Re: [PATCH v2] qdev: Report an error for machine without HotplugHandler

[Markus Armbruster]

Akihiko Odaki <akihiko.odaki@daynix.com> writes:

> On 2023/12/11 15:51, Markus Armbruster wrote:
>> Akihiko Odaki <akihiko.odaki@daynix.com> writes:
>> 
>>> The HotplugHandler of the machine will be used when the parent bus does
>>> not exist, but the machine may not have one. Report an error in such a
>>> case instead of aborting.
>>>
>>> Fixes: 7716b8ca74 ("qdev: HotplugHandler: Add support for unplugging 
>>> BUS-less devices")
>>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>>
>> Do you have a reproducer for the crash?
>> 
>>> ---
>>> Changes in v2:
>>> - Fixed indention.
>>> - Link to v1: 
>>> 20231202-bus-v1-1-f7540e3a8d62@daynix.com">https://lore.kernel.org/r/20231202-bus-v1-1-f7540e3a8d62@daynix.com
>>> ---
>>>   system/qdev-monitor.c | 13 ++++++++++---
>>>   1 file changed, 10 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/system/qdev-monitor.c b/system/qdev-monitor.c
>>> index a13db763e5..5fe5d49c20 100644
>>> --- a/system/qdev-monitor.c
>>> +++ b/system/qdev-monitor.c
>>> @@ -927,9 +927,16 @@ void qdev_unplug(DeviceState *dev, Error **errp)
>>   void qdev_unplug(DeviceState *dev, Error **errp)
>>   {
>>       DeviceClass *dc = DEVICE_GET_CLASS(dev);
>>       HotplugHandler *hotplug_ctrl;
>>       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;
>>       }
>>       if (!dc->hotpluggable) {
>>           error_setg(errp, QERR_DEVICE_NO_HOTPLUG,
>>                      object_get_typename(OBJECT(dev)));
>>           return;
>>       }
>>       if (!migration_is_idle() && !dev->allow_unplug_during_migration) {
>>           error_setg(errp, "device_del not allowed while migrating");
>>           return;
>>       }
>> 
>>>      qdev_hot_removed = true;
>>>         hotplug_ctrl = qdev_get_hotplug_handler(dev);
>>> -    /* hotpluggable device MUST have HotplugHandler, if it doesn't
>>> -     * then something is very wrong with it */
>>> -    g_assert(hotplug_ctrl);
>>> +    if (!hotplug_ctrl) {
>>> +        /*
>>> +         * hotpluggable bus MUST have HotplugHandler, if it doesn't
>>> +         * then something is very wrong with it
>>> +         */
>>> +        assert(!dev->parent_bus);
>>> +
>>> +        error_setg(errp, "The machine does not support hotplugging for a 
>>> device without parent bus");
>>> +        return;
>>> +    }
>>
>> Extended version of my question above: what are the devices where
>> qdev_get_hotplug_handler(dev) returns null here?
>
> Start a VM: qemu-system-aarch64 -M virt -nographic
> Run the following on its HMP: device_del /machine/unattached/device[0]
>
> It tries to unplug cortex-a15-arm-cpu and crashes.

This device has no parent bus (dev->parent_bus is null), but is marked
hot-pluggable (dc->hotpluggable is true).  Question for somebody
familiar with the hot-plug machinery: is this sane?