Re: [PATCH 01/11] qdev/qbus: add hidden device support

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 18 Oct 2019 22:20:30 +0200
Jens Freimann <jfreimann@redhat.com> wrote:

> This adds support for hiding a device to the qbus and qdev APIs.  The
> first user of this will be the virtio-net failover feature but the API
> introduced with this patch could be used to implement other features as
> well, for example hiding pci devices when a pci bus is powered off.
> 
> qdev_device_add() is modified to check for a net_failover_pair_id
> argument in the option string. A DeviceListener callback
> should_be_hidden() is added. It can be used by a standby device to
> inform qdev that this device should not be added now. The standby device
> handler can store the device options to plug the device in at a later
> point in time.
> 
> One reason for hiding the device is that we don't want to expose both
> devices to the guest kernel until the respective virtio feature bit
> VIRTIO_NET_F_STANDBY was negotiated and we know that the devices will be
> handled correctly by the guest.
> 
> More information on the kernel feature this is using:
>  https://www.kernel.org/doc/html/latest/networking/net_failover.html
> 
> An example where the primary device is a vfio-pci device and the standby
> device is a virtio-net device:
> 
> A device is hidden when it has an "net_failover_pair_id" option, e.g.
> 
>  -device virtio-net-pci,...,failover=on,...
>  -device vfio-pci,...,net_failover_pair_id=net1,...
> 
> Signed-off-by: Jens Freimann <jfreimann@redhat.com>
> ---
>  hw/core/qdev.c         | 23 +++++++++++++++++++++++
>  include/hw/qdev-core.h |  8 ++++++++
>  qdev-monitor.c         | 36 +++++++++++++++++++++++++++++++++---
>  vl.c                   |  6 ++++--
>  4 files changed, 68 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index cbad6c1d55..89c134ec53 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -212,6 +212,29 @@ void device_listener_unregister(DeviceListener *listener)
>      QTAILQ_REMOVE(&device_listeners, listener, link);
>  }
>  
> +bool qdev_should_hide_device(QemuOpts *opts)
> +{
> +    int rc;

Initialize to 0?

> +    DeviceListener *listener;
> +
> +    QTAILQ_FOREACH(listener, &device_listeners, link) {
> +       if (listener->should_be_hidden) {
> +            /* should_be_hidden_will return
> +             *  1 if device matches opts and it should be hidden
> +             *  0 if device matches opts and should not be hidden
> +             *  -1 if device doesn't match ops
> +             */
> +            rc = listener->should_be_hidden(listener, opts);
> +        }
> +
> +        if (rc > 0) {
> +            break;
> +        }
> +    }
> +
> +    return rc > 0;
> +}
> +
>  void qdev_set_legacy_instance_id(DeviceState *dev, int alias_id,
>                                   int required_for_version)
>  {

(...)

> +static bool should_hide_device(QemuOpts *opts)
> +{
> +    if (qemu_opt_foreach(opts, is_failover_device, opts, NULL) == 0) {
> +        return false;
> +    }
> +    return true;
> +}

I still think you should turn the check around to make it easier to
extend in the future, but this is fine as well.

(...)

With the rc thing changed,

Reviewed-by: Cornelia Huck <cohuck@redhat.com>