Re: [PATCH 1/1] spapr_vscsi: do not allow device hotplug

[Markus Armbruster]

Philippe Mathieu-Daudé <philmd@redhat.com> writes:

> Cc'ing Markus
>
> On 8/20/20 9:06 PM, Daniel Henrique Barboza wrote:
>> We do not implement hotplug in the vscsi bus, but we forgot to
>> tell qdev about it. The result is that users are able to hotplug
>> devices in the vscsi bus, the devices appear in qdev, but they
>> aren't usable by the guest OS unless the user reboots it first.
>> 
>> Setting qbus hotplug_handler to NULL will tell qdev-monitor, via
>> qbus_is_hotpluggable(), that we do not support hotplug operations
>> in spapr_vscsi.
>> 
>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1862059
>> 
>> Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
>> ---
>>  hw/scsi/spapr_vscsi.c | 3 +++
>>  1 file changed, 3 insertions(+)
>> 
>> diff --git a/hw/scsi/spapr_vscsi.c b/hw/scsi/spapr_vscsi.c
>> index d17dc03c73..57f0a1336f 100644
>> --- a/hw/scsi/spapr_vscsi.c
>> +++ b/hw/scsi/spapr_vscsi.c
>> @@ -1219,6 +1219,9 @@ static void spapr_vscsi_realize(SpaprVioDevice *dev, 
>> Error **errp)
>>  
>>      scsi_bus_new(&s->bus, sizeof(s->bus), DEVICE(dev),
>>                   &vscsi_scsi_info, NULL);
>> +
>> +    /* ibmvscsi SCSI bus does not allow hotplug. */
>> +    qbus_set_hotplug_handler(BUS(&s->bus), NULL);
>
> Can't this be a problem later in DeviceClass::unrealize()?

Can't say offhand.  Cc'ing QOM maintainers for actual expertise.

A quick grep coughs up virtio_serial_device_realize() /
virtio_serial_device_unrealize(), which set and unset the hotplug
handler of a bus their device provides.  Not the same as the code above,
just a clue that messing with a bus's hotplug handler in a realize
method can work.

qbus_set_hotplug_handler() is entirely undocumented.  It sets a link.
Might be intended for use by the device that provides the bus.

> I was expecting something like, overwriting the parent bus type:
>
> -- >8 --
> @@ -1271,6 +1271,7 @@ static void spapr_vscsi_class_init(ObjectClass
> *klass, void *data)
>      DeviceClass *dc = DEVICE_CLASS(klass);
>      SpaprVioDeviceClass *k = VIO_SPAPR_DEVICE_CLASS(klass);
>
> +    k->bus_type = NULL; /* ibmvscsi SCSI bus does not allow hotplug. */
>      k->realize = spapr_vscsi_realize;
>      k->reset = spapr_vscsi_reset;
>      k->devnode = spapr_vscsi_devnode;

k->bus_type does not exist.  Do you mean dc->bus_type?

TYPE_VIO_SPAPR_VSCSI_DEVICE is a subtype of TYPE_VIO_SPAPR_DEVICE is a
subtype of TYPE_DEVICE.

TYPE_DEVICE is bus-less, i.e. (concrete) instances do not plug into a
bus.

TYPE_VIO_SPAPR_DEVICE has bus_type TYPE_SPAPR_VIO_BUS, i.e. (concrete)
instances plug into a TYPE_SPAPR_VIO_BUS.  Both hot and cold plug.

Zapping TYPE_VIO_SPAPR_VSCSI_DEVICE's bus_type makes it bus-less.  I
doubt that's okay.  Also, it's not just about hot plug.  What are you
trying to accomplish?