virtio-bus: Plug devices after features are negotiated

Currently, devices are plugged before features are negotiated.
If the backend doesn't support VIRTIO_F_VERSION_1, the transport
needs to rewind some settings.

This is the case for CCW, for which a post_plugged callback had
been introduced, where max_rev field is just updated if
VIRTIO_F_VERSION_1 is not supported by the backend.
For PCI, implementing post_plugged would be much more
complicated, so it needs to know whether the backend supports
VIRTIO_F_VERSION_1 at plug time.

Currently, nothing is done for PCI. Modern capabilities get
exposed to the guest even if VIRTIO_F_VERSION_1 is not supported
by the backend, which confuses the guest.

This patch replaces existing post_plugged solution with an
approach that fits with both transports.
Features negotiation is performed before ->device_plugged() call.
A pre_plugged callback is introduced so that the transports can
set their supported features.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org
Tested-by: Cornelia Huck <cornelia.huck@de.ibm.com> [ccw]
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/include/hw/virtio/virtio-bus.h b/include/hw/virtio/virtio-bus.h
index f3e5ef3f5b..24caa0a6e5 100644
--- a/include/hw/virtio/virtio-bus.h
+++ b/include/hw/virtio/virtio-bus.h
@@ -54,16 +54,16 @@ typedef struct VirtioBusClass {
     int (*set_guest_notifiers)(DeviceState *d, int nvqs, bool assign);
     void (*vmstate_change)(DeviceState *d, bool running);
     /*
+     * Expose the features the transport layer supports before
+     * the negotiation takes place.
+     */
+    void (*pre_plugged)(DeviceState *d, Error **errp);
+    /*
      * transport independent init function.
      * This is called by virtio-bus just after the device is plugged.
      */
     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.
      */