Re: [PATCH v9 2/8] qapi: introduce query-backend-transfer-support

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> We are going to implement backend-transfer feature: some devices
> will be able to transfer their backend through migration stream
> for local migration through UNIX domain socket. For example,
> virtio-net will migrate its attached TAP netdev, with all its
> connected file descriptors.
>
> Let's add a command to list supporting devices (no one for now),
> together with necessary infrastructure in qdev code.

Use case?

> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
>  include/hw/qdev-core.h |  7 +++++++
>  qapi/qdev.json         | 26 +++++++++++++++++++++++++
>  system/qdev-monitor.c  | 43 ++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 76 insertions(+)
>
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index 2caa0cbd26..0551fbaa6c 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -96,6 +96,7 @@ typedef void (*DeviceReset)(DeviceState *dev);
>  typedef void (*BusRealize)(BusState *bus, Error **errp);
>  typedef void (*BusUnrealize)(BusState *bus);
>  typedef int (*DeviceSyncConfig)(DeviceState *dev, Error **errp);
> +typedef bool (*DeviceSupportBackendTransfer)(DeviceState *dev, Error **errp);
>  
>  /**
>   * struct DeviceClass - The base class for all devices.
> @@ -174,6 +175,12 @@ struct DeviceClass {
>      DeviceUnrealize unrealize;
>      DeviceSyncConfig sync_config;
>  
> +    /**
> +     * @backend_transfer_support: reports support for backend-transfer
> +     * migration of the device.
> +     */
> +    DeviceSupportBackendTransfer backend_transfer_support;
> +
>      /**
>       * @vmsd: device state serialisation description for
>       * migration/save/restore
> diff --git a/qapi/qdev.json b/qapi/qdev.json
> index e14a0c9259..d7e878d58d 100644
> --- a/qapi/qdev.json
> +++ b/qapi/qdev.json
> @@ -188,3 +188,29 @@
>  { 'command': 'device-sync-config',
>    'features': [ 'unstable' ],
>    'data': {'id': 'str'} }
> +
> +##
> +# @DevPath:
> +#
> +# @path: the device's QOM path
> +#
> +# Since: 10.2
> +##
> +{ 'struct': 'DevPath',
> +  'data': { 'path': 'str' } }
> +
> +##
> +# @query-backend-transfer-support:
> +#
> +# Returns list of devices, supporting backend-transfer
> +# migration.

Suggest

   # Return the devices that support backend-transfer migration.

> +#
> +# Features:
> +#
> +# @unstable: The command is experimental.

The conventional text ist "This command is experimental."

> +#
> +# Since: 10.2
> +##
> +{ 'command': 'query-backend-transfer-support',
> +  'features': [ 'unstable' ],
> +  'returns': [ 'DevPath' ] }
> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
> index ec4a2394ce..9d3d961c15 100644
> --- a/system/qdev-monitor.c
> +++ b/system/qdev-monitor.c
> @@ -939,6 +939,49 @@ void qmp_device_del(const char *id, Error **errp)
>      }
>  }
>  
> +static bool qdev_backend_transfer_support(DeviceState *dev, Error **errp)
> +{
> +    DeviceClass *dc = DEVICE_GET_CLASS(dev);
> +
> +    if (!dc->backend_transfer_support) {
> +        error_setg(errp, "backend-transfer is not supported for '%s'",
> +                   object_get_typename(OBJECT(dev)));
> +        return false;
> +    }
> +
> +    return dc->backend_transfer_support(dev, errp);
> +}

@errp is useless in this patch: the only caller passes NULL.  Took me a
minute to check it doesn't remain useless.  The next patch puts it to
use.

> +
> +static int qdev_add_if_backend_transfer_supported(Object *obj, void *opaque)
> +{
> +    DevPathList **list = opaque;
> +    DeviceState *dev = (DeviceState *)object_dynamic_cast(obj, TYPE_DEVICE);
> +
> +    if (dev != NULL && qdev_backend_transfer_support(dev, NULL)) {
> +        DevPath *el = g_new(DevPath, 1);
> +        *el = (DevPath) {
> +            .path = g_strdup(dev->canonical_path),
> +        };
> +        QAPI_LIST_PREPEND(*list, el);
> +    }
> +
> +    /* Recursively check all children */
> +    object_child_foreach(obj, qdev_add_if_backend_transfer_supported, opaque);
> +
> +    return 0;
> +}
> +
> +DevPathList *qmp_query_backend_transfer_support(Error **errp)
> +{
> +    DevPathList *result = NULL;
> +    Object *peripheral = machine_get_container("peripheral");
> +
> +    object_child_foreach(peripheral, qdev_add_if_backend_transfer_supported,
> +                         &result);
> +
> +    return result;
> +}
> +
>  int qdev_sync_config(DeviceState *dev, Error **errp)
>  {
>      DeviceClass *dc = DEVICE_GET_CLASS(dev);