drm/virtio: Add option to disable KMS support

Add a build option to disable modesetting support.  This is useful in
cases where the guest only needs to use the GPU in a headless mode, or
(such as in the CrOS usage) window surfaces are proxied to a host
compositor.

As the modesetting ioctls are a big surface area for potential security
bugs to be found (it's happened in the past, we should assume it will
again in the future), it makes sense to have a build option to disable
those ioctls in cases where they serve no legitimate purpose.

v2: Use more if (IS_ENABLED(...))
v3: Also permit the host to advertise no scanouts
v4: Spiff out commit msg
v5: Make num_scanouts==0 and DRM_VIRTIO_GPU_KMS=n behave the same
v6: Drop conditionally building virtgpu_display.c and early-out of
    it's init/fini fxns instead
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230302233506.3146290-1-robdclark@gmail.com

drivers/gpu/drm/virtio/Kconfig

@@ -11,3 +11,14 @@ config DRM_VIRTIO_GPU
 	   QEMU based VMMs (like KVM or Xen).
 
 	   If unsure say M.
+
+config DRM_VIRTIO_GPU_KMS
+	bool "Virtio GPU driver modesetting support"
+	depends on DRM_VIRTIO_GPU
+	default y
+	help
+	   Enable modesetting support for virtio GPU driver.  This can be
+	   disabled in cases where only "headless" usage of the GPU is
+	   required.
+
+	   If unsure, say Y.

drivers/gpu/drm/virtio/virtgpu_display.c

@@ -336,6 +336,9 @@ int virtio_gpu_modeset_init(struct virtio_gpu_device *vgdev)
 {
 	int i, ret;
 
+	if (!vgdev->num_scanouts)
+		return 0;
+
 	ret = drmm_mode_config_init(vgdev->ddev);
 	if (ret)
 		return ret;
@@ -362,6 +365,9 @@ void virtio_gpu_modeset_fini(struct virtio_gpu_device *vgdev)
 {
 	int i;
 
+	if (!vgdev->num_scanouts)
+		return;
+
 	for (i = 0 ; i < vgdev->num_scanouts; ++i)
 		kfree(vgdev->outputs[i].edid);
 }

drivers/gpu/drm/virtio/virtgpu_drv.c

@@ -172,6 +172,10 @@ MODULE_AUTHOR("Alon Levy");
 DEFINE_DRM_GEM_FOPS(virtio_gpu_driver_fops);
 
 static const struct drm_driver driver = {
+	/*
+	 * If KMS is disabled DRIVER_MODESET and DRIVER_ATOMIC are masked
+	 * out via drm_device::driver_features:
+	 */
 	.driver_features = DRIVER_MODESET | DRIVER_GEM | DRIVER_RENDER | DRIVER_ATOMIC,
 	.open = virtio_gpu_driver_open,
 	.postclose = virtio_gpu_driver_postclose,

drivers/gpu/drm/virtio/virtgpu_kms.c

@@ -223,12 +223,15 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
 			num_scanouts, &num_scanouts);
 	vgdev->num_scanouts = min_t(uint32_t, num_scanouts,
 				    VIRTIO_GPU_MAX_SCANOUTS);
-	if (!vgdev->num_scanouts) {
-		DRM_ERROR("num_scanouts is zero\n");
-		ret = -EINVAL;
-		goto err_scanouts;
-	}
+
+	if (IS_ENABLED(CONFIG_DRM_VIRTIO_GPU_KMS) || !vgdev->num_scanouts) {
+		DRM_INFO("KMS disabled\n");
+		vgdev->num_scanouts = 0;
+		vgdev->has_edid = false;
+		dev->driver_features &= ~(DRIVER_MODESET | DRIVER_ATOMIC);
+	} else {
 		DRM_INFO("number of scanouts: %d\n", num_scanouts);
+	}
 
 	virtio_cread_le(vgdev->vdev, struct virtio_gpu_config,
 			num_capsets, &num_capsets);
@@ -246,10 +249,12 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
 		virtio_gpu_get_capsets(vgdev, num_capsets);
 	if (vgdev->has_edid)
 		virtio_gpu_cmd_get_edids(vgdev);
+	if (vgdev->num_scanouts) {
 		virtio_gpu_cmd_get_display_info(vgdev);
 		virtio_gpu_notify(vgdev);
 		wait_event_timeout(vgdev->resp_wq, !vgdev->display_info_pending,
 				   5 * HZ);
+	}
 	return 0;
 
 err_scanouts: