drm/virtio: exbuf->fence_fd unmodified on interrupted wait

An interrupted dma_fence_wait() becomes an -ERESTARTSYS returned
to userspace ioctl(DRM_IOCTL_VIRTGPU_EXECBUFFER) calls, prompting to
retry the ioctl(), but the passed exbuf->fence_fd has been reset to -1,
making the retry attempt fail at sync_file_get_fence().

The uapi for DRM_IOCTL_VIRTGPU_EXECBUFFER is changed to retain the
passed value for exbuf->fence_fd when returning anything besides a
successful result from the ioctl.

Fixes: 2cd7b6f08bc4 ("drm/virtio: add in/out fence support for explicit synchronization")
Signed-off-by: Ryan Neph <[e-mail]>
Reviewed-by: Rob Clark <[e-mail]>
Reviewed-by: Dmitry Osipenko <[e-mail]>
Signed-off-by: Dmitry Osipenko <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/virtio/virtgpu_ioctl.c

···
 	void __user *user_bo_handles = NULL;
 	struct virtio_gpu_object_array *buflist = NULL;
 	struct sync_file *sync_file;
-	int in_fence_fd = exbuf->fence_fd;
 	int out_fence_fd = -1;
 	void *buf;
 	uint64_t fence_ctx;
···
 		ring_idx = exbuf->ring_idx;
 	}
 
-	exbuf->fence_fd = -1;
-
 	virtio_gpu_create_context(dev, file);
 	if (exbuf->flags & VIRTGPU_EXECBUF_FENCE_FD_IN) {
 		struct dma_fence *in_fence;
 
-		in_fence = sync_file_get_fence(in_fence_fd);
+		in_fence = sync_file_get_fence(exbuf->fence_fd);
 
 		if (!in_fence)
 			return -EINVAL;

include/uapi/drm/virtgpu_drm.h

···
 	__u32 pad;
 };
 
+/* fence_fd is modified on success if VIRTGPU_EXECBUF_FENCE_FD_OUT flag is set. */
 struct drm_virtgpu_execbuffer {
 	__u32 flags;
 	__u32 size;