drm/xe: Use NULL PTEs as scratch PTEs

Currently scratch PTEs are write-enabled and points to a single scratch
page. This has the side effect that buggy applications with out-of-bounds
memory accesses may not notice the bad access since what's written may
be read back.

Instead use NULL PTEs as scratch PTEs. These always return 0 when reading,
and writing has no effect. As a slight benefit, we can also use huge NULL
PTEs.

One drawback pointed out is that debugging may be hampered since previously
when inspecting the content of the scratch page, it might be possible to
detect writes to out-of-bound addresses and possibly also
from where the out-of-bounds address originated. However since the scratch
page-table structure is kept, it will be easy to add back the single
RW-enabled scratch page under a debug define if needed.

Also update the kerneldoc accordingly and move the function to create the
scratch page-tables from xe_pt.c to xe_pt.h since it is accessing
vm structure internals and this also makes it possible to make it static.

v2:
- Don't try to encode scratch PTEs larger than 1GiB.
- Move xe_pt_create_scratch(), Update kerneldoc.
v3:
- Rebase.

Cc: Brian Welty <brian.welty@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Acked-by: Lucas De Marchi <lucas.demarchi@intel.com> #for general direction.
Reviewed-by: Brian Welty <brian.welty@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231209151843.7903-3-thomas.hellstrom@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

1 files changed, 55 insertions, 11 deletions

diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
index d589beb99fe6..e190469ec03a 100644
--- a/drivers/gpu/drm/xe/xe_vm.c
+++ b/drivers/gpu/drm/xe/xe_vm.c
@@ -1348,6 +1348,57 @@ static const struct xe_pt_ops xelp_pt_ops = {
 
 static void vm_destroy_work_func(struct work_struct *w);
 
+/**
+ * xe_vm_create_scratch() - Setup a scratch memory pagetable tree for the
+ * given tile and vm.
+ * @xe: xe device.
+ * @tile: tile to set up for.
+ * @vm: vm to set up for.
+ *
+ * Sets up a pagetable tree with one page-table per level and a single
+ * leaf PTE. All pagetable entries point to the single page-table or,
+ * for MAX_HUGEPTE_LEVEL, a NULL huge PTE returning 0 on read and
+ * writes become NOPs.
+ *
+ * Return: 0 on success, negative error code on error.
+ */
+static int xe_vm_create_scratch(struct xe_device *xe, struct xe_tile *tile,
+				struct xe_vm *vm)
+{
+	u8 id = tile->id;
+	int i;
+
+	for (i = MAX_HUGEPTE_LEVEL; i < vm->pt_root[id]->level; i++) {
+		vm->scratch_pt[id][i] = xe_pt_create(vm, tile, i);
+		if (IS_ERR(vm->scratch_pt[id][i]))
+			return PTR_ERR(vm->scratch_pt[id][i]);
+
+		xe_pt_populate_empty(tile, vm, vm->scratch_pt[id][i]);
+	}
+
+	return 0;
+}
+
+static void xe_vm_free_scratch(struct xe_vm *vm)
+{
+	struct xe_tile *tile;
+	u8 id;
+
+	if (!xe_vm_has_scratch(vm))
+		return;
+
+	for_each_tile(tile, vm->xe, id) {
+		u32 i;
+
+		if (!vm->pt_root[id])
+			continue;
+
+		for (i = MAX_HUGEPTE_LEVEL; i < vm->pt_root[id]->level; ++i)
+			if (vm->scratch_pt[id][i])
+				xe_pt_destroy(vm->scratch_pt[id][i], vm->flags, NULL);
+	}
+}
+
 struct xe_vm *xe_vm_create(struct xe_device *xe, u32 flags)
 {
 	struct drm_gem_object *vm_resv_obj;
@@ -1424,12 +1475,12 @@ struct xe_vm *xe_vm_create(struct xe_device *xe, u32 flags)
 		}
 	}
 
-	if (flags & XE_VM_FLAG_SCRATCH_PAGE) {
+	if (xe_vm_has_scratch(vm)) {
 		for_each_tile(tile, xe, id) {
 			if (!vm->pt_root[id])
 				continue;
 
-			err = xe_pt_create_scratch(xe, tile, vm);
+			err = xe_vm_create_scratch(xe, tile, vm);
 			if (err)
 				goto err_unlock_close;
 		}
@@ -1575,16 +1626,9 @@ void xe_vm_close_and_put(struct xe_vm *vm)
 	 * install a fence to resv. Hence it's safe to
 	 * destroy the pagetables immediately.
 	 */
-	for_each_tile(tile, xe, id) {
-		if (vm->scratch_bo[id]) {
-			u32 i;
+	xe_vm_free_scratch(vm);
 
-			xe_bo_unpin(vm->scratch_bo[id]);
-			xe_bo_put(vm->scratch_bo[id]);
-			for (i = 0; i < vm->pt_root[id]->level; i++)
-				xe_pt_destroy(vm->scratch_pt[id][i], vm->flags,
-					      NULL);
-		}
+	for_each_tile(tile, xe, id) {
 		if (vm->pt_root[id]) {
 			xe_pt_destroy(vm->pt_root[id], vm->flags, NULL);
 			vm->pt_root[id] = NULL;