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have been queried that can handle the VERSION_1 bit being withdrawn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>

1 files changed, 5 insertions, 0 deletions

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.
      */