Re: [Qemu-devel] [PATCH 1/6] s390x/virtio-ccw: enable has_dynamic_sysbus

[Alexander Graf <agraf@suse.de>]

On 04/24/2015 11:07 AM, Cornelia Huck wrote:
> On Wed, 22 Apr 2015 14:21:36 +0200
> Alexander Graf <agraf@suse.de> wrote:
>
>>
>>> Am 22.04.2015 um 13:40 schrieb Cornelia Huck <cornelia.huck@de.ibm.com>:
>>>
>>> On Wed, 22 Apr 2015 11:14:40 +0200
>>> Alexander Graf <agraf@suse.de> wrote:
>>>
>>>>> On 04/22/2015 10:25 AM, Cornelia Huck wrote:
>>>>> On Tue, 21 Apr 2015 21:06:42 +0200
>>>>> Alexander Graf <agraf@suse.de> wrote:
>>>>>
>>>>>>> On 04/17/2015 09:52 AM, Cornelia Huck wrote:
>>>>>>> From: Xu Wang <gesaint@linux.vnet.ibm.com>
>>>>>>>
>>>>>>> We have to enable this flag to support dynamically adding devices to the
>>>>>>> sysbus. This change is needed for the the upcoming diag288 watchdog.
>>>>>> s390 doesn't have a "sysbus" per se. Please create a new bus type.
>>>>> So what's wrong with the sysbus? I don't see why we should be different
>>>>> than everyone else.
>>>> The idea behind sysbus is that you have MMIO, PIO and IRQ pins
>>>> connecting to a PIC. It provides a lot of infrastructure for those
>>>> interfaces. S390 doesn't use any of them and instead wants registration
>>>> on "diag" interfaces for example which I'd put on the same layer as PIO
>>>> or MMIO registration.
>>> I don't think a "diag" bus makes sense.
>> You don't need a bus necessarily, just a parent class.
>>
>>> The individual diagnoses are
>>> way too heterogenous beyond the fact that they use the same base
>>> instruction.
>>>
>>> So where's the proper place for "misc" devices? My impression was that
>>> they can go on the sysbus.
>>>
>> If you really don't want to create your own class, how about you inherit from the DeviceState class?
> I tried that for the watchdog, and it certainly works, but some things
> end up odd:
>
> - in 'info qtree', the watchdog device does not show up at all

Please try "info qom-tree". Andreas also has a patch outstanding that 
shows properties along the way with a verbose switch.

> - in the list of devices printed by "-device help", diag288 is now the
>    only device without any bus

But it's not attached to a bus, so that's reasonable, no?

> I would have thought that any device not attached to a specialized bus
> should end up on the main system bus, which brings me back to adding it
> as a sysbus device ;)

Not really, sysbus is QEMU's wording for what Linux calls "platform 
bus". It's where devices go to that are attached to MMIO/PIO/IRQ lines 
via some fabric that we don't model.

The target for devices that are not the above we now have non-bus'ed 
devices.


Alex


> Does sysbus instead need to get more generic? The platform stuff seems
> to be a better fit for MMIO, PIO et al.?