[v8] vfio-user server in QEMU

[PATCH v8 02/17] qdev: unplug blocker for devices

Posted by Jagannathan Raman 3 years, 9 months ago

Add blocker to prevent hot-unplug of devices

Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
---
 include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
 hw/core/qdev.c         | 24 ++++++++++++++++++++++++
 softmmu/qdev-monitor.c |  4 ++++
 3 files changed, 57 insertions(+)

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 92c3d65208..1b9fa25e5c 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -193,6 +193,7 @@ struct DeviceState {
     int instance_id_alias;
     int alias_required_for_version;
     ResettableState reset;
+    GSList *unplug_blockers;
 };
 
 struct DeviceListener {
@@ -419,6 +420,34 @@ void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
 void qdev_machine_creation_done(void);
 bool qdev_machine_modified(void);
 
+/*
+ * qdev_add_unplug_blocker: Adds an unplug blocker to a device
+ *
+ * @dev: Device to be blocked from unplug
+ * @reason: Reason for blocking
+ */
+void qdev_add_unplug_blocker(DeviceState *dev, Error *reason);
+
+/*
+ * qdev_del_unplug_blocker: Removes an unplug blocker from a device
+ *
+ * @dev: Device to be unblocked
+ * @reason: Pointer to the Error used with qdev_add_unplug_blocker.
+ *          Used as a handle to lookup the blocker for deletion.
+ */
+void qdev_del_unplug_blocker(DeviceState *dev, Error *reason);
+
+/*
+ * qdev_unplug_blocked: Confirms if a device is blocked from unplug
+ *
+ * @dev: Device to be tested
+ * @reason: Returns one of the reasons why the device is blocked,
+ *          if any
+ *
+ * Returns: true if device is blocked from unplug, false otherwise
+ */
+bool qdev_unplug_blocked(DeviceState *dev, Error **errp);
+
 /**
  * GpioPolarity: Polarity of a GPIO line
  *
diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 84f3019440..0806d8fcaa 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -468,6 +468,28 @@ char *qdev_get_dev_path(DeviceState *dev)
     return NULL;
 }
 
+void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
+{
+    dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);
+}
+
+void qdev_del_unplug_blocker(DeviceState *dev, Error *reason)
+{
+    dev->unplug_blockers = g_slist_remove(dev->unplug_blockers, reason);
+}
+
+bool qdev_unplug_blocked(DeviceState *dev, Error **errp)
+{
+    ERRP_GUARD();
+
+    if (dev->unplug_blockers) {
+        error_propagate(errp, error_copy(dev->unplug_blockers->data));
+        return true;
+    }
+
+    return false;
+}
+
 static bool device_get_realized(Object *obj, Error **errp)
 {
     DeviceState *dev = DEVICE(obj);
@@ -704,6 +726,8 @@ static void device_finalize(Object *obj)
 
     DeviceState *dev = DEVICE(obj);
 
+    g_assert(!dev->unplug_blockers);
+
     QLIST_FOREACH_SAFE(ngl, &dev->gpios, node, next) {
         QLIST_REMOVE(ngl, node);
         qemu_free_irqs(ngl->in, ngl->num_in);
diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
index 12fe60c467..9cfd59d17c 100644
--- a/softmmu/qdev-monitor.c
+++ b/softmmu/qdev-monitor.c
@@ -898,6 +898,10 @@ void qdev_unplug(DeviceState *dev, Error **errp)
     HotplugHandlerClass *hdc;
     Error *local_err = NULL;
 
+    if (qdev_unplug_blocked(dev, errp)) {
+        return;
+    }
+
     if (dev->parent_bus && !qbus_is_hotpluggable(dev->parent_bus)) {
         error_setg(errp, QERR_BUS_NO_HOTPLUG, dev->parent_bus->name);
         return;
-- 
2.20.1

Re: [PATCH v8 02/17] qdev: unplug blocker for devices

Posted by Markus Armbruster 3 years, 9 months ago

Jagannathan Raman <jag.raman@oracle.com> writes:

> Add blocker to prevent hot-unplug of devices

Why do you need this?  I'm not doubting you do, I just want to read your
reasons here :)

>
> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
> ---
>  include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
>  hw/core/qdev.c         | 24 ++++++++++++++++++++++++
>  softmmu/qdev-monitor.c |  4 ++++
>  3 files changed, 57 insertions(+)
>
> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
> index 92c3d65208..1b9fa25e5c 100644
> --- a/include/hw/qdev-core.h
> +++ b/include/hw/qdev-core.h
> @@ -193,6 +193,7 @@ struct DeviceState {
>      int instance_id_alias;
>      int alias_required_for_version;
>      ResettableState reset;
> +    GSList *unplug_blockers;
>  };
>  
>  struct DeviceListener {
> @@ -419,6 +420,34 @@ void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
>  void qdev_machine_creation_done(void);
>  bool qdev_machine_modified(void);
>  
> +/*
> + * qdev_add_unplug_blocker: Adds an unplug blocker to a device
> + *
> + * @dev: Device to be blocked from unplug
> + * @reason: Reason for blocking
> + */
> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason);
> +
> +/*
> + * qdev_del_unplug_blocker: Removes an unplug blocker from a device
> + *
> + * @dev: Device to be unblocked
> + * @reason: Pointer to the Error used with qdev_add_unplug_blocker.
> + *          Used as a handle to lookup the blocker for deletion.
> + */
> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason);
> +
> +/*
> + * qdev_unplug_blocked: Confirms if a device is blocked from unplug
> + *
> + * @dev: Device to be tested
> + * @reason: Returns one of the reasons why the device is blocked,
> + *          if any
> + *
> + * Returns: true if device is blocked from unplug, false otherwise
> + */
> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp);
> +
>  /**
>   * GpioPolarity: Polarity of a GPIO line
>   *
> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
> index 84f3019440..0806d8fcaa 100644
> --- a/hw/core/qdev.c
> +++ b/hw/core/qdev.c
> @@ -468,6 +468,28 @@ char *qdev_get_dev_path(DeviceState *dev)
>      return NULL;
>  }
>  
> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
> +{
> +    dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);
> +}
> +
> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason)
> +{
> +    dev->unplug_blockers = g_slist_remove(dev->unplug_blockers, reason);
> +}
> +
> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp)
> +{
> +    ERRP_GUARD();
> +
> +    if (dev->unplug_blockers) {
> +        error_propagate(errp, error_copy(dev->unplug_blockers->data));
> +        return true;
> +    }
> +
> +    return false;
> +}

This cites the most recently added blocker as reason.  Your function
comment covers it: "Returns one of the reasons".  Okay.

> +
>  static bool device_get_realized(Object *obj, Error **errp)
>  {
>      DeviceState *dev = DEVICE(obj);
> @@ -704,6 +726,8 @@ static void device_finalize(Object *obj)
>  
>      DeviceState *dev = DEVICE(obj);
>  
> +    g_assert(!dev->unplug_blockers);
> +
>      QLIST_FOREACH_SAFE(ngl, &dev->gpios, node, next) {
>          QLIST_REMOVE(ngl, node);
>          qemu_free_irqs(ngl->in, ngl->num_in);
> diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
> index 12fe60c467..9cfd59d17c 100644
> --- a/softmmu/qdev-monitor.c
> +++ b/softmmu/qdev-monitor.c
> @@ -898,6 +898,10 @@ void qdev_unplug(DeviceState *dev, Error **errp)
>      HotplugHandlerClass *hdc;
>      Error *local_err = NULL;
>  
> +    if (qdev_unplug_blocked(dev, errp)) {
> +        return;
> +    }
> +
>      if (dev->parent_bus && !qbus_is_hotpluggable(dev->parent_bus)) {
>          error_setg(errp, QERR_BUS_NO_HOTPLUG, dev->parent_bus->name);
>          return;

Re: [PATCH v8 02/17] qdev: unplug blocker for devices

Posted by Jag Raman 3 years, 9 months ago


> On Apr 21, 2022, at 10:55 AM, Markus Armbruster <armbru@redhat.com> wrote:
> 
> Jagannathan Raman <jag.raman@oracle.com> writes:
> 
>> Add blocker to prevent hot-unplug of devices
> 
> Why do you need this?  I'm not doubting you do, I just want to read your
> reasons here :)

Hi Markus, :)

The x-vfio-user-server depends on an attached PCIDevice. As long as x-vfio-user-server
is used, we don’t want the PCIDevice to be unplugged. This blocker prevents an user
from removing PCIDevice while the vfio-user server is in use.

> 
>> 
>> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
>> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
>> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>

I recall receiving a “Reviewed-by” from Stefan previously.

I’m very sorry I didn’t add that here. I’ll go over all the patches once again to confirm that
the “Reviewed-by” status reflects accurately.

>> ---
>> include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
>> hw/core/qdev.c         | 24 ++++++++++++++++++++++++
>> softmmu/qdev-monitor.c |  4 ++++
>> 3 files changed, 57 insertions(+)
>> 
>> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
>> index 92c3d65208..1b9fa25e5c 100644
>> --- a/include/hw/qdev-core.h
>> +++ b/include/hw/qdev-core.h
>> @@ -193,6 +193,7 @@ struct DeviceState {
>>     int instance_id_alias;
>>     int alias_required_for_version;
>>     ResettableState reset;
>> +    GSList *unplug_blockers;
>> };
>> 
>> struct DeviceListener {
>> @@ -419,6 +420,34 @@ void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
>> void qdev_machine_creation_done(void);
>> bool qdev_machine_modified(void);
>> 
>> +/*
>> + * qdev_add_unplug_blocker: Adds an unplug blocker to a device
>> + *
>> + * @dev: Device to be blocked from unplug
>> + * @reason: Reason for blocking
>> + */
>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason);
>> +
>> +/*
>> + * qdev_del_unplug_blocker: Removes an unplug blocker from a device
>> + *
>> + * @dev: Device to be unblocked
>> + * @reason: Pointer to the Error used with qdev_add_unplug_blocker.
>> + *          Used as a handle to lookup the blocker for deletion.
>> + */
>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason);
>> +
>> +/*
>> + * qdev_unplug_blocked: Confirms if a device is blocked from unplug
>> + *
>> + * @dev: Device to be tested
>> + * @reason: Returns one of the reasons why the device is blocked,
>> + *          if any
>> + *
>> + * Returns: true if device is blocked from unplug, false otherwise
>> + */
>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp);
>> +
>> /**
>>  * GpioPolarity: Polarity of a GPIO line
>>  *
>> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>> index 84f3019440..0806d8fcaa 100644
>> --- a/hw/core/qdev.c
>> +++ b/hw/core/qdev.c
>> @@ -468,6 +468,28 @@ char *qdev_get_dev_path(DeviceState *dev)
>>     return NULL;
>> }
>> 
>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
>> +{
>> +    dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);
>> +}
>> +
>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason)
>> +{
>> +    dev->unplug_blockers = g_slist_remove(dev->unplug_blockers, reason);
>> +}
>> +
>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp)
>> +{
>> +    ERRP_GUARD();
>> +
>> +    if (dev->unplug_blockers) {
>> +        error_propagate(errp, error_copy(dev->unplug_blockers->data));
>> +        return true;
>> +    }
>> +
>> +    return false;
>> +}
> 
> This cites the most recently added blocker as reason.  Your function
> comment covers it: "Returns one of the reasons".  Okay.

I could change the comment to say that it returns the recently added reason.

Thank you!
--
Jag

> 
>> +
>> static bool device_get_realized(Object *obj, Error **errp)
>> {
>>     DeviceState *dev = DEVICE(obj);
>> @@ -704,6 +726,8 @@ static void device_finalize(Object *obj)
>> 
>>     DeviceState *dev = DEVICE(obj);
>> 
>> +    g_assert(!dev->unplug_blockers);
>> +
>>     QLIST_FOREACH_SAFE(ngl, &dev->gpios, node, next) {
>>         QLIST_REMOVE(ngl, node);
>>         qemu_free_irqs(ngl->in, ngl->num_in);
>> diff --git a/softmmu/qdev-monitor.c b/softmmu/qdev-monitor.c
>> index 12fe60c467..9cfd59d17c 100644
>> --- a/softmmu/qdev-monitor.c
>> +++ b/softmmu/qdev-monitor.c
>> @@ -898,6 +898,10 @@ void qdev_unplug(DeviceState *dev, Error **errp)
>>     HotplugHandlerClass *hdc;
>>     Error *local_err = NULL;
>> 
>> +    if (qdev_unplug_blocked(dev, errp)) {
>> +        return;
>> +    }
>> +
>>     if (dev->parent_bus && !qbus_is_hotpluggable(dev->parent_bus)) {
>>         error_setg(errp, QERR_BUS_NO_HOTPLUG, dev->parent_bus->name);
>>         return;
>

Re: [PATCH v8 02/17] qdev: unplug blocker for devices

Posted by Markus Armbruster 3 years, 9 months ago

Jag Raman <jag.raman@oracle.com> writes:

>> On Apr 21, 2022, at 10:55 AM, Markus Armbruster <armbru@redhat.com> wrote:
>> 
>> Jagannathan Raman <jag.raman@oracle.com> writes:
>> 
>>> Add blocker to prevent hot-unplug of devices
>> 
>> Why do you need this?  I'm not doubting you do, I just want to read your
>> reasons here :)
>
> Hi Markus, :)
>
> The x-vfio-user-server depends on an attached PCIDevice. As long as x-vfio-user-server
> is used, we don’t want the PCIDevice to be unplugged. This blocker prevents an user
> from removing PCIDevice while the vfio-user server is in use.

Please work that into your commit message.  Perhaps along the lines of

    One of the next commits will do <stuff>.  <badness> will happen when
    the PCI device is unplugged.  Create the means to prevent that.

>>> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
>>> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
>>> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
>
> I recall receiving a “Reviewed-by” from Stefan previously.
>
> I’m very sorry I didn’t add that here. I’ll go over all the patches once again to confirm that
> the “Reviewed-by” status reflects accurately.
>
>>> ---
>>> include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
>>> hw/core/qdev.c         | 24 ++++++++++++++++++++++++
>>> softmmu/qdev-monitor.c |  4 ++++
>>> 3 files changed, 57 insertions(+)
>>> 
>>> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
>>> index 92c3d65208..1b9fa25e5c 100644
>>> --- a/include/hw/qdev-core.h
>>> +++ b/include/hw/qdev-core.h
>>> @@ -193,6 +193,7 @@ struct DeviceState {
>>>     int instance_id_alias;
>>>     int alias_required_for_version;
>>>     ResettableState reset;
>>> +    GSList *unplug_blockers;
>>> };
>>> 
>>> struct DeviceListener {
>>> @@ -419,6 +420,34 @@ void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
>>> void qdev_machine_creation_done(void);
>>> bool qdev_machine_modified(void);
>>> 
>>> +/*
>>> + * qdev_add_unplug_blocker: Adds an unplug blocker to a device
>>> + *
>>> + * @dev: Device to be blocked from unplug
>>> + * @reason: Reason for blocking
>>> + */
>>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason);
>>> +
>>> +/*
>>> + * qdev_del_unplug_blocker: Removes an unplug blocker from a device
>>> + *
>>> + * @dev: Device to be unblocked
>>> + * @reason: Pointer to the Error used with qdev_add_unplug_blocker.
>>> + *          Used as a handle to lookup the blocker for deletion.
>>> + */
>>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason);
>>> +
>>> +/*
>>> + * qdev_unplug_blocked: Confirms if a device is blocked from unplug
>>> + *
>>> + * @dev: Device to be tested
>>> + * @reason: Returns one of the reasons why the device is blocked,
>>> + *          if any
>>> + *
>>> + * Returns: true if device is blocked from unplug, false otherwise
>>> + */
>>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp);
>>> +
>>> /**
>>>  * GpioPolarity: Polarity of a GPIO line
>>>  *
>>> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>>> index 84f3019440..0806d8fcaa 100644
>>> --- a/hw/core/qdev.c
>>> +++ b/hw/core/qdev.c
>>> @@ -468,6 +468,28 @@ char *qdev_get_dev_path(DeviceState *dev)
>>>     return NULL;
>>> }
>>> 
>>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
>>> +{
>>> +    dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);
>>> +}
>>> +
>>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason)
>>> +{
>>> +    dev->unplug_blockers = g_slist_remove(dev->unplug_blockers, reason);
>>> +}
>>> +
>>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp)
>>> +{
>>> +    ERRP_GUARD();
>>> +
>>> +    if (dev->unplug_blockers) {
>>> +        error_propagate(errp, error_copy(dev->unplug_blockers->data));
>>> +        return true;
>>> +    }
>>> +
>>> +    return false;
>>> +}
>> 
>> This cites the most recently added blocker as reason.  Your function
>> comment covers it: "Returns one of the reasons".  Okay.
>
> I could change the comment to say that it returns the recently added reason.

Up to you.

Re: [PATCH v8 02/17] qdev: unplug blocker for devices

Posted by Jag Raman 3 years, 9 months ago


> On Apr 22, 2022, at 1:18 AM, Markus Armbruster <armbru@redhat.com> wrote:
> 
> Jag Raman <jag.raman@oracle.com> writes:
> 
>>> On Apr 21, 2022, at 10:55 AM, Markus Armbruster <armbru@redhat.com> wrote:
>>> 
>>> Jagannathan Raman <jag.raman@oracle.com> writes:
>>> 
>>>> Add blocker to prevent hot-unplug of devices
>>> 
>>> Why do you need this?  I'm not doubting you do, I just want to read your
>>> reasons here :)
>> 
>> Hi Markus, :)
>> 
>> The x-vfio-user-server depends on an attached PCIDevice. As long as x-vfio-user-server
>> is used, we don’t want the PCIDevice to be unplugged. This blocker prevents an user
>> from removing PCIDevice while the vfio-user server is in use.
> 
> Please work that into your commit message.  Perhaps along the lines of
> 
>    One of the next commits will do <stuff>.  <badness> will happen when
>    the PCI device is unplugged.  Create the means to prevent that.

OK, will do.

Thank you!
--
Jag

> 
>>>> Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>
>>>> Signed-off-by: John G Johnson <john.g.johnson@oracle.com>
>>>> Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
>> 
>> I recall receiving a “Reviewed-by” from Stefan previously.
>> 
>> I’m very sorry I didn’t add that here. I’ll go over all the patches once again to confirm that
>> the “Reviewed-by” status reflects accurately.
>> 
>>>> ---
>>>> include/hw/qdev-core.h | 29 +++++++++++++++++++++++++++++
>>>> hw/core/qdev.c         | 24 ++++++++++++++++++++++++
>>>> softmmu/qdev-monitor.c |  4 ++++
>>>> 3 files changed, 57 insertions(+)
>>>> 
>>>> diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
>>>> index 92c3d65208..1b9fa25e5c 100644
>>>> --- a/include/hw/qdev-core.h
>>>> +++ b/include/hw/qdev-core.h
>>>> @@ -193,6 +193,7 @@ struct DeviceState {
>>>>    int instance_id_alias;
>>>>    int alias_required_for_version;
>>>>    ResettableState reset;
>>>> +    GSList *unplug_blockers;
>>>> };
>>>> 
>>>> struct DeviceListener {
>>>> @@ -419,6 +420,34 @@ void qdev_simple_device_unplug_cb(HotplugHandler *hotplug_dev,
>>>> void qdev_machine_creation_done(void);
>>>> bool qdev_machine_modified(void);
>>>> 
>>>> +/*
>>>> + * qdev_add_unplug_blocker: Adds an unplug blocker to a device
>>>> + *
>>>> + * @dev: Device to be blocked from unplug
>>>> + * @reason: Reason for blocking
>>>> + */
>>>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason);
>>>> +
>>>> +/*
>>>> + * qdev_del_unplug_blocker: Removes an unplug blocker from a device
>>>> + *
>>>> + * @dev: Device to be unblocked
>>>> + * @reason: Pointer to the Error used with qdev_add_unplug_blocker.
>>>> + *          Used as a handle to lookup the blocker for deletion.
>>>> + */
>>>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason);
>>>> +
>>>> +/*
>>>> + * qdev_unplug_blocked: Confirms if a device is blocked from unplug
>>>> + *
>>>> + * @dev: Device to be tested
>>>> + * @reason: Returns one of the reasons why the device is blocked,
>>>> + *          if any
>>>> + *
>>>> + * Returns: true if device is blocked from unplug, false otherwise
>>>> + */
>>>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp);
>>>> +
>>>> /**
>>>> * GpioPolarity: Polarity of a GPIO line
>>>> *
>>>> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>>>> index 84f3019440..0806d8fcaa 100644
>>>> --- a/hw/core/qdev.c
>>>> +++ b/hw/core/qdev.c
>>>> @@ -468,6 +468,28 @@ char *qdev_get_dev_path(DeviceState *dev)
>>>>    return NULL;
>>>> }
>>>> 
>>>> +void qdev_add_unplug_blocker(DeviceState *dev, Error *reason)
>>>> +{
>>>> +    dev->unplug_blockers = g_slist_prepend(dev->unplug_blockers, reason);
>>>> +}
>>>> +
>>>> +void qdev_del_unplug_blocker(DeviceState *dev, Error *reason)
>>>> +{
>>>> +    dev->unplug_blockers = g_slist_remove(dev->unplug_blockers, reason);
>>>> +}
>>>> +
>>>> +bool qdev_unplug_blocked(DeviceState *dev, Error **errp)
>>>> +{
>>>> +    ERRP_GUARD();
>>>> +
>>>> +    if (dev->unplug_blockers) {
>>>> +        error_propagate(errp, error_copy(dev->unplug_blockers->data));
>>>> +        return true;
>>>> +    }
>>>> +
>>>> +    return false;
>>>> +}
>>> 
>>> This cites the most recently added blocker as reason.  Your function
>>> comment covers it: "Returns one of the reasons".  Okay.
>> 
>> I could change the comment to say that it returns the recently added reason.
> 
> Up to you.
>