Re: [PATCH v6 01/11] qdev/qbus: add hidden device support

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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 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 "failover_pair_id" option, e.g.
> 
>  -device virtio-net-pci,...,failover=on,...
>  -device vfio-pci,...,failover_pair_id=net1,...
> 
> Signed-off-by: Jens Freimann <jfreimann@redhat.com>
> Reviewed-by: Cornelia Huck <cohuck@redhat.com>

I think I see why you've done this, but I'd lay odds on that we're
going to find some odd corners of other things in qemu prodding hidden
devices.  We'll see!

Dave

> ---
>  hw/core/qdev.c         | 24 ++++++++++++++++++++++++
>  include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
>  qdev-monitor.c         | 41 +++++++++++++++++++++++++++++++++++++----
>  vl.c                   |  6 ++++--
>  4 files changed, 94 insertions(+), 6 deletions(-)
> 
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index cbad6c1d55..3b8d43d0fd 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -212,6 +212,30 @@ void device_listener_unregister(DeviceListener *listener)
>      QTAILQ_REMOVE(&device_listeners, listener, link);
>  }
>  
> +bool qdev_should_hide_device(QemuOpts *opts)
> +{
> +    int rc = -1;
> +    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)
>  {
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index aa123f88cb..710981af36 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -78,6 +78,19 @@ typedef void (*BusUnrealize)(BusState *bus, Error **errp);
>   * respective parent types.
>   *   </para>
>   * </note>
> + *
> + * # Hiding a device #
> + * To hide a device, a DeviceListener function should_be_hidden() needs to
> + * be registered.
> + * It can be used to defer adding a device and therefore hide it from the
> + * guest. The handler registering to this DeviceListener can save the QOpts
> + * passed to it for re-using it later and must return that it wants the device
> + * to be/remain hidden or not. When the handler function decides the device
> + * shall not be hidden it will be added in qdev_device_add() and
> + * realized as any other device. Otherwise qdev_device_add() will return early
> + * without adding the device. The guest will not see a "hidden" device
> + * until it was marked don't hide and qdev_device_add called again.
> + *
>   */
>  typedef struct DeviceClass {
>      /*< private >*/
> @@ -154,6 +167,12 @@ struct DeviceState {
>  struct DeviceListener {
>      void (*realize)(DeviceListener *listener, DeviceState *dev);
>      void (*unrealize)(DeviceListener *listener, DeviceState *dev);
> +    /*
> +     * This callback is called upon init of the DeviceState and allows to
> +     * inform qdev that a device should be hidden, depending on the device
> +     * opts, for example, to hide a standby device.
> +     */
> +    int (*should_be_hidden)(DeviceListener *listener, QemuOpts *device_opts);
>      QTAILQ_ENTRY(DeviceListener) link;
>  };
>  
> @@ -451,4 +470,14 @@ static inline bool qbus_is_hotpluggable(BusState *bus)
>  void device_listener_register(DeviceListener *listener);
>  void device_listener_unregister(DeviceListener *listener);
>  
> +/**
> + * @qdev_should_hide_device:
> + * @opts: QemuOpts as passed on cmdline.
> + *
> + * Check if a device should be added.
> + * When a device is added via qdev_device_add() this will be called,
> + * and return if the device should be added now or not.
> + */
> +bool qdev_should_hide_device(QemuOpts *opts);
> +
>  #endif
> diff --git a/qdev-monitor.c b/qdev-monitor.c
> index 148df9cacf..ffa08c670f 100644
> --- a/qdev-monitor.c
> +++ b/qdev-monitor.c
> @@ -32,9 +32,11 @@
>  #include "qemu/help_option.h"
>  #include "qemu/option.h"
>  #include "qemu/qemu-print.h"
> +#include "qemu/option_int.h"
>  #include "sysemu/block-backend.h"
>  #include "sysemu/sysemu.h"
>  #include "migration/misc.h"
> +#include "migration/migration.h"
>  
>  /*
>   * Aliases were a bad idea from the start.  Let's keep them
> @@ -562,13 +564,36 @@ void qdev_set_id(DeviceState *dev, const char *id)
>      }
>  }
>  
> +static int is_failover_device(void *opaque, const char *name, const char *value,
> +                        Error **errp)
> +{
> +    if (strcmp(name, "failover_pair_id") == 0) {
> +        QemuOpts *opts = (QemuOpts *)opaque;
> +
> +        if (qdev_should_hide_device(opts)) {
> +            return 1;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +static bool should_hide_device(QemuOpts *opts)
> +{
> +    if (qemu_opt_foreach(opts, is_failover_device, opts, NULL) == 0) {
> +        return false;
> +    }
> +    return true;
> +}
> +
>  DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
>  {
>      DeviceClass *dc;
>      const char *driver, *path;
> -    DeviceState *dev;
> +    DeviceState *dev = NULL;
>      BusState *bus = NULL;
>      Error *err = NULL;
> +    bool hide;
>  
>      driver = qemu_opt_get(opts, "driver");
>      if (!driver) {
> @@ -602,11 +627,17 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
>              return NULL;
>          }
>      }
> -    if (qdev_hotplug && bus && !qbus_is_hotpluggable(bus)) {
> +    hide = should_hide_device(opts);
> +
> +    if ((hide || qdev_hotplug) && bus && !qbus_is_hotpluggable(bus)) {
>          error_setg(errp, QERR_BUS_NO_HOTPLUG, bus->name);
>          return NULL;
>      }
>  
> +    if (hide) {
> +        return NULL;
> +    }
> +
>      if (!migration_is_idle()) {
>          error_setg(errp, "device_add not allowed while migrating");
>          return NULL;
> @@ -648,8 +679,10 @@ DeviceState *qdev_device_add(QemuOpts *opts, Error **errp)
>  
>  err_del_dev:
>      error_propagate(errp, err);
> -    object_unparent(OBJECT(dev));
> -    object_unref(OBJECT(dev));
> +    if (dev) {
> +        object_unparent(OBJECT(dev));
> +        object_unref(OBJECT(dev));
> +    }
>      return NULL;
>  }
>  
> diff --git a/vl.c b/vl.c
> index 4489cfb2bb..62c388cb49 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -2204,10 +2204,12 @@ static int device_init_func(void *opaque, QemuOpts *opts, Error **errp)
>      DeviceState *dev;
>  
>      dev = qdev_device_add(opts, errp);
> -    if (!dev) {
> +    if (!dev && *errp) {
> +        error_report_err(*errp);
>          return -1;
> +    } else if (dev) {
> +        object_unref(OBJECT(dev));
>      }
> -    object_unref(OBJECT(dev));
>      return 0;
>  }
>  
> -- 
> 2.21.0
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK