vhost-net: cleanup host notifiers at last step

When the vhost notifier is disabled, the userspace handler runs
immediately: virtio_pci_set_host_notifier_internal might
call virtio_queue_notify_vq.
Since the VQ state and the tap backend state aren't
recovered yet, this causes
"Guest moved used index from XXX to YYY" assertions.

The solution is to split out host notifier handling
from vhost VQ setup and disable notifiers as our last step
when we stop vhost-net. For symmetry enable them first thing
on start.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 13 insertions, 3 deletions

diff --git a/hw/vhost_net.c b/hw/vhost_net.c
index a55981200d..950a6b8d99 100644
--- a/hw/vhost_net.c
+++ b/hw/vhost_net.c
@@ -139,16 +139,22 @@ int vhost_net_start(struct vhost_net *net,
 {
     struct vhost_vring_file file = { };
     int r;
+
+    net->dev.nvqs = 2;
+    net->dev.vqs = net->vqs;
+
+    r = vhost_dev_enable_notifiers(&net->dev, dev);
+    if (r < 0) {
+        goto fail_notifiers;
+    }
     if (net->dev.acked_features & (1 << VIRTIO_NET_F_MRG_RXBUF)) {
         tap_set_vnet_hdr_len(net->vc,
                              sizeof(struct virtio_net_hdr_mrg_rxbuf));
     }
 
-    net->dev.nvqs = 2;
-    net->dev.vqs = net->vqs;
     r = vhost_dev_start(&net->dev, dev);
     if (r < 0) {
-        return r;
+        goto fail_start;
     }
 
     net->vc->info->poll(net->vc, false);
@@ -173,6 +179,9 @@ fail:
     if (net->dev.acked_features & (1 << VIRTIO_NET_F_MRG_RXBUF)) {
         tap_set_vnet_hdr_len(net->vc, sizeof(struct virtio_net_hdr));
     }
+fail_start:
+    vhost_dev_disable_notifiers(&net->dev, dev);
+fail_notifiers:
     return r;
 }
 
@@ -190,6 +199,7 @@ void vhost_net_stop(struct vhost_net *net,
     if (net->dev.acked_features & (1 << VIRTIO_NET_F_MRG_RXBUF)) {
         tap_set_vnet_hdr_len(net->vc, sizeof(struct virtio_net_hdr));
     }
+    vhost_dev_disable_notifiers(&net->dev, dev);
 }
 
 void vhost_net_cleanup(struct vhost_net *net)