int virtio_bus_start_ioeventfd(VirtioBusState *bus)
{
    VirtioBusClass *k = VIRTIO_BUS_GET_CLASS(bus);
    DeviceState *proxy = DEVICE(BUS(bus)->parent);
    VirtIODevice *vdev = virtio_bus_get_device(bus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
    int r;

    if (!k->ioeventfd_assign || !k->ioeventfd_enabled(proxy)) {
        return -ENOSYS;
    }
    if (bus->ioeventfd_started) {
        return 0;
    }

    /* Only set our notifier if we have ownership.  */
    if (!bus->ioeventfd_grabbed) {
        r = vdc->start_ioeventfd(vdev);
        if (r < 0) {
            error_report("%s: failed. Fallback to userspace (slower).", __func__);
            return r;
        }
    }
    bus->ioeventfd_started = true;
    return 0;
}

static int virtio_mmio_set_guest_notifier(DeviceState *d, int n, bool assign,
                                          bool with_irqfd)
{
    VirtIOMMIOProxy *proxy = VIRTIO_MMIO(d);
    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
    VirtQueue *vq = virtio_get_queue(vdev, n);
    EventNotifier *notifier = virtio_queue_get_guest_notifier(vq);

    if (assign) {
        int r = event_notifier_init(notifier, 0);
        if (r < 0) {
            return r;
        }
        virtio_queue_set_guest_notifier_fd_handler(vq, true, with_irqfd);
    } else {
        virtio_queue_set_guest_notifier_fd_handler(vq, false, with_irqfd);
        event_notifier_cleanup(notifier);
    }

    if (vdc->guest_notifier_mask && vdev->use_guest_notifier_mask) {
        vdc->guest_notifier_mask(vdev, n, !assign);
    }

    return 0;
}

/* Get config of the plugged device. */
void virtio_bus_get_vdev_config(VirtioBusState *bus, uint8_t *config)
{
    VirtioDeviceClass *k;
    assert(bus->vdev != NULL);
    k = VIRTIO_DEVICE_GET_CLASS(bus->vdev);
    if (k->get_config != NULL) {
        k->get_config(bus->vdev, config);
    }
}

/* Get the features of the plugged device. */
uint32_t virtio_bus_get_vdev_features(VirtioBusState *bus,
                                    uint32_t requested_features)
{
    VirtioDeviceClass *k;
    assert(bus->vdev != NULL);
    k = VIRTIO_DEVICE_GET_CLASS(bus->vdev);
    assert(k->get_features != NULL);
    return k->get_features(bus->vdev, requested_features);
}

/* Get bad features of the plugged device. */
uint32_t virtio_bus_get_vdev_bad_features(VirtioBusState *bus)
{
    VirtioDeviceClass *k;
    assert(bus->vdev != NULL);
    k = VIRTIO_DEVICE_GET_CLASS(bus->vdev);
    if (k->bad_features != NULL) {
        return k->bad_features(bus->vdev);
    } else {
        return 0;
    }
}

/* Set the features of the plugged device. */
void virtio_bus_set_vdev_features(VirtioBusState *bus,
                                      uint32_t requested_features)
{
    VirtIODevice *vdev = virtio_bus_get_device(bus);
    VirtioDeviceClass *k;

    assert(vdev != NULL);
    k = VIRTIO_DEVICE_GET_CLASS(vdev);
    if (k->set_features != NULL) {
        k->set_features(vdev, requested_features);
    }
}

void virtio_bus_stop_ioeventfd(VirtioBusState *bus)
{
    VirtIODevice *vdev;
    VirtioDeviceClass *vdc;

    if (!bus->ioeventfd_started) {
        return;
    }

    /* Only remove our notifier if we have ownership.  */
    if (!bus->ioeventfd_grabbed) {
        vdev = virtio_bus_get_device(bus);
        vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
        vdc->stop_ioeventfd(vdev);
    }
    bus->ioeventfd_started = false;
}

/* A VirtIODevice is being plugged */
void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
{
    DeviceState *qdev = DEVICE(vdev);
    BusState *qbus = BUS(qdev_get_parent_bus(qdev));
    VirtioBusState *bus = VIRTIO_BUS(qbus);
    VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);
    bool has_iommu = virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM);
    Error *local_err = NULL;

    DPRINTF("%s: plug device.\n", qbus->name);

    if (klass->pre_plugged != NULL) {
        klass->pre_plugged(qbus->parent, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
            return;
        }
    }

    /* Get the features of the plugged device. */
    assert(vdc->get_features != NULL);
    vdev->host_features = vdc->get_features(vdev, vdev->host_features,
                                            &local_err);
    if (local_err) {
        error_propagate(errp, local_err);
        return;
    }

    if (klass->device_plugged != NULL) {
        klass->device_plugged(qbus->parent, &local_err);
    }
    if (local_err) {
        error_propagate(errp, local_err);
        return;
    }

    if (klass->get_dma_as != NULL && has_iommu) {
        virtio_add_feature(&vdev->host_features, VIRTIO_F_IOMMU_PLATFORM);
        vdev->dma_as = klass->get_dma_as(qbus->parent);
    } else {
        vdev->dma_as = &address_space_memory;
    }
}

/* A VirtIODevice is being plugged */
void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
{
    DeviceState *qdev = DEVICE(vdev);
    BusState *qbus = BUS(qdev_get_parent_bus(qdev));
    VirtioBusState *bus = VIRTIO_BUS(qbus);
    VirtioBusClass *klass = VIRTIO_BUS_GET_CLASS(bus);
    VirtioDeviceClass *vdc = VIRTIO_DEVICE_GET_CLASS(vdev);

    DPRINTF("%s: plug device.\n", qbus->name);

    if (klass->device_plugged != NULL) {
        klass->device_plugged(qbus->parent, errp);
    }

    /* Get the features of the plugged device. */
    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);
    }
}

static int virtio_ccw_set_guest_notifier(VirtioCcwDevice *dev, int n,
                                         bool assign, bool with_irqfd)
{
    VirtQueue *vq = virtio_get_queue(dev->vdev, n);
    EventNotifier *notifier = virtio_queue_get_guest_notifier(vq);
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(dev->vdev);

    if (assign) {
        int r = event_notifier_init(notifier, 0);

        if (r < 0) {
            return r;
        }
        virtio_queue_set_guest_notifier_fd_handler(vq, true, with_irqfd);
        /* We do not support irqfd for classic I/O interrupts, because the
         * classic interrupts are intermixed with the subchannel status, that
         * is queried with test subchannel. We want to use vhost, though.
         * Lets make sure to have vhost running and wire up the irq fd to
         * land in qemu (and only the irq fd) in this code.
         */
        if (k->guest_notifier_mask) {
            k->guest_notifier_mask(dev->vdev, n, false);
        }
        /* get lost events and re-inject */
        if (k->guest_notifier_pending &&
            k->guest_notifier_pending(dev->vdev, n)) {
            event_notifier_set(notifier);
        }
    } else {
        if (k->guest_notifier_mask) {
            k->guest_notifier_mask(dev->vdev, n, true);
        }
        virtio_queue_set_guest_notifier_fd_handler(vq, false, with_irqfd);
        event_notifier_cleanup(notifier);
    }
    return 0;
}