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From: David Hildenbrand <david@redhat.com>
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Date: Sat, 12 May 2018 18:45:50 +0200
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Subject: Re: [Qemu-devel] [PATCH v2 04/17] pc: route all memory devices
 through the machine hotplug handler
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To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: qemu-s390x@nongnu.org, "Michael S . Tsirkin" <mst@redhat.com>, Igor Mammedov <imammedo@redhat.com>, Marcel Apfelbaum <marcel@redhat.com>, Richard Henderson <rth@twiddle.net>, Eduardo Habkost <ehabkost@redhat.com>, David Gibson <david@gibson.dropbear.id.au>, Markus Armbruster <armbru@redhat.com>, qemu-ppc@nongnu.org, Pankaj Gupta <pagupta@redhat.com>, Alexander Graf <agraf@suse.de>, Cornelia Huck <cohuck@redhat.com>, Christian Borntraeger <borntraeger@de.ibm.com>, Luiz Capitulino <lcapitulino@redhat.com>

On 12.05.2018 16:47, Paolo Bonzini wrote:
> On 11/05/2018 15:19, David Hildenbrand wrote:
>> +    if (dev->parent_bus) {
>> +        if (object_dynamic_cast(OBJECT(dev), TYPE_MEMORY_DEVICE)) {
>> +            return HOTPLUG_HANDLER(machine);
>> +        }
>> +    }
>> +
>=20
> How do you get here with a MemoryDevice that has !dev->parent_bus?
>=20

Excellent question :)

This is for now (for pc and spapr) a theoretical case, but I
included it to make all hotplug handler look alike and also show for
other potential device (interfaces) how it should be handled.

I'll give you the s390x example I had in mind:

s390x cannot hotplug dimms. dimms, however are busless devices that
implement the MemoryDevice interface.

If we would simply always indicate this way that we have a hotplug
handler, e.g. the check in qdev_device_add() would not trigger:

...
if (bus) {
    qdev_set_parent_bus(dev, bus);
} else if (qdev_hotplug && !qdev_get_machine_hotplug_handler(dev)) {
    /* No bus, no machine hotplug handler --> device is not hotpluggable =
*/
    error_setg(&err, "Device '%s' can not be hotplugged on this machine",
               driver);
    goto err_del_dev;
}
...

So the rational is "if its a busless device and I (the machine)
am not able to fully plug it, I must also not partially plug it."

However, right now I am not sure (due to qdev_hotplug) if this
is enough.


> Thanks,
>=20
> Paolo
>=20


--=20

Thanks,

David / dhildenb