drm/i915/gt: Always try to reserve GGTT address 0x0

Since writing to address 0 is a very common mistake, let's try to avoid
putting anything sensitive there.

References: https://gitlab.freedesktop.org/drm/intel/-/issues/2989Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125125033.23656-1-chris@chris-wilson.co.uk
Cc: stable@vger.kernel.org
(cherry picked from commit 56b429cc584c6ed8b895d8d8540959655db1ff73)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/gt/intel_ggtt.c

@@ -526,16 +526,39 @@ static int init_ggtt(struct i915_ggtt *ggtt)
 
 	mutex_init(&ggtt->error_mutex);
 	if (ggtt->mappable_end) {
-		/* Reserve a mappable slot for our lockless error capture */
-		ret = drm_mm_insert_node_in_range(&ggtt->vm.mm,
-						  &ggtt->error_capture,
-						  PAGE_SIZE, 0,
-						  I915_COLOR_UNEVICTABLE,
-						  0, ggtt->mappable_end,
-						  DRM_MM_INSERT_LOW);
-		if (ret)
-			return ret;
+		/*
+		 * Reserve a mappable slot for our lockless error capture.
+		 *
+		 * We strongly prefer taking address 0x0 in order to protect
+		 * other critical buffers against accidental overwrites,
+		 * as writing to address 0 is a very common mistake.
+		 *
+		 * Since 0 may already be in use by the system (e.g. the BIOS
+		 * framebuffer), we let the reservation fail quietly and hope
+		 * 0 remains reserved always.
+		 *
+		 * If we fail to reserve 0, and then fail to find any space
+		 * for an error-capture, remain silent. We can afford not
+		 * to reserve an error_capture node as we have fallback
+		 * paths, and we trust that 0 will remain reserved. However,
+		 * the only likely reason for failure to insert is a driver
+		 * bug, which we expect to cause other failures...
+		 */
+		ggtt->error_capture.size = I915_GTT_PAGE_SIZE;
+		ggtt->error_capture.color = I915_COLOR_UNEVICTABLE;
+		if (drm_mm_reserve_node(&ggtt->vm.mm, &ggtt->error_capture))
+			drm_mm_insert_node_in_range(&ggtt->vm.mm,
+						    &ggtt->error_capture,
+						    ggtt->error_capture.size, 0,
+						    ggtt->error_capture.color,
+						    0, ggtt->mappable_end,
+						    DRM_MM_INSERT_LOW);
 	}
+	if (drm_mm_node_allocated(&ggtt->error_capture))
+		drm_dbg(&ggtt->vm.i915->drm,
+			"Reserved GGTT:[%llx, %llx] for use by error capture\n",
+			ggtt->error_capture.start,
+			ggtt->error_capture.start + ggtt->error_capture.size);
 
 	/*
 	 * The upper portion of the GuC address space has a sizeable hole
@@ -548,9 +571,9 @@ static int init_ggtt(struct i915_ggtt *ggtt)
 
 	/* Clear any non-preallocated blocks */
 	drm_mm_for_each_hole(entry, &ggtt->vm.mm, hole_start, hole_end) {
-		drm_dbg_kms(&ggtt->vm.i915->drm,
-			    "clearing unused GTT space: [%lx, %lx]\n",
-			    hole_start, hole_end);
+		drm_dbg(&ggtt->vm.i915->drm,
+			"clearing unused GTT space: [%lx, %lx]\n",
+			hole_start, hole_end);
 		ggtt->vm.clear_range(&ggtt->vm, hole_start,
 				     hole_end - hole_start);
 	}