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/hw/core/qdev.c b/hw/core/qdev.c
index cbad6c1d55..84fac591ca 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -212,6 +212,25 @@ void device_listener_unregister(DeviceListener *listener)
     QTAILQ_REMOVE(&device_listeners, listener, link);
 }
 
+bool qdev_should_hide_device(QemuOpts *opts, Error **errp)
+{
+    bool res = false;
+    bool match_found = false;
+    DeviceListener *listener;
+
+    QTAILQ_FOREACH(listener, &device_listeners, link) {
+       if (listener->should_be_hidden) {
+            listener->should_be_hidden(listener, opts, &match_found, &res);
+        }
+
+        if (match_found) {
+            break;
+        }
+    }
+
+    return res;
+}
+
 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..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
+     */
+    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;
+    }
+    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())) {
         error_setg(errp, "device_del not allowed while migrating");
         return;
     }
diff --git a/vl.c b/vl.c
index 002bf4919e..898b9901c8 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

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;
>      }

(...)

On Mon, Oct 14, 2019 at 11:46:22AM +0200, Cornelia Huck wrote:
>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.

Sounds good. 
>
>> +    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.

Done.

>> +    }
>> +    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?

Yes it should :)

Thanks for the review!

regards,
Jens 

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/hw/core/qdev.c b/hw/core/qdev.c
> index cbad6c1d55..84fac591ca 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -212,6 +212,25 @@ void device_listener_unregister(DeviceListener *listener)
>      QTAILQ_REMOVE(&device_listeners, listener, link);
>  }
>  
> +bool qdev_should_hide_device(QemuOpts *opts, Error **errp)
> +{
> +    bool res = false;
> +    bool match_found = false;
> +    DeviceListener *listener;
> +
> +    QTAILQ_FOREACH(listener, &device_listeners, link) {
> +       if (listener->should_be_hidden) {
> +            listener->should_be_hidden(listener, opts, &match_found, &res);
> +        }
> +
> +        if (match_found) {
> +            break;
> +        }

Calling convention here seems overly complicated, couldn't
should_be_hidden() just return >0 (should be hidden), 0 (should not be
hidden), <0 (don't care), ie. continue until >=0?  The errp arg is
unused and using "res" to return should/shouldn't hide is very unclear.
The virtio callback renames this to hide, which makes more sense, but
as above, both the stop and hidden state could be conveyed with a
simple int return value.  Thanks,

Alex

> +    }
> +
> +    return res;
> +}
> +
>  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..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
> +     */
> +    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;
> +    }
> +    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())) {
>          error_setg(errp, "device_del not allowed while migrating");
>          return;
>      }
> diff --git a/vl.c b/vl.c
> index 002bf4919e..898b9901c8 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;
>  }
>  

On Tue, Oct 15, 2019 at 01:19:33PM -0600, Alex Williamson wrote:
>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/hw/core/qdev.c b/hw/core/qdev.c
>> index cbad6c1d55..84fac591ca 100644
>> --- a/hw/core/qdev.c
>> +++ b/hw/core/qdev.c
>> @@ -212,6 +212,25 @@ void device_listener_unregister(DeviceListener *listener)
>>      QTAILQ_REMOVE(&device_listeners, listener, link);
>>  }
>>
>> +bool qdev_should_hide_device(QemuOpts *opts, Error **errp)
>> +{
>> +    bool res = false;
>> +    bool match_found = false;
>> +    DeviceListener *listener;
>> +
>> +    QTAILQ_FOREACH(listener, &device_listeners, link) {
>> +       if (listener->should_be_hidden) {
>> +            listener->should_be_hidden(listener, opts, &match_found, &res);
>> +        }
>> +
>> +        if (match_found) {
>> +            break;
>> +        }
>
>Calling convention here seems overly complicated, couldn't
>should_be_hidden() just return >0 (should be hidden), 0 (should not be
>hidden), <0 (don't care), ie. continue until >=0?  The errp arg is
>unused and using "res" to return should/shouldn't hide is very unclear.
>The virtio callback renames this to hide, which makes more sense, but
>as above, both the stop and hidden state could be conveyed with a
>simple int return value.  Thanks,

Yes, this can be simplified. Will be in the next version.

Thanks!

regards,
Jens