[Qemu-devel] [PATCH v2 1/1] virtio: fix vq->inuse recalc after migr

[Halil Pasic]

Correct recalculation of vq->inuse after migration for the corner case
where the avail_idx has already wrapped but used_idx not yet.

Also change the type of the VirtQueue.inuse to unsigned int. This is
done to be consistent with other members representing sizes (VRing.num),
and because C99 guarantees max ring size < UINT_MAX but does not
guarantee max ring size < INT_MAX.

Signed-off-by: Halil Pasic <address@hidden>
Fixes: bccdef6b ("virtio: recalculate vq->inuse after migration")
CC: address@hidden
---
v1 -> v2:
* Reworded comment explaining the cast. (thanks Stefan)
* Changed type of vq->inuse from signed to unsigned
* Fixed misnomer %s/vring->inuse/vq->inuse/
---
 hw/virtio/virtio.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 1af2de2..e37641a 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -92,7 +92,7 @@ struct VirtQueue
 
     uint16_t queue_index;
 
-    int inuse;
+    unsigned int inuse;
 
     uint16_t vector;
     VirtIOHandleOutput handle_output;
@@ -1855,9 +1855,11 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
version_id)
             /*
              * Some devices migrate VirtQueueElements that have been popped
              * from the avail ring but not yet returned to the used ring.
+             * Since max ring size < UINT16_MAX it's safe to use modulo
+             * UINT16_MAX + 1 subtraction.
              */
-            vdev->vq[i].inuse = vdev->vq[i].last_avail_idx -
-                                vdev->vq[i].used_idx;
+            vdev->vq[i].inuse = (uint16_t)(vdev->vq[i].last_avail_idx -
+                                vdev->vq[i].used_idx);
             if (vdev->vq[i].inuse > vdev->vq[i].vring.num) {
                 error_report("VQ %d size 0x%x < last_avail_idx 0x%x - "
                              "used_idx 0x%x",
-- 
2.8.4