virtio: handle non-virtio-1-capable backend for ccw

If you run a qemu advertising VERSION_1 with an old kernel where
vhost did not yet support VERSION_1, you'll end up with a device
that is {modern pci|ccw revision 1} but does not advertise VERSION_1.
This is not a sensible configuration and is rejected by the Linux
guest drivers.

To fix this, add a ->post_plugged() callback invoked after features
have been queried that can handle the VERSION_1 bit being withdrawn
and change ccw to fall back to revision 0 if VERSION_1 is gone.

Note that pci is _not_ fixed; we'll need to rethink the approach
for the next release but at least for pci it's not a regression.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

3 files changed, 20 insertions, 0 deletions

diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
index fb103b78ac..63da303864 100644
--- a/hw/s390x/virtio-ccw.c
+++ b/hw/s390x/virtio-ccw.c
@@ -1555,6 +1555,17 @@ static void virtio_ccw_device_plugged(DeviceState *d, Error **errp)
                           d->hotplugged, 1);
 }
 
+static void virtio_ccw_post_plugged(DeviceState *d, Error **errp)
+{
+   VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
+   VirtIODevice *vdev = virtio_bus_get_device(&dev->bus);
+
+   if (!virtio_host_has_feature(vdev, VIRTIO_F_VERSION_1)) {
+        /* A backend didn't support modern virtio. */
+       dev->max_rev = 0;
+   }
+}
+
 static void virtio_ccw_device_unplugged(DeviceState *d)
 {
     VirtioCcwDevice *dev = VIRTIO_CCW_DEVICE(d);
@@ -1891,6 +1902,7 @@ static void virtio_ccw_bus_class_init(ObjectClass *klass, void *data)
     k->save_config = virtio_ccw_save_config;
     k->load_config = virtio_ccw_load_config;
     k->device_plugged = virtio_ccw_device_plugged;
+    k->post_plugged = virtio_ccw_post_plugged;
     k->device_unplugged = virtio_ccw_device_unplugged;
 }
 
diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index febda76b94..81c7cdd507 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -56,6 +56,9 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
     assert(vdc->get_features != NULL);
     vdev->host_features = vdc->get_features(vdev, vdev->host_features,
                                             errp);
+    if (klass->post_plugged != NULL) {
+        klass->post_plugged(qbus->parent, errp);
+    }
 }
 
 /* Reset the virtio_bus */
diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
index 6c3d4cb19e..3f2c1363d0 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -60,6 +60,11 @@ typedef struct VirtioBusClass {
      */
     void (*device_plugged)(DeviceState *d, Error **errp);
     /*
+     * Re-evaluate setup after feature bits have been validated
+     * by the device backend.
+     */
+    void (*post_plugged)(DeviceState *d, Error **errp);
+    /*
      * transport independent exit function.
      * This is called by virtio-bus just before the device is unplugged.
      */