Re: [Qemu-devel] [RFC PATCH] qdev: Mark devices as non-hotpluggable by default

[Peter Maydell]

On 20 September 2017 at 08:50, Marcel Apfelbaum <address@hidden> wrote:
> Hi Thomas,
>
>
> On 19/09/2017 11:55, Thomas Huth wrote:
>>
>> Historically we've marked all devices as hotpluggable by default. However,
>> most devices are not hotpluggable, and you also need a HotplugHandler to
>> support these devices. So if the user tries to "device_add" or
>> "device_del"
>> such a non-hotpluggable device during runtime, either nothing really
>> usable
>> happens, or QEMU even crashes/aborts unexpectedly (see for example commit
>> 84ebd3e8c7d4fe955b - "Mark diag288 watchdog as non-hotpluggable").
>> So let's change this dangerous default behaviour and mark the devices as
>> non-hotpluggable by default. Certain parent devices classes which are
>> known
>> as hotpluggable (e.g. PCI, USB, etc.) are marked with "hotpluggable =
>> true",
>> so that devices that are derived from these classes continue to work as
>> expected.
>>
>> Signed-off-by: Thomas Huth <address@hidden>
>> ---
>>   Note: I've marked the patch as RFC since I'm not 100% sure whether I've
>>   correctly identified all devices that should still be marked as hot-
>>   pluggable. Feedback is welcome!
>>
>>   hw/core/qdev.c        | 10 ++++------
>>   hw/cpu/core.c         |  1 +
>>   hw/mem/nvdimm.c       |  3 +++
>>   hw/mem/pc-dimm.c      |  1 +
>>   hw/pci/pci.c          |  1 +
>>   hw/s390x/ccw-device.c |  1 +
>>   hw/scsi/scsi-bus.c    |  1 +
>>   hw/usb/bus.c          |  1 +
>>   8 files changed, 13 insertions(+), 6 deletions(-)
>>
>> diff --git a/hw/core/qdev.c b/hw/core/qdev.c
>> index 606ab53..c4f1902 100644
>> --- a/hw/core/qdev.c
>> +++ b/hw/core/qdev.c
>> @@ -1120,13 +1120,11 @@ static void device_class_init(ObjectClass *class,
>> void *data)
>>       dc->realize = device_realize;
>>       dc->unrealize = device_unrealize;
>>   -    /* by default all devices were considered as hotpluggable,
>> -     * so with intent to check it in generic qdev_unplug() /
>> -     * device_set_realized() functions make every device
>> -     * hotpluggable. Devices that shouldn't be hotpluggable,
>> -     * should override it in their class_init()
>> +    /*
>> +     * All devices are considered as cold-pluggable by default. The
>> devices
>> +     * that are hotpluggable should override it in their class_init().
>>        */
>> -    dc->hotpluggable = true;
>> +    dc->hotpluggable = false;
>
>
> I agree with the defensive mode as long as we don't
> introduce any regression...

Is it possible to hack together some kind of test code that
can give us a list of the devices compiled in that have
hotpluggable enabled? Then we could compare before and
after to see which devices we've changed.

thanks
-- PMM