[PATCH 4.4 15/44] drm/virtio: fix vq wait_event condition

[Greg Kroah-Hartman]

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gerd Hoffmann <kraxel@xxxxxxxxxx>

commit d02d270014f70dcab0117776b81a37b6fca745ae upstream.

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@xxxxxxxxxxxxxxx
Reported-by: Alain Magloire <amagloire@xxxxxxxxxxxxxx>
Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Reviewed-by: Dave Airlie <airlied@xxxxxxxxxx>
Link: http://patchwork.freedesktop.org/patch/msgid/20180403095904.11152-1-kraxel@xxxxxxxxxx
Signed-off-by: Sean Paul <seanpaul@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/gpu/drm/virtio/virtgpu_vq.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -324,7 +324,7 @@ 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 {
@@ -399,7 +399,7 @@ 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 {