[PATCH 2/3] virtio-net: Only enable userland vq if using tap backend

["Eugenio Pérez" <eperezma@redhat.com>]

Qemu falls back on userland handlers even if vhost-user and vhost-vdpa
cases. These assumes a tap device can handle the packets.

If a vdpa device fail to start, it can trigger a sigsegv because of
that. Do not resort on them unless actually possible.

Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 include/hw/virtio/virtio.h |  2 ++
 hw/net/virtio-net.c        |  4 ++++
 hw/virtio/virtio.c         | 21 +++++++++++++--------
 3 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index 8bab9cfb75..1712ba0b4c 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -105,6 +105,8 @@ struct VirtIODevice
     VMChangeStateEntry *vmstate;
     char *bus_name;
     uint8_t device_endian;
+    /* backend does not support userspace handler */
+    bool disable_ioeventfd_handler;
     bool use_guest_notifier_mask;
     AddressSpace *dma_as;
     QLIST_HEAD(, VirtQueue) *vector_queues;
diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 004acf858f..8c5c4e5a9d 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -3501,6 +3501,10 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
     nc = qemu_get_queue(n->nic);
     nc->rxfilter_notify_enabled = 1;
 
+    if (!nc->peer || nc->peer->info->type != NET_CLIENT_DRIVER_TAP) {
+        /* Only tap can use userspace networking */
+        vdev->disable_ioeventfd_handler = true;
+    }
     if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
         struct virtio_net_config netcfg = {};
         memcpy(&netcfg.mac, &n->nic_conf.macaddr, ETH_ALEN);
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index ea7c079fb0..1e04db6650 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -3734,17 +3734,22 @@ static int virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
             err = r;
             goto assign_error;
         }
-        event_notifier_set_handler(&vq->host_notifier,
-                                   virtio_queue_host_notifier_read);
+
+        if (!vdev->disable_ioeventfd_handler) {
+            event_notifier_set_handler(&vq->host_notifier,
+                                       virtio_queue_host_notifier_read);
+        }
     }
 
-    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
-        /* Kick right away to begin processing requests already in vring */
-        VirtQueue *vq = &vdev->vq[n];
-        if (!vq->vring.num) {
-            continue;
+    if (!vdev->disable_ioeventfd_handler) {
+        for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
+            /* Kick right away to begin processing requests already in vring */
+            VirtQueue *vq = &vdev->vq[n];
+            if (!vq->vring.num) {
+                continue;
+            }
+            event_notifier_set(&vq->host_notifier);
         }
-        event_notifier_set(&vq->host_notifier);
     }
     memory_region_transaction_commit();
     return 0;
-- 
2.27.0