drivers/xen/gntdev.c

/******************************************************************************
 * gntdev.c
 *
 * Device for accessing (in user-space) pages that have been granted by other
 * domains.
 *
 * Copyright (c) 2006-2007, D G Murray.
 *           (c) 2009 Gerd Hoffmann <kraxel@redhat.com>
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

#undef DEBUG

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

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/miscdevice.h>
#include <linux/fs.h>
#include <linux/mm.h>
#include <linux/mman.h>
#include <linux/mmu_notifier.h>
#include <linux/types.h>
#include <linux/uaccess.h>
#include <linux/sched.h>
#include <linux/spinlock.h>
#include <linux/slab.h>
#include <linux/highmem.h>

#include <xen/xen.h>
#include <xen/grant_table.h>
#include <xen/balloon.h>
#include <xen/gntdev.h>
#include <xen/events.h>
#include <xen/page.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Derek G. Murray <Derek.Murray@cl.cam.ac.uk>, "
	      "Gerd Hoffmann <kraxel@redhat.com>");
MODULE_DESCRIPTION("User-space granted page access driver");

static int limit = 1024*1024;
module_param(limit, int, 0644);
MODULE_PARM_DESC(limit, "Maximum number of grants that may be mapped by "
		"the gntdev device");

static atomic_t pages_mapped = ATOMIC_INIT(0);

static int use_ptemod;
#define populate_freeable_maps use_ptemod

struct gntdev_priv {
	/* maps with visible offsets in the file descriptor */
	struct list_head maps;
	/* maps that are not visible; will be freed on munmap.
	 * Only populated if populate_freeable_maps == 1 */
	struct list_head freeable_maps;
	/* lock protects maps and freeable_maps */
	struct mutex lock;
	struct mm_struct *mm;
	struct mmu_notifier mn;
};

struct unmap_notify {
	int flags;
	/* Address relative to the start of the grant_map */
	int addr;
	int event;
};

struct grant_map {
	struct list_head next;
	struct vm_area_struct *vma;
	int index;
	int count;
	int flags;
	atomic_t users;
	struct unmap_notify notify;
	struct ioctl_gntdev_grant_ref *grants;
	struct gnttab_map_grant_ref   *map_ops;
	struct gnttab_unmap_grant_ref *unmap_ops;
	struct gnttab_map_grant_ref   *kmap_ops;
	struct gnttab_unmap_grant_ref *kunmap_ops;
	struct page **pages;
	unsigned long pages_vm_start;
};

static int unmap_grant_pages(struct grant_map *map, int offset, int pages);

/* ------------------------------------------------------------------ */

static void gntdev_print_maps(struct gntdev_priv *priv,
			      char *text, int text_index)
{
#ifdef DEBUG
	struct grant_map *map;

	pr_debug("%s: maps list (priv %p)\n", __func__, priv);
	list_for_each_entry(map, &priv->maps, next)
		pr_debug("  index %2d, count %2d %s\n",
		       map->index, map->count,
		       map->index == text_index && text ? text : "");
#endif
}

static void gntdev_free_map(struct grant_map *map)
{
	if (map == NULL)
		return;

	if (map->pages)
		gnttab_free_pages(map->count, map->pages);
	kfree(map->pages);
	kfree(map->grants);
	kfree(map->map_ops);
	kfree(map->unmap_ops);
	kfree(map->kmap_ops);
	kfree(map->kunmap_ops);
	kfree(map);
}

static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
{
	struct grant_map *add;
	int i;

	add = kzalloc(sizeof(struct grant_map), GFP_KERNEL);
	if (NULL == add)
		return NULL;

	add->grants    = kcalloc(count, sizeof(add->grants[0]), GFP_KERNEL);
	add->map_ops   = kcalloc(count, sizeof(add->map_ops[0]), GFP_KERNEL);
	add->unmap_ops = kcalloc(count, sizeof(add->unmap_ops[0]), GFP_KERNEL);
	add->kmap_ops  = kcalloc(count, sizeof(add->kmap_ops[0]), GFP_KERNEL);
	add->kunmap_ops = kcalloc(count, sizeof(add->kunmap_ops[0]), GFP_KERNEL);
	add->pages     = kcalloc(count, sizeof(add->pages[0]), GFP_KERNEL);
	if (NULL == add->grants    ||
	    NULL == add->map_ops   ||
	    NULL == add->unmap_ops ||
	    NULL == add->kmap_ops  ||
	    NULL == add->kunmap_ops ||
	    NULL == add->pages)
		goto err;

	if (gnttab_alloc_pages(count, add->pages))
		goto err;

	for (i = 0; i < count; i++) {
		add->map_ops[i].handle = -1;
		add->unmap_ops[i].handle = -1;
		add->kmap_ops[i].handle = -1;
		add->kunmap_ops[i].handle = -1;
	}

	add->index = 0;
	add->count = count;
	atomic_set(&add->users, 1);

	return add;

err:
	gntdev_free_map(add);
	return NULL;
}

static void gntdev_add_map(struct gntdev_priv *priv, struct grant_map *add)
{
	struct grant_map *map;

	list_for_each_entry(map, &priv->maps, next) {
		if (add->index + add->count < map->index) {
			list_add_tail(&add->next, &map->next);
			goto done;
		}
		add->index = map->index + map->count;
	}
	list_add_tail(&add->next, &priv->maps);

done:
	gntdev_print_maps(priv, "[new]", add->index);
}

static struct grant_map *gntdev_find_map_index(struct gntdev_priv *priv,
		int index, int count)
{
	struct grant_map *map;

	list_for_each_entry(map, &priv->maps, next) {
		if (map->index != index)
			continue;
		if (count && map->count != count)
			continue;
		return map;
	}
	return NULL;
}

static void gntdev_put_map(struct gntdev_priv *priv, struct grant_map *map)
{
	if (!map)
		return;

	if (!atomic_dec_and_test(&map->users))
		return;

	atomic_sub(map->count, &pages_mapped);

	if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
		notify_remote_via_evtchn(map->notify.event);
		evtchn_put(map->notify.event);
	}

	if (populate_freeable_maps && priv) {
		mutex_lock(&priv->lock);
		list_del(&map->next);
		mutex_unlock(&priv->lock);
	}

	if (map->pages && !use_ptemod)
		unmap_grant_pages(map, 0, map->count);
	gntdev_free_map(map);
}

/* ------------------------------------------------------------------ */

static int find_grant_ptes(pte_t *pte, pgtable_t token,
		unsigned long addr, void *data)
{
	struct grant_map *map = data;
	unsigned int pgnr = (addr - map->vma->vm_start) >> PAGE_SHIFT;
	int flags = map->flags | GNTMAP_application_map | GNTMAP_contains_pte;
	u64 pte_maddr;

	BUG_ON(pgnr >= map->count);
	pte_maddr = arbitrary_virt_to_machine(pte).maddr;

	/*
	 * Set the PTE as special to force get_user_pages_fast() fall
	 * back to the slow path.  If this is not supported as part of
	 * the grant map, it will be done afterwards.
	 */
	if (xen_feature(XENFEAT_gnttab_map_avail_bits))
		flags |= (1 << _GNTMAP_guest_avail0);

	gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr, flags,
			  map->grants[pgnr].ref,
			  map->grants[pgnr].domid);
	gnttab_set_unmap_op(&map->unmap_ops[pgnr], pte_maddr, flags,
			    -1 /* handle */);
	return 0;
}

#ifdef CONFIG_X86
static int set_grant_ptes_as_special(pte_t *pte, pgtable_t token,
				     unsigned long addr, void *data)
{
	set_pte_at(current->mm, addr, pte, pte_mkspecial(*pte));
	return 0;
}
#endif

static int map_grant_pages(struct grant_map *map)
{
	int i, err = 0;

	if (!use_ptemod) {
		/* Note: it could already be mapped */
		if (map->map_ops[0].handle != -1)
			return 0;
		for (i = 0; i < map->count; i++) {
			unsigned long addr = (unsigned long)
				pfn_to_kaddr(page_to_pfn(map->pages[i]));
			gnttab_set_map_op(&map->map_ops[i], addr, map->flags,
				map->grants[i].ref,
				map->grants[i].domid);
			gnttab_set_unmap_op(&map->unmap_ops[i], addr,
				map->flags, -1 /* handle */);
		}
	} else {
		/*
		 * Setup the map_ops corresponding to the pte entries pointing
		 * to the kernel linear addresses of the struct pages.
		 * These ptes are completely different from the user ptes dealt
		 * with find_grant_ptes.
		 */
		for (i = 0; i < map->count; i++) {
			unsigned long address = (unsigned long)
				pfn_to_kaddr(page_to_pfn(map->pages[i]));
			BUG_ON(PageHighMem(map->pages[i]));

			gnttab_set_map_op(&map->kmap_ops[i], address,
				map->flags | GNTMAP_host_map,
				map->grants[i].ref,
				map->grants[i].domid);
			gnttab_set_unmap_op(&map->kunmap_ops[i], address,
				map->flags | GNTMAP_host_map, -1);
		}
	}

	pr_debug("map %d+%d\n", map->index, map->count);
	err = gnttab_map_refs(map->map_ops, use_ptemod ? map->kmap_ops : NULL,
			map->pages, map->count);
	if (err)
		return err;

	for (i = 0; i < map->count; i++) {
		if (map->map_ops[i].status) {
			err = -EINVAL;
			continue;
		}

		map->unmap_ops[i].handle = map->map_ops[i].handle;
		if (use_ptemod)
			map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
	}
	return err;
}

static int __unmap_grant_pages(struct grant_map *map, int offset, int pages)
{
	int i, err = 0;
	struct gntab_unmap_queue_data unmap_data;

	if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
		int pgno = (map->notify.addr >> PAGE_SHIFT);
		if (pgno >= offset && pgno < offset + pages) {
			/* No need for kmap, pages are in lowmem */
			uint8_t *tmp = pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
			tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
			map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
		}
	}

	unmap_data.unmap_ops = map->unmap_ops + offset;
	unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
	unmap_data.pages = map->pages + offset;
	unmap_data.count = pages;

	err = gnttab_unmap_refs_sync(&unmap_data);
	if (err)
		return err;

	for (i = 0; i < pages; i++) {
		if (map->unmap_ops[offset+i].status)
			err = -EINVAL;
		pr_debug("unmap handle=%d st=%d\n",
			map->unmap_ops[offset+i].handle,
			map->unmap_ops[offset+i].status);
		map->unmap_ops[offset+i].handle = -1;
	}
	return err;
}

static int unmap_grant_pages(struct grant_map *map, int offset, int pages)
{
	int range, err = 0;

	pr_debug("unmap %d+%d [%d+%d]\n", map->index, map->count, offset, pages);

	/* It is possible the requested range will have a "hole" where we
	 * already unmapped some of the grants. Only unmap valid ranges.
	 */
	while (pages && !err) {
		while (pages && map->unmap_ops[offset].handle == -1) {
			offset++;
			pages--;
		}
		range = 0;
		while (range < pages) {
			if (map->unmap_ops[offset+range].handle == -1) {
				range--;
				break;
			}
			range++;
		}
		err = __unmap_grant_pages(map, offset, range);
		offset += range;
		pages -= range;
	}

	return err;
}

/* ------------------------------------------------------------------ */

static void gntdev_vma_open(struct vm_area_struct *vma)
{
	struct grant_map *map = vma->vm_private_data;

	pr_debug("gntdev_vma_open %p\n", vma);
	atomic_inc(&map->users);
}

static void gntdev_vma_close(struct vm_area_struct *vma)
{
	struct grant_map *map = vma->vm_private_data;
	struct file *file = vma->vm_file;
	struct gntdev_priv *priv = file->private_data;

	pr_debug("gntdev_vma_close %p\n", vma);
	if (use_ptemod) {
		/* It is possible that an mmu notifier could be running
		 * concurrently, so take priv->lock to ensure that the vma won't
		 * vanishing during the unmap_grant_pages call, since we will
		 * spin here until that completes. Such a concurrent call will
		 * not do any unmapping, since that has been done prior to
		 * closing the vma, but it may still iterate the unmap_ops list.
		 */
		mutex_lock(&priv->lock);
		map->vma = NULL;
		mutex_unlock(&priv->lock);
	}
	vma->vm_private_data = NULL;
	gntdev_put_map(priv, map);
}

static struct page *gntdev_vma_find_special_page(struct vm_area_struct *vma,
						 unsigned long addr)
{
	struct grant_map *map = vma->vm_private_data;

	return map->pages[(addr - map->pages_vm_start) >> PAGE_SHIFT];
}

static const struct vm_operations_struct gntdev_vmops = {
	.open = gntdev_vma_open,
	.close = gntdev_vma_close,
	.find_special_page = gntdev_vma_find_special_page,
};

/* ------------------------------------------------------------------ */

static void unmap_if_in_range(struct grant_map *map,
			      unsigned long start, unsigned long end)
{
	unsigned long mstart, mend;
	int err;

	if (!map->vma)
		return;
	if (map->vma->vm_start >= end)
		return;
	if (map->vma->vm_end <= start)
		return;
	mstart = max(start, map->vma->vm_start);
	mend   = min(end,   map->vma->vm_end);
	pr_debug("map %d+%d (%lx %lx), range %lx %lx, mrange %lx %lx\n",
			map->index, map->count,
			map->vma->vm_start, map->vma->vm_end,
			start, end, mstart, mend);
	err = unmap_grant_pages(map,
				(mstart - map->vma->vm_start) >> PAGE_SHIFT,
				(mend - mstart) >> PAGE_SHIFT);
	WARN_ON(err);
}

static void mn_invl_range_start(struct mmu_notifier *mn,
				struct mm_struct *mm,
				unsigned long start, unsigned long end)
{
	struct gntdev_priv *priv = container_of(mn, struct gntdev_priv, mn);
	struct grant_map *map;

	mutex_lock(&priv->lock);
	list_for_each_entry(map, &priv->maps, next) {
		unmap_if_in_range(map, start, end);
	}
	list_for_each_entry(map, &priv->freeable_maps, next) {
		unmap_if_in_range(map, start, end);
	}
	mutex_unlock(&priv->lock);
}

static void mn_invl_page(struct mmu_notifier *mn,
			 struct mm_struct *mm,
			 unsigned long address)
{
	mn_invl_range_start(mn, mm, address, address + PAGE_SIZE);
}

static void mn_release(struct mmu_notifier *mn,
		       struct mm_struct *mm)
{
	struct gntdev_priv *priv = container_of(mn, struct gntdev_priv, mn);
	struct grant_map *map;
	int err;

	mutex_lock(&priv->lock);
	list_for_each_entry(map, &priv->maps, next) {
		if (!map->vma)
			continue;
		pr_debug("map %d+%d (%lx %lx)\n",
				map->index, map->count,
				map->vma->vm_start, map->vma->vm_end);
		err = unmap_grant_pages(map, /* offset */ 0, map->count);
		WARN_ON(err);
	}
	list_for_each_entry(map, &priv->freeable_maps, next) {
		if (!map->vma)
			continue;
		pr_debug("map %d+%d (%lx %lx)\n",
				map->index, map->count,
				map->vma->vm_start, map->vma->vm_end);
		err = unmap_grant_pages(map, /* offset */ 0, map->count);
		WARN_ON(err);
	}
	mutex_unlock(&priv->lock);
}

static struct mmu_notifier_ops gntdev_mmu_ops = {
	.release                = mn_release,
	.invalidate_page        = mn_invl_page,
	.invalidate_range_start = mn_invl_range_start,
};

/* ------------------------------------------------------------------ */

static int gntdev_open(struct inode *inode, struct file *flip)
{
	struct gntdev_priv *priv;
	int ret = 0;

	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
	if (!priv)
		return -ENOMEM;

	INIT_LIST_HEAD(&priv->maps);
	INIT_LIST_HEAD(&priv->freeable_maps);
	mutex_init(&priv->lock);

	if (use_ptemod) {
		priv->mm = get_task_mm(current);
		if (!priv->mm) {
			kfree(priv);
			return -ENOMEM;
		}
		priv->mn.ops = &gntdev_mmu_ops;
		ret = mmu_notifier_register(&priv->mn, priv->mm);
		mmput(priv->mm);
	}

	if (ret) {
		kfree(priv);
		return ret;
	}

	flip->private_data = priv;
	pr_debug("priv %p\n", priv);

	return 0;
}

static int gntdev_release(struct inode *inode, struct file *flip)
{
	struct gntdev_priv *priv = flip->private_data;
	struct grant_map *map;

	pr_debug("priv %p\n", priv);

	mutex_lock(&priv->lock);
	while (!list_empty(&priv->maps)) {
		map = list_entry(priv->maps.next, struct grant_map, next);
		list_del(&map->next);
		gntdev_put_map(NULL /* already removed */, map);
	}
	WARN_ON(!list_empty(&priv->freeable_maps));
	mutex_unlock(&priv->lock);

	if (use_ptemod)
		mmu_notifier_unregister(&priv->mn, priv->mm);
	kfree(priv);
	return 0;
}

static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv,
				       struct ioctl_gntdev_map_grant_ref __user *u)
{
	struct ioctl_gntdev_map_grant_ref op;
	struct grant_map *map;
	int err;

	if (copy_from_user(&op, u, sizeof(op)) != 0)
		return -EFAULT;
	pr_debug("priv %p, add %d\n", priv, op.count);
	if (unlikely(op.count <= 0))
		return -EINVAL;

	err = -ENOMEM;
	map = gntdev_alloc_map(priv, op.count);
	if (!map)
		return err;

	if (unlikely(atomic_add_return(op.count, &pages_mapped) > limit)) {
		pr_debug("can't map: over limit\n");
		gntdev_put_map(NULL, map);
		return err;
	}

	if (copy_from_user(map->grants, &u->refs,
			   sizeof(map->grants[0]) * op.count) != 0) {
		gntdev_put_map(NULL, map);
		return -EFAULT;
	}

	mutex_lock(&priv->lock);
	gntdev_add_map(priv, map);
	op.index = map->index << PAGE_SHIFT;
	mutex_unlock(&priv->lock);

	if (copy_to_user(u, &op, sizeof(op)) != 0)
		return -EFAULT;

	return 0;
}

static long gntdev_ioctl_unmap_grant_ref(struct gntdev_priv *priv,
					 struct ioctl_gntdev_unmap_grant_ref __user *u)
{
	struct ioctl_gntdev_unmap_grant_ref op;
	struct grant_map *map;
	int err = -ENOENT;

	if (copy_from_user(&op, u, sizeof(op)) != 0)
		return -EFAULT;
	pr_debug("priv %p, del %d+%d\n", priv, (int)op.index, (int)op.count);

	mutex_lock(&priv->lock);
	map = gntdev_find_map_index(priv, op.index >> PAGE_SHIFT, op.count);
	if (map) {
		list_del(&map->next);
		if (populate_freeable_maps)
			list_add_tail(&map->next, &priv->freeable_maps);
		err = 0;
	}
	mutex_unlock(&priv->lock);
	if (map)
		gntdev_put_map(priv, map);
	return err;
}

static long gntdev_ioctl_get_offset_for_vaddr(struct gntdev_priv *priv,
					      struct ioctl_gntdev_get_offset_for_vaddr __user *u)
{
	struct ioctl_gntdev_get_offset_for_vaddr op;
	struct vm_area_struct *vma;
	struct grant_map *map;
	int rv = -EINVAL;

	if (copy_from_user(&op, u, sizeof(op)) != 0)
		return -EFAULT;
	pr_debug("priv %p, offset for vaddr %lx\n", priv, (unsigned long)op.vaddr);

	down_read(&current->mm->mmap_sem);
	vma = find_vma(current->mm, op.vaddr);
	if (!vma || vma->vm_ops != &gntdev_vmops)
		goto out_unlock;

	map = vma->vm_private_data;
	if (!map)
		goto out_unlock;

	op.offset = map->index << PAGE_SHIFT;
	op.count = map->count;
	rv = 0;

 out_unlock:
	up_read(&current->mm->mmap_sem);

	if (rv == 0 && copy_to_user(u, &op, sizeof(op)) != 0)
		return -EFAULT;
	return rv;
}

static long gntdev_ioctl_notify(struct gntdev_priv *priv, void __user *u)
{
	struct ioctl_gntdev_unmap_notify op;
	struct grant_map *map;
	int rc;
	int out_flags;
	unsigned int out_event;

	if (copy_from_user(&op, u, sizeof(op)))
		return -EFAULT;

	if (op.action & ~(UNMAP_NOTIFY_CLEAR_BYTE|UNMAP_NOTIFY_SEND_EVENT))
		return -EINVAL;

	/* We need to grab a reference to the event channel we are going to use
	 * to send the notify before releasing the reference we may already have
	 * (if someone has called this ioctl twice). This is required so that
	 * it is possible to change the clear_byte part of the notification
	 * without disturbing the event channel part, which may now be the last
	 * reference to that event channel.
	 */
	if (op.action & UNMAP_NOTIFY_SEND_EVENT) {
		if (evtchn_get(op.event_channel_port))
			return -EINVAL;
	}

	out_flags = op.action;
	out_event = op.event_channel_port;

	mutex_lock(&priv->lock);

	list_for_each_entry(map, &priv->maps, next) {
		uint64_t begin = map->index << PAGE_SHIFT;
		uint64_t end = (map->index + map->count) << PAGE_SHIFT;
		if (op.index >= begin && op.index < end)
			goto found;
	}
	rc = -ENOENT;
	goto unlock_out;

 found:
	if ((op.action & UNMAP_NOTIFY_CLEAR_BYTE) &&
			(map->flags & GNTMAP_readonly)) {
		rc = -EINVAL;
		goto unlock_out;
	}

	out_flags = map->notify.flags;
	out_event = map->notify.event;

	map->notify.flags = op.action;
	map->notify.addr = op.index - (map->index << PAGE_SHIFT);
	map->notify.event = op.event_channel_port;

	rc = 0;

 unlock_out:
	mutex_unlock(&priv->lock);

	/* Drop the reference to the event channel we did not save in the map */
	if (out_flags & UNMAP_NOTIFY_SEND_EVENT)
		evtchn_put(out_event);

	return rc;
}

static long gntdev_ioctl(struct file *flip,
			 unsigned int cmd, unsigned long arg)
{
	struct gntdev_priv *priv = flip->private_data;
	void __user *ptr = (void __user *)arg;

	switch (cmd) {
	case IOCTL_GNTDEV_MAP_GRANT_REF:
		return gntdev_ioctl_map_grant_ref(priv, ptr);

	case IOCTL_GNTDEV_UNMAP_GRANT_REF:
		return gntdev_ioctl_unmap_grant_ref(priv, ptr);

	case IOCTL_GNTDEV_GET_OFFSET_FOR_VADDR:
		return gntdev_ioctl_get_offset_for_vaddr(priv, ptr);

	case IOCTL_GNTDEV_SET_UNMAP_NOTIFY:
		return gntdev_ioctl_notify(priv, ptr);

	default:
		pr_debug("priv %p, unknown cmd %x\n", priv, cmd);
		return -ENOIOCTLCMD;
	}

	return 0;
}

static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
{
	struct gntdev_priv *priv = flip->private_data;
	int index = vma->vm_pgoff;
	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
	struct grant_map *map;
	int i, err = -EINVAL;

	if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
		return -EINVAL;

	pr_debug("map %d+%d at %lx (pgoff %lx)\n",
			index, count, vma->vm_start, vma->vm_pgoff);

	mutex_lock(&priv->lock);
	map = gntdev_find_map_index(priv, index, count);
	if (!map)
		goto unlock_out;
	if (use_ptemod && map->vma)
		goto unlock_out;
	if (use_ptemod && priv->mm != vma->vm_mm) {
		pr_warn("Huh? Other mm?\n");
		goto unlock_out;
	}

	atomic_inc(&map->users);

	vma->vm_ops = &gntdev_vmops;

	vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;

	if (use_ptemod)
		vma->vm_flags |= VM_DONTCOPY;

	vma->vm_private_data = map;

	if (use_ptemod)
		map->vma = vma;

	if (map->flags) {
		if ((vma->vm_flags & VM_WRITE) &&
				(map->flags & GNTMAP_readonly))
			goto out_unlock_put;
	} else {
		map->flags = GNTMAP_host_map;
		if (!(vma->vm_flags & VM_WRITE))
			map->flags |= GNTMAP_readonly;
	}

	mutex_unlock(&priv->lock);

	if (use_ptemod) {
		err = apply_to_page_range(vma->vm_mm, vma->vm_start,
					  vma->vm_end - vma->vm_start,
					  find_grant_ptes, map);
		if (err) {
			pr_warn("find_grant_ptes() failure.\n");
			goto out_put_map;
		}
	}

	err = map_grant_pages(map);
	if (err)
		goto out_put_map;

	if (!use_ptemod) {
		for (i = 0; i < count; i++) {
			err = vm_insert_page(vma, vma->vm_start + i*PAGE_SIZE,
				map->pages[i]);
			if (err)
				goto out_put_map;
		}
	} else {
#ifdef CONFIG_X86
		/*
		 * If the PTEs were not made special by the grant map
		 * hypercall, do so here.
		 *
		 * This is racy since the mapping is already visible
		 * to userspace but userspace should be well-behaved
		 * enough to not touch it until the mmap() call
		 * returns.
		 */
		if (!xen_feature(XENFEAT_gnttab_map_avail_bits)) {
			apply_to_page_range(vma->vm_mm, vma->vm_start,
					    vma->vm_end - vma->vm_start,
					    set_grant_ptes_as_special, NULL);
		}
#endif
		map->pages_vm_start = vma->vm_start;
	}

	return 0;

unlock_out:
	mutex_unlock(&priv->lock);
	return err;

out_unlock_put:
	mutex_unlock(&priv->lock);
out_put_map:
	if (use_ptemod)
		map->vma = NULL;
	gntdev_put_map(priv, map);
	return err;
}

static const struct file_operations gntdev_fops = {
	.owner = THIS_MODULE,
	.open = gntdev_open,
	.release = gntdev_release,
	.mmap = gntdev_mmap,
	.unlocked_ioctl = gntdev_ioctl
};

static struct miscdevice gntdev_miscdev = {
	.minor        = MISC_DYNAMIC_MINOR,
	.name         = "xen/gntdev",
	.fops         = &gntdev_fops,
};

/* ------------------------------------------------------------------ */

static int __init gntdev_init(void)
{
	int err;

	if (!xen_domain())
		return -ENODEV;

	use_ptemod = !xen_feature(XENFEAT_auto_translated_physmap);

	err = misc_register(&gntdev_miscdev);
	if (err != 0) {
		pr_err("Could not register gntdev device\n");
		return err;
	}
	return 0;
}

static void __exit gntdev_exit(void)
{
	misc_deregister(&gntdev_miscdev);
}

module_init(gntdev_init);
module_exit(gntdev_exit);

/* ------------------------------------------------------------------ */