drm/i915/gt: Always poison the kernel_context image before unparking

Keep scrubbing the kernel_context image with poison before we reset it
in order to demonstrate that we will be resilient in the case where it
is accidentally overwritten on idle.
Suggested-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102131707.1463945-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_context_types.h

@@ -17,6 +17,8 @@
 #include "intel_engine_types.h"
 #include "intel_sseu.h"
 
+#define CONTEXT_REDZONE POISON_INUSE
+
 struct i915_gem_context;
 struct i915_vma;
 struct intel_context;

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

@@ -37,8 +37,24 @@ static int __engine_unpark(struct intel_wakeref *wf)
 
 	/* Discard stale context state from across idling */
 	ce = engine->kernel_context;
-	if (ce)
+	if (ce) {
+		GEM_BUG_ON(test_bit(CONTEXT_VALID_BIT, &ce->flags));
+
+		/* First poison the image to verify we never fully trust it */
+		if (IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM) && ce->state) {
+			struct drm_i915_gem_object *obj = ce->state->obj;
+			int type = i915_coherent_map_type(engine->i915);
+
+			map = i915_gem_object_pin_map(obj, type);
+			if (!IS_ERR(map)) {
+				memset(map, CONTEXT_REDZONE, obj->base.size);
+				i915_gem_object_flush_map(obj);
+				i915_gem_object_unpin_map(obj);
+			}
+		}
+
 		ce->ops->reset(ce);
+	}
 
 	if (engine->unpark)
 		engine->unpark(engine);

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

@@ -2507,7 +2507,7 @@ set_redzone(void *vaddr, const struct intel_engine_cs *engine)
 
 	vaddr += engine->context_size;
 
-	memset(vaddr, POISON_INUSE, I915_GTT_PAGE_SIZE);
+	memset(vaddr, CONTEXT_REDZONE, I915_GTT_PAGE_SIZE);
 }
 
 static void
@@ -2518,7 +2518,7 @@ check_redzone(const void *vaddr, const struct intel_engine_cs *engine)
 
 	vaddr += engine->context_size;
 
-	if (memchr_inv(vaddr, POISON_INUSE, I915_GTT_PAGE_SIZE))
+	if (memchr_inv(vaddr, CONTEXT_REDZONE, I915_GTT_PAGE_SIZE))
 		dev_err_once(engine->i915->drm.dev,
 			     "%s context redzone overwritten!\n",
 			     engine->name);