drm/i915/gtt: Reuse the read-only 64KiB scratch page and directories

If we can prevent stray writes from landing in the scratch page, we can
reuse the same page and same scratch PT for all contexts without fear of
information leaks and side-channels.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181029182721.29568-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -580,10 +580,9 @@ setup_scratch_page(struct i915_address_space *vm, gfp_t gfp)
 	 * region, including any PTEs which happen to point to scratch.
 	 *
 	 * This is only relevant for the 48b PPGTT where we support
-	 * huge-gtt-pages, see also i915_vma_insert().
-	 *
-	 * TODO: we should really consider write-protecting the scratch-page and
-	 * sharing between ppgtt
+	 * huge-gtt-pages, see also i915_vma_insert(). However, as we share the
+	 * scratch (read-only) between all vm, we create one 64k scratch page
+	 * for all.
 	 */
 	size = I915_GTT_PAGE_SIZE_4K;
 	if (i915_vm_is_48bit(vm) &&
@@ -1209,6 +1208,26 @@ static int gen8_init_scratch(struct i915_address_space *vm)
 {
 	int ret;
 
+	/*
+	 * If everybody agrees to not to write into the scratch page,
+	 * we can reuse it for all vm, keeping contexts and processes separate.
+	 */
+	if (vm->has_read_only &&
+	    vm->i915->kernel_context &&
+	    vm->i915->kernel_context->ppgtt) {
+		struct i915_address_space *clone =
+			&vm->i915->kernel_context->ppgtt->vm;
+
+		GEM_BUG_ON(!clone->has_read_only);
+
+		vm->scratch_page.order = clone->scratch_page.order;
+		vm->scratch_pte = clone->scratch_pte;
+		vm->scratch_pt  = clone->scratch_pt;
+		vm->scratch_pd  = clone->scratch_pd;
+		vm->scratch_pdp = clone->scratch_pdp;
+		return 0;
+	}
+
 	ret = setup_scratch_page(vm, __GFP_HIGHMEM);
 	if (ret)
 		return ret;
@@ -1289,6 +1308,9 @@ static int gen8_ppgtt_notify_vgt(struct i915_hw_ppgtt *ppgtt, bool create)
 
 static void gen8_free_scratch(struct i915_address_space *vm)
 {
+	if (!vm->scratch_page.daddr)
+		return;
+
 	if (use_4lvl(vm))
 		free_pdp(vm, vm->scratch_pdp);
 	free_pd(vm, vm->scratch_pd);