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

diff --git a/drivers/gpu/drm/virtio/Kconfig b/drivers/gpu/drm/virtio/Kconfig
index 51ec7c3240c91c2b0540a29d0feac31ff1267cfe..ea06ff2aa4b457d09c251cc0baadc2c2850fd86d 100644 (file)
--- a/drivers/gpu/drm/virtio/Kconfig
+++ b/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.

diff --git a/drivers/gpu/drm/virtio/virtgpu_display.c b/drivers/gpu/drm/virtio/virtgpu_display.c
index 9ea7611a9e0fc1eccb0b11036dbcf9c0964b501a..ad924a8502e9025cd4c99a4342dee7c41965c409 100644 (file)
--- a/drivers/gpu/drm/virtio/virtgpu_display.c
+++ b/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);
 }

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.c b/drivers/gpu/drm/virtio/virtgpu_drv.c
index ae97b98750b6d5f4baf5ed9d2f7c663d738bfaf1..add075681e18f80b3b42afc6cb31ff38e8fa937b 100644 (file)
--- a/drivers/gpu/drm/virtio/virtgpu_drv.c
+++ b/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,

diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index 27b7f14dae89a167d5cdc824888eb0c1caeab14b..02e5c18c2c75085e163cf10d246554b319a6e3f6 100644 (file)
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/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);
        }
-       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);
-       virtio_gpu_cmd_get_display_info(vgdev);
-       virtio_gpu_notify(vgdev);
-       wait_event_timeout(vgdev->resp_wq, !vgdev->display_info_pending,
-                          5 * HZ);
+       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: