xen-gntdev: Support mapping in HVM domains

HVM does not allow direct PTE modification, so instead we request
that Xen change its internal p2m mappings on the allocated pages and
map the memory into userspace normally.

Note:
The HVM path for map and unmap is slightly different: HVM keeps the pages
mapped until the area is deleted, while the PV case (use_ptemod being true)
must unmap them when userspace unmaps the range. In the normal use case,
this makes no difference to users since unmap time is deletion time.

[v2: Expanded commit descr.]
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/gntdev.c

@@ -32,6 +32,7 @@
 #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>
@@ -52,6 +53,8 @@ MODULE_PARM_DESC(limit, "Maximum number of grants that may be mapped by "
 
 static atomic_t pages_mapped = ATOMIC_INIT(0);
 
+static int use_ptemod;
+
 struct gntdev_priv {
 	struct list_head maps;
 	/* lock protects maps from concurrent changes */
@@ -74,6 +77,8 @@ struct grant_map {
 	struct page **pages;
 };
 
+static int unmap_grant_pages(struct grant_map *map, int offset, int pages);
+
 /* ------------------------------------------------------------------ */
 
 static void gntdev_print_maps(struct gntdev_priv *priv,
@@ -179,11 +184,34 @@ static void gntdev_put_map(struct grant_map *map)
 
 	atomic_sub(map->count, &pages_mapped);
 
-	if (map->pages)
+	if (map->pages) {
+		if (!use_ptemod)
+			unmap_grant_pages(map, 0, map->count);
+
 		for (i = 0; i < map->count; i++) {
-			if (map->pages[i])
+			uint32_t check, *tmp;
+			if (!map->pages[i])
+				continue;
+			/* XXX When unmapping in an HVM domain, Xen will
+			 * sometimes end up mapping the GFN to an invalid MFN.
+			 * In this case, writes will be discarded and reads will
+			 * return all 0xFF bytes.  Leak these unusable GFNs
+			 * until Xen supports fixing their p2m mapping.
+			 *
+			 * Confirmed present in Xen 4.1-RC3 with HVM source
+			 */
+			tmp = kmap(map->pages[i]);
+			*tmp = 0xdeaddead;
+			mb();
+			check = *tmp;
+			kunmap(map->pages[i]);
+			if (check == 0xdeaddead)
 				__free_page(map->pages[i]);
+			else
+				pr_debug("Discard page %d=%ld\n", i,
+					page_to_pfn(map->pages[i]));
 		}
+	}
 	kfree(map->pages);
 	kfree(map->grants);
 	kfree(map->map_ops);
@@ -198,17 +226,16 @@ static int find_grant_ptes(pte_t *pte, pgtable_t token,
 {
 	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;
 
-	gnttab_set_map_op(&map->map_ops[pgnr], pte_maddr,
-			  GNTMAP_contains_pte | map->flags,
+	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,
-			    GNTMAP_contains_pte | map->flags,
+	gnttab_set_unmap_op(&map->unmap_ops[pgnr], pte_maddr, flags,
 			    0 /* handle */);
 	return 0;
 }
@@ -216,6 +243,19 @@ static int find_grant_ptes(pte_t *pte, pgtable_t token,
 static int map_grant_pages(struct grant_map *map)
 {
 	int i, err = 0;
+	phys_addr_t addr;
+
+	if (!use_ptemod) {
+		for (i = 0; i < map->count; i++) {
+			addr = (phys_addr_t)
+				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, 0 /* handle */);
+		}
+	}
 
 	pr_debug("map %d+%d\n", map->index, map->count);
 	err = gnttab_map_refs(map->map_ops, map->pages, map->count);
@@ -260,17 +300,8 @@ static void gntdev_vma_close(struct vm_area_struct *vma)
 	gntdev_put_map(map);
 }
 
-static int gntdev_vma_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
-{
-	pr_debug("vaddr %p, pgoff %ld (shouldn't happen)\n",
-			vmf->virtual_address, vmf->pgoff);
-	vmf->flags = VM_FAULT_ERROR;
-	return 0;
-}
-
 static struct vm_operations_struct gntdev_vmops = {
 	.close = gntdev_vma_close,
-	.fault = gntdev_vma_fault,
 };
 
 /* ------------------------------------------------------------------ */
@@ -355,14 +386,16 @@ static int gntdev_open(struct inode *inode, struct file *flip)
 	INIT_LIST_HEAD(&priv->maps);
 	spin_lock_init(&priv->lock);
 
-	priv->mm = get_task_mm(current);
-	if (!priv->mm) {
-		kfree(priv);
-		return -ENOMEM;
+	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);
 	}
-	priv->mn.ops = &gntdev_mmu_ops;
-	ret = mmu_notifier_register(&priv->mn, priv->mm);
-	mmput(priv->mm);
 
 	if (ret) {
 		kfree(priv);
@@ -390,7 +423,8 @@ static int gntdev_release(struct inode *inode, struct file *flip)
 	}
 	spin_unlock(&priv->lock);
 
-	mmu_notifier_unregister(&priv->mn, priv->mm);
+	if (use_ptemod)
+		mmu_notifier_unregister(&priv->mn, priv->mm);
 	kfree(priv);
 	return 0;
 }
@@ -515,7 +549,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	int index = vma->vm_pgoff;
 	int count = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
 	struct grant_map *map;
-	int err = -EINVAL;
+	int i, err = -EINVAL;
 
 	if ((vma->vm_flags & VM_WRITE) && !(vma->vm_flags & VM_SHARED))
 		return -EINVAL;
@@ -527,9 +561,9 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	map = gntdev_find_map_index(priv, index, count);
 	if (!map)
 		goto unlock_out;
-	if (map->vma)
+	if (use_ptemod && map->vma)
 		goto unlock_out;
-	if (priv->mm != vma->vm_mm) {
+	if (use_ptemod && priv->mm != vma->vm_mm) {
 		printk(KERN_WARNING "Huh? Other mm?\n");
 		goto unlock_out;
 	}
@@ -541,20 +575,24 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 	vma->vm_flags |= VM_RESERVED|VM_DONTCOPY|VM_DONTEXPAND|VM_PFNMAP;
 
 	vma->vm_private_data = map;
-	map->vma = vma;
 
-	map->flags = GNTMAP_host_map | GNTMAP_application_map;
+	if (use_ptemod)
+		map->vma = vma;
+
+	map->flags = GNTMAP_host_map;
 	if (!(vma->vm_flags & VM_WRITE))
 		map->flags |= GNTMAP_readonly;
 
 	spin_unlock(&priv->lock);
 
-	err = apply_to_page_range(vma->vm_mm, vma->vm_start,
-				  vma->vm_end - vma->vm_start,
-				  find_grant_ptes, map);
-	if (err) {
-		printk(KERN_WARNING "find_grant_ptes() failure.\n");
-		return err;
+	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) {
+			printk(KERN_WARNING "find_grant_ptes() failure.\n");
+			return err;
+		}
 	}
 
 	err = map_grant_pages(map);
@@ -565,6 +603,15 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 
 	map->is_mapped = 1;
 
+	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)
+				return err;
+		}
+	}
+
 	return 0;
 
 unlock_out:
@@ -595,6 +642,8 @@ static int __init gntdev_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+	use_ptemod = xen_pv_domain();
+
 	err = misc_register(&gntdev_miscdev);
 	if (err != 0) {
 		printk(KERN_ERR "Could not register gntdev device\n");

drivers/xen/grant-table.c

@@ -458,6 +458,9 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
 	if (ret)
 		return ret;
 
+	if (xen_feature(XENFEAT_auto_translated_physmap))
+		return ret;
+
 	for (i = 0; i < count; i++) {
 		/* m2p override only supported for GNTMAP_contains_pte mappings */
 		if (!(map_ops[i].flags & GNTMAP_contains_pte))
@@ -483,6 +486,9 @@ int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
 	if (ret)
 		return ret;
 
+	if (xen_feature(XENFEAT_auto_translated_physmap))
+		return ret;
+
 	for (i = 0; i < count; i++) {
 		ret = m2p_remove_override(pages[i]);
 		if (ret)