Re: [PATCH v3 01/10] qdev/qbus: add hidden device support

[Cornelia Huck <cohuck@redhat.com>]

On Fri, 11 Oct 2019 13:20:06 +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         | 19 +++++++++++++++++++
>  include/hw/qdev-core.h |  9 +++++++++
>  qdev-monitor.c         | 43 ++++++++++++++++++++++++++++++++++++++----
>  vl.c                   |  6 ++++--
>  4 files changed, 71 insertions(+), 6 deletions(-)
> 

(...)

> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index aa123f88cb..b61cf82ded 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -154,6 +154,13 @@ struct DeviceState {
>  struct DeviceListener {
>      void (*realize)(DeviceListener *listener, DeviceState *dev);
>      void (*unrealize)(DeviceListener *listener, DeviceState *dev);
> +    /*
> +     * This callback is called just upon init of the DeviceState
> +     * and can be used by a standby device for informing qdev if this
> +     * device should be hidden by checking the device opts
> +     */

Maybe tweak this comment a bit:

/*
 * 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.
 */

This makes it clearer that this interface could be reused for other
purposes.

> +    void (*should_be_hidden)(DeviceListener *listener, QemuOpts *device_opts,
> +            bool *match_found, bool *res);
>      QTAILQ_ENTRY(DeviceListener) link;
>  };
>  
> @@ -451,4 +458,6 @@ static inline bool qbus_is_hotpluggable(BusState *bus)
>  void device_listener_register(DeviceListener *listener);
>  void device_listener_unregister(DeviceListener *listener);
>  
> +bool qdev_should_hide_device(QemuOpts *opts, Error **errp);
> +
>  #endif
> diff --git a/qdev-monitor.c b/qdev-monitor.c
> index 148df9cacf..9fc8331157 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
> @@ -562,14 +564,45 @@ 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, "net_failover_pair_id") == 0) {
> +        QemuOpts *opts = (QemuOpts *)opaque;
> +
> +        if (qdev_should_hide_device(opts, errp) && errp && !*errp) {
> +            return 1;
> +        } else if (errp && *errp) {
> +            return -1;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +static bool should_hide_device(QemuOpts *opts, Error **err)
> +{
> +    if (qemu_opt_foreach(opts, is_failover_device, opts, err) == 0) {
> +        return false;

Maybe turn that check around? I.e. return true if the failover property
is present, else return false. Makes it easier to add checks for other
properties in the future.

> +    }
> +    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;
>  
> +    if (opts && should_hide_device(opts, &err)) {
> +        if (err) {
> +            goto err_del_dev;
> +        }
> +        return NULL;
> +    }
> +
>      driver = qemu_opt_get(opts, "driver");
>      if (!driver) {
>          error_setg(errp, QERR_MISSING_PARAMETER, "driver");
> @@ -648,8 +681,10 @@ 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;
>  }
>  
> @@ -818,7 +853,7 @@ void qdev_unplug(DeviceState *dev, Error **errp)
>          return;
>      }
>  
> -    if (!migration_is_idle()) {
> +    if (!migration_is_idle() && !migration_in_setup(migrate_get_current())) {

Hm, should that hunk go into another patch?

>          error_setg(errp, "device_del not allowed while migrating");
>          return;
>      }

(...)