[PATCH 09/10] drm/i915/gt: Always try to reserve GGTT address 0x0

[Chris Wilson]

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/2989
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/gt/intel_ggtt.c | 40 +++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_ggtt.c b/drivers/gpu/drm/i915/gt/intel_ggtt.c
index dac07d66f658..4072b522f88e 100644
--- a/drivers/gpu/drm/i915/gt/intel_ggtt.c
+++ b/drivers/gpu/drm/i915/gt/intel_ggtt.c
@@ -535,15 +535,35 @@ 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);
+		/*
+		 * 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.
+		 */
+		ret = 0;
+		if (i915_gem_gtt_reserve(&ggtt->vm, &ggtt->error_capture,
+					 0, I915_GTT_PAGE_SIZE,
+					 I915_COLOR_UNEVICTABLE,
+					 PIN_NOEVICT))
+			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;
+
+		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);
 	}
 
 	/*
@@ -557,9 +577,9 @@ 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);
 	}
-- 
2.20.1