Re: [Qemu-devel] [PATCH for 2.11] virtio-net: don't touch virtqueue if vm is stopped

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Nov 23, 2017 at 11:37:46AM +0800, Jason Wang wrote:
> Guest state should not be touched if VM is stopped, unfortunately we
> didn't check running state and tried to drain tx queue unconditionally
> in virtio_net_set_status(). A crash was then noticed as a migration
> destination when user type quit after virtqueue state is loaded but
> before region cache is initialized. In this case,
> virtio_net_drop_tx_queue_data() tries to access the uninitialized
> region cache.
> 
> Fix this by only dropping tx queue data when vm is running.

hw/virtio/virtio.c:virtio_load() does the following:

  for (i = 0; i < num; i++) {
      if (vdev->vq[i].vring.desc) {
          uint16_t nheads;

          /*
           * VIRTIO-1 devices migrate desc, used, and avail ring addresses so
           * only the region cache needs to be set up.  Legacy devices need
           * to calculate used and avail ring addresses based on the desc
           * address.
           */
          if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
              virtio_init_region_cache(vdev, i);
          } else {
              virtio_queue_update_rings(vdev, i);
          }

So the region caches should be initialized after virtqueue state is
loaded.

It's unclear to me which code path triggers this issue.  Can you add a
backtrace or an explanation?

Thanks,
Stefan

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