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

[Cornelia Huck]

On Mon, 27 Apr 2015 15:57:04 +0200
Alexander Graf <address@hidden> wrote:

> On 04/24/2015 11:07 AM, Cornelia Huck wrote:
> > On Wed, 22 Apr 2015 14:21:36 +0200
> > Alexander Graf <address@hidden> wrote:
> >
> >>
> >>> Am 22.04.2015 um 13:40 schrieb Cornelia Huck <address@hidden>:
> >>>
> >>> On Wed, 22 Apr 2015 11:14:40 +0200
> >>> Alexander Graf <address@hidden> wrote:
> >>>
> >>>>> On 04/22/2015 10:25 AM, Cornelia Huck wrote:
> >>>>> On Tue, 21 Apr 2015 21:06:42 +0200
> >>>>> Alexander Graf <address@hidden> wrote:
> >>>>>
> >>>>>>> On 04/17/2015 09:52 AM, Cornelia Huck wrote:
> >>>>>>> From: Xu Wang <address@hidden>
> >>>>>>>
> >>>>>>> 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.

While it does show up in info qom-tree, is hiding it from info qtree a
good idea? I'd think that it is still widely used.

> 
> > - 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?

Hm. Are there bus-less devices on other platforms?

> 
> > 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.

But in practice sysbus seems to be more like a catch-all: On s390x,
there are already things like the flic, various sclp-related devices,
the virtio bridges or the ipl device sitting on the sysbus. Should they
really be thrown out from the sysbus and dangle as bus-less devices? I
think there is a case to be made for a catch-all bus, even if it is not
the sysbus.

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

I'm afraid I can't parse that sentence :)