virtio: update MemoryRegionCaches when guest negotiates features

Because the cache is sized to include the rings and the event indices,
negotiating the VIRTIO_RING_F_EVENT_IDX feature will result in the size
of the cache changing.  And because MemoryRegionCache accesses are
range-checked, if we skip this we end up with an assertion failure.
This happens with OpenBSD 6.3.

Reported-by: Fam Zheng <famz@redhat.com>
Fixes: 97cd965c070152bc626c7507df9fb356bbe1cd81
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/virtio/virtio.c

@@ -2006,14 +2006,25 @@ static int virtio_set_features_nocheck(VirtIODevice *vdev, uint64_t val)
 
 int virtio_set_features(VirtIODevice *vdev, uint64_t val)
 {
-   /*
+    int ret;
+    /*
      * The driver must not attempt to set features after feature negotiation
      * has finished.
      */
     if (vdev->status & VIRTIO_CONFIG_S_FEATURES_OK) {
         return -EINVAL;
     }
-    return virtio_set_features_nocheck(vdev, val);
+    ret = virtio_set_features_nocheck(vdev, val);
+    if (!ret && virtio_vdev_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX)) {
+        /* VIRTIO_RING_F_EVENT_IDX changes the size of the caches.  */
+        int i;
+        for (i = 0; i < VIRTIO_QUEUE_MAX; i++) {
+            if (vdev->vq[i].vring.num != 0) {
+                virtio_init_region_cache(vdev, i);
+            }
+        }
+    }
+    return ret;
 }
 
 int virtio_load(VirtIODevice *vdev, QEMUFile *f, int version_id)