KVM: s390 allow virtio_ccw status writes to fail

Gracefully handle failure to write device status.
We really should handle other errors as well, but this one is needed for
virtio 1.0 compliance.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>

drivers/s390/kvm/virtio_ccw.c

@@ -862,7 +862,9 @@ static u8 virtio_ccw_get_status(struct virtio_device *vdev)
 static void virtio_ccw_set_status(struct virtio_device *vdev, u8 status)
 {
 	struct virtio_ccw_device *vcdev = to_vc_device(vdev);
+	u8 old_status = *vcdev->status;
 	struct ccw1 *ccw;
+	int ret;
 
 	ccw = kzalloc(sizeof(*ccw), GFP_DMA | GFP_KERNEL);
 	if (!ccw)
@@ -874,7 +876,10 @@ static void virtio_ccw_set_status(struct virtio_device *vdev, u8 status)
 	ccw->flags = 0;
 	ccw->count = sizeof(status);
 	ccw->cda = (__u32)(unsigned long)vcdev->status;
-	ccw_io_helper(vcdev, ccw, VIRTIO_CCW_DOING_WRITE_STATUS);
+	ret = ccw_io_helper(vcdev, ccw, VIRTIO_CCW_DOING_WRITE_STATUS);
+	/* Write failed? We assume status is unchanged. */
+	if (ret)
+		*vcdev->status = old_status;
 	kfree(ccw);
 }