon_stats.max_retry_count != RETRY_UNLIMITED &&
                        balloon_stats.retry_count > balloon_stats.max_retry_count) {
                balloon_stats.schedule_delay = 1;
                balloon_stats.retry_count = 1;
                return BP_ECANCELED;
        }

        balloon_stats.schedule_delay <<= 1;

        if (balloon_stats.schedule_delay > balloon_stats.max_schedule_delay)
                balloon_stats.schedule_delay = balloon_stats.max_schedule_delay;

        return BP_EAGAIN;
}

#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
static void release_memory_resource(struct resource *resource)
{
        if (!resource)
                return;

        /*
         * No need to reset region to identity mapped since we now
         * know that no I/O can be in this region
         */
        release_resource(resource);
        kfree(resource);
}

/*
 * Host memory not allocated to dom0. We can use this range for hotplug-based
 * ballooning.
 *
 * It's a type-less resource. Setting IORESOURCE_MEM will make resource
 * management algorithms (arch_remove_reservations()) look into guest e820,
 * which we don't want.
 */
static struct resource hostmem_resource = {
        .name   = "Host RAM",
};

void __attribute__((weak)) __init arch_xen_balloon_init(struct resource *res)
{}

static struct resource *additional_memory_resource(phys_addr_t size)
{
        struct resource *res, *res_hostmem;
        int ret = -ENOMEM;

        res = kzalloc(sizeof(*res), GFP_KERNEL);
        if (!res)
                return NULL;

        res->name = "System RAM";
        res->flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_BUSY;

        res_hostmem = kzalloc(sizeof(*res), GFP_KERNEL);
        if (res_hostmem) {
                /* Try to grab a range from hostmem */
                res_hostmem->name = "Host memory";
                ret = allocate_resource(&hostmem_resource, res_hostmem,
                                        size, 0, -1,
                                        PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
        }

        if (!ret) {
                /*
                 * Insert this resource into iomem. Because hostmem_resource
                 * tracks portion of guest e820 marked as UNUSABLE noone else
                 * should try to use it.
                 */
                res->start = res_hostmem->start;
                res->end = res_hostmem->end;
                ret = insert_resource(&iomem_resource, res);
                if (ret < 0) {
                        pr_err("Can't insert iomem_resource [%llx - %llx]\n",
                                res->start, res->end);
                        release_memory_resource(res_hostmem);
                        res_hostmem = NULL;
                        res->start = res->end = 0;
                }
        }

        if (ret) {
                ret = allocate_resource(&iomem_resource, res,
                                        size, 0, -1,
                                        PAGES_PER_SECTION * PAGE_SIZE, NULL, NULL);
                if (ret < 0) {
                        pr_err("Cannot allocate new System RAM resource\n");
                        kfree(res);
                        return NULL;
                }
        }

#ifdef CONFIG_SPARSEMEM
        {
                unsigned long limit = 1UL << (MAX_PHYSMEM_BITS - PAGE_SHIFT);
                unsigned long pfn = res->start >> PAGE_SHIFT;

                if (pfn > limit) {
                        pr_err("New System RAM resource outside addressable RAM (%lu > %lu)\n",
                               pfn, limit);
                        release_memory_resource(res);
                        release_memory_resource(res_hostmem);
                        return NULL;
                }
        }
#endif

        return res;
}

static enum bp_state reserve_additional_memory(void)
{
        long credit;
        struct resource *resource;
        int nid, rc;
        unsigned long balloon_hotplug;

        credit = balloon_stats.target_pages + balloon_stats.target_unpopulated
                - balloon_stats.total_pages;

        /*
         * Already hotplugged enough pages?  Wait for them to be
         * onlined.
         */
        if (credit <= 0)
                return BP_WAIT;

        balloon_hotplug = round_up(credit, PAGES_PER_SECTION);

        resource = additional_memory_resource(balloon_hotplug * PAGE_SIZE);
        if (!resource)
                goto err;

        nid = memory_add_physaddr_to_nid(resource->start);

#ifdef CONFIG_XEN_HAVE_PVMMU
        /*
         * We don't support PV MMU when Linux and Xen is using
         * different page granularity.
         */
        BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);

        /*
         * add_memory() will build page tables for the new memory so
         * the p2m must contain invalid entries so the correct
         * non-present PTEs will be written.
         *
         * If a failure occurs, the original (identity) p2m entries
         * are not restored since this region is now known not to
         * conflict with any devices.
         */ 
        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                unsigned long pfn, i;

                pfn = PFN_DOWN(resource->start);
                for (i = 0; i < balloon_hotplug; i++) {
                        if (!set_phys_to_machine(pfn + i, INVALID_P2M_ENTRY)) {
                                pr_warn("set_phys_to_machine() failed, no memory added\n");
                                goto err;
                        }
                }
        }
#endif

        /*
         * add_memory_resource() will call online_pages() which in its turn
         * will call xen_online_page() callback causing deadlock if we don't
         * release balloon_mutex here. Unlocking here is safe because the
         * callers drop the mutex before trying again.
         */
        mutex_unlock(&balloon_mutex);
        rc = add_memory_resource(nid, resource, memhp_auto_online);
        mutex_lock(&balloon_mutex);

        if (rc) {
                pr_warn("Cannot add additional memory (%i)\n", rc);
                goto err;
        }

        balloon_stats.total_pages += balloon_hotplug;

        return BP_WAIT;
  err:
        release_memory_resource(resource);
        return BP_ECANCELED;
}

static void xen_online_page(struct page *page)
{
        __online_page_set_limits(page);

        mutex_lock(&balloon_mutex);

        __balloon_append(page);

        mutex_unlock(&balloon_mutex);
}

static int xen_memory_notifier(struct notifier_block *nb, unsigned long val, void *v)
{
        if (val == MEM_ONLINE)
                schedule_delayed_work(&balloon_worker, 0);

        return NOTIFY_OK;
}

static struct notifier_block xen_memory_nb = {
        .notifier_call = xen_memory_notifier,
        .priority = 0
};
#else
static enum bp_state reserve_additional_memory(void)
{
        balloon_stats.target_pages = balloon_stats.current_pages;
        return BP_ECANCELED;
}
#endif /* CONFIG_XEN_BALLOON_MEMORY_HOTPLUG */

static long current_credit(void)
{
        return balloon_stats.target_pages - balloon_stats.current_pages;
}

static bool balloon_is_inflated(void)
{
        return balloon_stats.balloon_low || balloon_stats.balloon_high;
}

static enum bp_state increase_reservation(unsigned long nr_pages)
{
        int rc;
        unsigned long i;
        struct page   *page;
        struct xen_memory_reservation reservation = {
                .address_bits = 0,
                .extent_order = EXTENT_ORDER,
                .domid        = DOMID_SELF
        };

        if (nr_pages > ARRAY_SIZE(frame_list))
                nr_pages = ARRAY_SIZE(frame_list);

        page = list_first_entry_or_null(&ballooned_pages, struct page, lru);
        for (i = 0; i < nr_pages; i++) {
                if (!page) {
                        nr_pages = i;
                        break;
                }

                /* XENMEM_populate_physmap requires a PFN based on Xen
                 * granularity.
                 */
                frame_list[i] = page_to_xen_pfn(page);
                page = balloon_next_page(page);
        }

        set_xen_guest_handle(reservation.extent_start, frame_list);
        reservation.nr_extents = nr_pages;
        rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
        if (rc <= 0)
                return BP_EAGAIN;

        for (i = 0; i < rc; i++) {
                page = balloon_retrieve(false);
                BUG_ON(page == NULL);

#ifdef CONFIG_XEN_HAVE_PVMMU
                /*
                 * We don't support PV MMU when Linux and Xen is using
                 * different page granularity.
                 */
                BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);

                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                        unsigned long pfn = page_to_pfn(page);

                        set_phys_to_machine(pfn, frame_list[i]);

                        /* Link back into the page tables if not highmem. */
                        if (!PageHighMem(page)) {
                                int ret;
                                ret = HYPERVISOR_update_va_mapping(
                                                (unsigned long)__va(pfn << PAGE_SHIFT),
                                                mfn_pte(frame_list[i], PAGE_KERNEL),
                                                0);
                                BUG_ON(ret);
                        }
                }
#endif

                /* Relinquish the page back to the allocator. */
                free_reserved_page(page);
        }

        balloon_stats.current_pages += rc;

        return BP_DONE;
}

static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
{
        enum bp_state state = BP_DONE;
        unsigned long i;
        struct page *page, *tmp;
        int ret;
        struct xen_memory_reservation reservation = {
                .address_bits = 0,
                .extent_order = EXTENT_ORDER,
                .domid        = DOMID_SELF
        };
        LIST_HEAD(pages);

        if (nr_pages > ARRAY_SIZE(frame_list))
                nr_pages = ARRAY_SIZE(frame_list);

        for (i = 0; i < nr_pages; i++) {
                page = alloc_page(gfp);
                if (page == NULL) {
                        nr_pages = i;
                        state = BP_EAGAIN;
                        break;
                }
                adjust_managed_page_count(page, -1);
                scrub_page(page);
                list_add(&page->lru, &pages);
        }

        /*
         * Ensure that ballooned highmem pages don't have kmaps.
         *
         * Do this before changing the p2m as kmap_flush_unused()
         * reads PTEs to obtain pages (and hence needs the original
         * p2m entry).
         */
        kmap_flush_unused();

        /*
         * Setup the frame, update direct mapping, invalidate P2M,
         * and add to balloon.
         */
        i = 0;
        list_for_each_entry_safe(page, tmp, &pages, lru) {
                /* XENMEM_decrease_reservation requires a GFN */
                frame_list[i++] = xen_page_to_gfn(page);

#ifdef CONFIG_XEN_HAVE_PVMMU
                /*
                 * We don't support PV MMU when Linux and Xen is using
                 * different page granularity.
                 */
                BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);

                if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                        unsigned long pfn = page_to_pfn(page);

                        if (!PageHighMem(page)) {
                                ret = HYPERVISOR_update_va_mapping(
                                                (unsigned long)__va(pfn << PAGE_SHIFT),
                                                __pte_ma(0), 0);
                                BUG_ON(ret);
                        }
                        __set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
                }
#endif
                list_del(&page->lru);

                balloon_append(page);
        }

        flush_tlb_all();

        set_xen_guest_handle(reservation.extent_start, 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;

        return state;
}

/*
 * As this is a work item it is guaranteed to run as a single instance only.
 * 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)
{
        enum bp_state state = BP_DONE;
        long credit;


        do {
                mutex_lock(&balloon_mutex);

                credit = current_credit();

                if (credit > 0) {
                        if (balloon_is_inflated())
                                state = increase_reservation(credit);
                        else
                                state = reserve_additional_memory();
                }

                if (credit < 0)
                        state = decrease_reservation(-credit, GFP_BALLOON);

                state = update_schedule(state);

                mutex_unlock(&balloon_mutex);

                cond_resched();

        } while (credit && state == BP_DONE);

        /* Schedule more work if there is some still to be done. */
        if (state == BP_EAGAIN)
                schedule_delayed_work(&balloon_worker, balloon_stats.schedule_delay * HZ);
}

/* 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.target_pages = target;
        schedule_delayed_work(&balloon_worker, 0);
}
EXPORT_SYMBOL_GPL(balloon_set_new_target);

static int add_ballooned_pages(int nr_pages)
{
        enum bp_state st;

        if (xen_hotplug_unpopulated) {
                st = reserve_additional_memory();
                if (st != BP_ECANCELED) {
                        mutex_unlock(&balloon_mutex);
                        wait_event(balloon_wq,
                                   !list_empty(&ballooned_pages));
                        mutex_lock(&balloon_mutex);
                        return 0;
                }
        }

        st = decrease_reservation(nr_pages, GFP_USER);
        if (st != BP_DONE)
                return -ENOMEM;

        return 0;
}

/**
 * alloc_xenballooned_pages - get pages that have been ballooned out
 * @nr_pages: Number of pages to get
 * @pages: pages returned
 * @return 0 on success, error otherwise
 */
int alloc_xenballooned_pages(int nr_pages, struct page **pages)
{
        int pgno = 0;
        struct page *page;
        int ret;

        mutex_lock(&balloon_mutex);

        balloon_stats.target_unpopulated += nr_pages;

        while (pgno < nr_pages) {
                page = balloon_retrieve(true);
                if (page) {
                        pages[pgno++] = page;
#ifdef CONFIG_XEN_HAVE_PVMMU
                        /*
                         * We don't support PV MMU when Linux and Xen is using
                         * different page granularity.
                         */
                        BUILD_BUG_ON(XEN_PAGE_SIZE != PAGE_SIZE);

                        if (!xen_feature(XENFEAT_auto_translated_physmap)) {
                                ret = xen_alloc_p2m_entry(page_to_pfn(page));
                                if (ret < 0)
                                        goto out_undo;
                        }
#endif
                } else {
                        ret = add_ballooned_pages(nr_pages - pgno);
                        if (ret < 0)
                                goto out_undo;
                }
        }
        mutex_unlock(&balloon_mutex);
        return 0;
 out_undo:
        mutex_unlock(&balloon_mutex);
        free_xenballooned_pages(pgno, pages);
        return ret;
}
EXPORT_SYMBOL(alloc_xenballooned_pages);

/**
 * free_xenballooned_pages - return pages retrieved with get_ballooned_pages
 * @nr_pages: Number of pages
 * @pages: pages to return
 */
void free_xenballooned_pages(int nr_pages, struct page **pages)
{
        int i;

        mutex_lock(&balloon_mutex);

        for (i = 0; i < nr_pages; i++) {
                if (pages[i])
                        balloon_append(pages[i]);
        }

        balloon_stats.target_unpopulated -= nr_pages;

        /* The balloon may be too large now. Shrink it if needed. */
        if (current_credit())
                schedule_delayed_work(&balloon_worker, 0);

        mutex_unlock(&balloon_mutex);
}
EXPORT_SYMBOL(free_xenballooned_pages);

#ifdef CONFIG_XEN_PV
static void __init balloon_add_region(unsigned long start_pfn,
                                      unsigned long pages)
{
        unsigned long pfn, extra_pfn_end;
        struct page *page;

        /*
         * If the amount of usable memory has been limited (e.g., with
         * the 'mem' command line parameter), don't add pages beyond
         * this limit.
         */
        extra_pfn_end = min(max_pfn, start_pfn + pages);

        for (pfn = start_pfn; pfn < extra_pfn_end; pfn++) {
                page = pfn_to_page(pfn);
                /* totalram_pages and totalhigh_pages do not
                   include the boot-time balloon extension, so
                   don't subtract from it. */
                __balloon_append(page);
        }

        balloon_stats.total_pages += extra_pfn_end - start_pfn;
}
#endif

static int __init balloon_init(void)
{
        if (!xen_domain())
                return -ENODEV;

        pr_info("Initialising balloon driver\n");

#ifdef CONFIG_XEN_PV
        balloon_stats.current_pages = xen_pv_domain()
                ? min(xen_start_info->nr_pages - xen_released_pages, max_pfn)
                : get_num_physpages();
#else
        balloon_stats.current_pages = get_num_physpages();
#endif
        balloon_stats.target_pages  = balloon_stats.current_pages;
        balloon_stats.balloon_low   = 0;
        balloon_stats.balloon_high  = 0;
        balloon_stats.total_pages   = balloon_stats.current_pages;

        balloon_stats.schedule_delay = 1;
        balloon_stats.max_schedule_delay = 32;
        balloon_stats.retry_count = 1;
        balloon_stats.max_retry_count = RETRY_UNLIMITED;

#ifdef CONFIG_XEN_BALLOON_MEMORY_HOTPLUG
        set_online_page_callback(&xen_online_page);
        register_memory_notifier(&xen_memory_nb);
        register_sysctl_table(xen_root);

        arch_xen_balloon_init(&hostmem_resource);
#endif

#ifdef CONFIG_XEN_PV
        {
                int i;

                /*
                 * Initialize the balloon with pages from the extra memory
                 * regions (see arch/x86/xen/setup.c).
                 */
                for (i = 0; i < XEN_EXTRA_MEM_MAX_REGIONS; i++)
                        if (xen_extra_mem[i].n_pfns)
                                balloon_add_region(xen_extra_mem[i].start_pfn,
                                                   xen_extra_mem[i].n_pfns);
        }
#endif

        /* Init the xen-balloon driver. */
        xen_balloon_init();

        return 0;
}
subsys_initcall(balloon_init);