drm/virtio: fix vq wait_event condition

Wait until we have enough space in the virt queue to actually queue up
our request.  Avoids the guest spinning in case we have a non-zero
amount of free entries but not enough for the request.

Cc: stable@vger.kernel.org
Reported-by: Alain Magloire <amagloire@blackberry.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180403095904.11152-1-kraxel@redhat.com
Signed-off-by: Sean Paul <seanpaul@chromium.org>

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -293,7 +293,7 @@ retry:
 	ret = virtqueue_add_sgs(vq, sgs, outcnt, incnt, vbuf, GFP_ATOMIC);
 	if (ret == -ENOSPC) {
 		spin_unlock(&vgdev->ctrlq.qlock);
-		wait_event(vgdev->ctrlq.ack_queue, vq->num_free);
+		wait_event(vgdev->ctrlq.ack_queue, vq->num_free >= outcnt + incnt);
 		spin_lock(&vgdev->ctrlq.qlock);
 		goto retry;
 	} else {
@@ -368,7 +368,7 @@ retry:
 	ret = virtqueue_add_sgs(vq, sgs, outcnt, 0, vbuf, GFP_ATOMIC);
 	if (ret == -ENOSPC) {
 		spin_unlock(&vgdev->cursorq.qlock);
-		wait_event(vgdev->cursorq.ack_queue, vq->num_free);
+		wait_event(vgdev->cursorq.ack_queue, vq->num_free >= outcnt);
 		spin_lock(&vgdev->cursorq.qlock);
 		goto retry;
 	} else {