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

[Cornelia Huck <cornelia.huck@de.ibm.com>]

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

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 ;)

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