Re: [PATCH v4 3/3] qapi: introduce device-sync-config

[Markus Armbruster <armbru@redhat.com>]

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

> Add command to sync config from vhost-user backend to the device. It
> may be helpful when VHOST_USER_SLAVE_CONFIG_CHANGE_MSG failed or not
> triggered interrupt to the guest or just not available (not supported
> by vhost-user server).
>
> Command result is racy if allow it during migration. Let's allow the
> sync only in RUNNING state.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> ---
>  hw/block/vhost-user-blk.c |  1 +
>  hw/virtio/virtio-pci.c    |  9 ++++++++
>  include/hw/qdev-core.h    |  3 +++
>  qapi/qdev.json            | 23 +++++++++++++++++++
>  system/qdev-monitor.c     | 48 +++++++++++++++++++++++++++++++++++++++
>  5 files changed, 84 insertions(+)
>
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 091d2c6acf..2f301f380c 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -588,6 +588,7 @@ static void vhost_user_blk_class_init(ObjectClass *klass, void *data)
>  
>      device_class_set_props(dc, vhost_user_blk_properties);
>      dc->vmsd = &vmstate_vhost_user_blk;
> +    dc->sync_config = vhost_user_blk_sync_config;
>      set_bit(DEVICE_CATEGORY_STORAGE, dc->categories);
>      vdc->realize = vhost_user_blk_device_realize;
>      vdc->unrealize = vhost_user_blk_device_unrealize;
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index b1d02f4b3d..0d91e8b5dc 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -2351,6 +2351,14 @@ static void virtio_pci_dc_realize(DeviceState *qdev, Error **errp)
>      vpciklass->parent_dc_realize(qdev, errp);
>  }
>  
> +static int virtio_pci_sync_config(DeviceState *dev, Error **errp)
> +{
> +    VirtIOPCIProxy *proxy = VIRTIO_PCI(dev);
> +    VirtIODevice *vdev = virtio_bus_get_device(&proxy->bus);
> +
> +    return qdev_sync_config(DEVICE(vdev), errp);
> +}
> +
>  static void virtio_pci_class_init(ObjectClass *klass, void *data)
>  {
>      DeviceClass *dc = DEVICE_CLASS(klass);
> @@ -2367,6 +2375,7 @@ static void virtio_pci_class_init(ObjectClass *klass, void *data)
>      device_class_set_parent_realize(dc, virtio_pci_dc_realize,
>                                      &vpciklass->parent_dc_realize);
>      rc->phases.hold = virtio_pci_bus_reset_hold;
> +    dc->sync_config = virtio_pci_sync_config;
>  }
>  
>  static const TypeInfo virtio_pci_info = {
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index 9228e96c87..87135bdcdf 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -95,6 +95,7 @@ typedef void (*DeviceUnrealize)(DeviceState *dev);
>  typedef void (*DeviceReset)(DeviceState *dev);
>  typedef void (*BusRealize)(BusState *bus, Error **errp);
>  typedef void (*BusUnrealize)(BusState *bus);
> +typedef int (*DeviceSyncConfig)(DeviceState *dev, Error **errp);
>  
>  /**
>   * struct DeviceClass - The base class for all devices.
> @@ -162,6 +163,7 @@ struct DeviceClass {
>      DeviceReset reset;
>      DeviceRealize realize;
>      DeviceUnrealize unrealize;
> +    DeviceSyncConfig sync_config;
>  
>      /**
>       * @vmsd: device state serialisation description for
> @@ -546,6 +548,7 @@ bool qdev_hotplug_allowed(DeviceState *dev, Error **errp);
>   */
>  HotplugHandler *qdev_get_hotplug_handler(DeviceState *dev);
>  void qdev_unplug(DeviceState *dev, Error **errp);
> +int qdev_sync_config(DeviceState *dev, Error **errp);
>  void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
>                                    DeviceState *dev, Error **errp);
>  void qdev_machine_creation_done(void);
> diff --git a/qapi/qdev.json b/qapi/qdev.json
> index facaa0bc6a..fc5e125a45 100644
> --- a/qapi/qdev.json
> +++ b/qapi/qdev.json
> @@ -161,3 +161,26 @@
>  ##
>  { 'event': 'DEVICE_UNPLUG_GUEST_ERROR',
>    'data': { '*device': 'str', 'path': 'str' } }
> +
> +##
> +# @device-sync-config:
> +#
> +# Synchronize device configuration from host to guest part.  First,
> +# copy the configuration from the host part (backend) to the guest
> +# part (frontend).  Then notify guest software that device
> +# configuration changed.

Blank line here, please.

> +# The command may be used to notify the guest about block device
> +# capcity change.  Currently only vhost-user-blk device supports
> +# this.
> +#
> +# @id: the device's ID or QOM path
> +#
> +# Features:
> +#
> +# @unstable: The command is experimental.
> +#
> +# Since: 9.1
> +##
> +{ 'command': 'device-sync-config',
> +  'features': [ 'unstable' ],
> +  'data': {'id': 'str'} }
> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
> index 264978aa40..47bfc0506e 100644
> --- a/system/qdev-monitor.c
> +++ b/system/qdev-monitor.c
> @@ -23,6 +23,7 @@
>  #include "monitor/monitor.h"
>  #include "monitor/qdev.h"
>  #include "sysemu/arch_init.h"
> +#include "sysemu/runstate.h"
>  #include "qapi/error.h"
>  #include "qapi/qapi-commands-qdev.h"
>  #include "qapi/qmp/dispatch.h"
> @@ -971,6 +972,53 @@ void qmp_device_del(const char *id, Error **errp)
>      }
>  }
>  
> +int qdev_sync_config(DeviceState *dev, Error **errp)
> +{
> +    DeviceClass *dc = DEVICE_GET_CLASS(dev);
> +
> +    if (!dc->sync_config) {
> +        error_setg(errp, "device-sync-config is not supported for '%s'",
> +                   object_get_typename(OBJECT(dev)));
> +        return -ENOTSUP;
> +    }
> +
> +    return dc->sync_config(dev, errp);
> +}
> +
> +void qmp_device_sync_config(const char *id, Error **errp)
> +{
> +    DeviceState *dev;
> +
> +    /*
> +     * During migration there is a race between syncing`configuration and
> +     * migrating it (if migrate first, that target would get outdated version),
> +     * so let's just not allow it.

Wrap comment lines around column 70 for legibility, please.

> +     *
> +     * Moreover, let's not rely on setting up interrupts in paused
> +     * state, which may be a part of migration process.

We discussed this in review of v3.  You wanted to check whether the
problem is real.  Is it?

> +     */
> +
> +    if (migration_is_running()) {
> +        error_setg(errp, "Config synchronization is not allowed "
> +                   "during migration");
> +        return;
> +    }
> +
> +    if (!runstate_is_running()) {
> +        error_setg(errp, "Config synchronization allowed only in '%s' state, "

Suggest a line break after errp,

> +                   "current state is '%s'", RunState_str(RUN_STATE_RUNNING),
> +                   RunState_str(runstate_get()));
> +        return;
> +    }
> +
> +    dev = find_device_state(id, true, errp);
> +    if (!dev) {
> +        return;
> +    }
> +
> +    qdev_sync_config(dev, errp);
> +}
> +
>  void hmp_device_add(Monitor *mon, const QDict *qdict)
>  {
>      Error *err = NULL;