drm/i915/selftest: Clear the output buffers before GPU writes

When testing whether we can get the GPU to leak information about
non-privileged state, we first need to ensure that the output buffer is
set to a known value as the HW may opt to skip the write into memory for
a non-privileged read of a sensitive register. We chose POISON_INUSE (0x5a)
so that is both non-zero and distinct from the poison values used during
the test.

v2:
  Use i915_gem_object_pin_map_unlocked
Reported-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: CQ Tang <cq.tang@intel.com>
cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Signed-off-by: Karolina Drobnik <karolina.drobnik@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5cebab02d182c171cf40cb5b73d6c3eeb7619360.1663081418.git.karolina.drobnik@intel.com

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

@@ -1395,6 +1395,30 @@ static int compare_isolation(struct intel_engine_cs *engine,
 	return err;
 }
 
+static struct i915_vma *
+create_result_vma(struct i915_address_space *vm, unsigned long sz)
+{
+	struct i915_vma *vma;
+	void *ptr;
+
+	vma = create_user_vma(vm, sz);
+	if (IS_ERR(vma))
+		return vma;
+
+	/* Set the results to a known value distinct from the poison */
+	ptr = i915_gem_object_pin_map_unlocked(vma->obj, I915_MAP_WC);
+	if (IS_ERR(ptr)) {
+		i915_vma_put(vma);
+		return ERR_CAST(ptr);
+	}
+
+	memset(ptr, POISON_INUSE, vma->size);
+	i915_gem_object_flush_map(vma->obj);
+	i915_gem_object_unpin_map(vma->obj);
+
+	return vma;
+}
+
 static int __lrc_isolation(struct intel_engine_cs *engine, u32 poison)
 {
 	u32 *sema = memset32(engine->status_page.addr + 1000, 0, 1);
@@ -1413,13 +1437,13 @@ static int __lrc_isolation(struct intel_engine_cs *engine, u32 poison)
 		goto err_A;
 	}
 
-	ref[0] = create_user_vma(A->vm, SZ_64K);
+	ref[0] = create_result_vma(A->vm, SZ_64K);
 	if (IS_ERR(ref[0])) {
 		err = PTR_ERR(ref[0]);
 		goto err_B;
 	}
 
-	ref[1] = create_user_vma(A->vm, SZ_64K);
+	ref[1] = create_result_vma(A->vm, SZ_64K);
 	if (IS_ERR(ref[1])) {
 		err = PTR_ERR(ref[1]);
 		goto err_ref0;
@@ -1441,13 +1465,13 @@ static int __lrc_isolation(struct intel_engine_cs *engine, u32 poison)
 	}
 	i915_request_put(rq);
 
-	result[0] = create_user_vma(A->vm, SZ_64K);
+	result[0] = create_result_vma(A->vm, SZ_64K);
 	if (IS_ERR(result[0])) {
 		err = PTR_ERR(result[0]);
 		goto err_ref1;
 	}
 
-	result[1] = create_user_vma(A->vm, SZ_64K);
+	result[1] = create_result_vma(A->vm, SZ_64K);
 	if (IS_ERR(result[1])) {
 		err = PTR_ERR(result[1]);
 		goto err_result0;