vdpa: add get_backend_features vdpa operation

This operation allow vdpa parent to expose its own backend feature bits.

Next patches introduce a feature not compatible with all parent drivers:
the ability to enable vq after driver_ok. Each parent must declare if
it allows it or not.

Signed-off-by: Eugenio Pérez <[e-mail]>
Acked-by: Shannon Nelson <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vhost/vdpa.c

···
 	return 0;
 }
 
+static u64 vhost_vdpa_get_backend_features(const struct vhost_vdpa *v)
+{
+	struct vdpa_device *vdpa = v->vdpa;
+	const struct vdpa_config_ops *ops = vdpa->config;
+
+	if (!ops->get_backend_features)
+		return 0;
+	else
+		return ops->get_backend_features(vdpa);
+}
+
 static long vhost_vdpa_set_features(struct vhost_vdpa *v, u64 __user *featurep)
 {
 	struct vdpa_device *vdpa = v->vdpa;
···
 			features |= BIT_ULL(VHOST_BACKEND_F_SUSPEND);
 		if (vhost_vdpa_can_resume(v))
 			features |= BIT_ULL(VHOST_BACKEND_F_RESUME);
+		features |= vhost_vdpa_get_backend_features(v);
 		if (copy_to_user(featurep, &features, sizeof(features)))
 			r = -EFAULT;
 		break;

include/linux/vdpa.h

···
  *				@vdev: vdpa device
  *				Returns the virtio features support by the
  *				device
+ * @get_backend_features:	Get parent-specific backend features (optional)
+ *				Returns the vdpa features supported by the
+ *				device.
  * @set_driver_features:	Set virtio features supported by the driver
  *				@vdev: vdpa device
  *				@features: feature support by the driver
···
 	u32 (*get_vq_align)(struct vdpa_device *vdev);
 	u32 (*get_vq_group)(struct vdpa_device *vdev, u16 idx);
 	u64 (*get_device_features)(struct vdpa_device *vdev);
+	u64 (*get_backend_features)(const struct vdpa_device *vdev);
 	int (*set_driver_features)(struct vdpa_device *vdev, u64 features);
 	u64 (*get_driver_features)(struct vdpa_device *vdev);
 	void (*set_config_cb)(struct vdpa_device *vdev,