Re: [Qemu-devel] -nodefaults and available buses (was Re: [RFC 00/15] qmp: Report supported device types on 'query-machines')

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

On Thu, 24 Nov 2016 12:51:19 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Wed, Nov 23, 2016 at 03:10:47PM -0200, Eduardo Habkost wrote:
> > (CCing the maintainers of the machines that crash when using
> > -nodefaults)
> > 
> > On Tue, Nov 22, 2016 at 08:34:50PM -0200, Eduardo Habkost wrote:
> > [...]
> > > "default defaults" vs "-nodefault defaults"
> > > -------------------------------------------
> > > 
> > > Two bad news:
> > > 
> > > 1) We need to differentiate buses created by the machine with
> > >    "-nodefaults" and buses that are created only without
> > >    "-nodefaults".
> > > 
> > > libvirt use -nodefaults when starting QEMU, so knowing which
> > > buses are available when using -nodefaults is more interesting
> > > for them.
> > > 
> > > Other software, on the other hand, might be interested in the
> > > results without -nodefaults.
> > > 
> > > We need to be able model both cases in the new interface.
> > > Suggestions are welcome.
> > 
> > The good news is that the list is short. The only[1] machines
> > where the list of buses seem to change when using -nodefaults
> > are:
> > 
> > * mpc8544ds
> > * ppce500
> > * mpc8544ds
> > * ppce500
> > * s390-ccw-virtio-*
> > 
> > On all cases above, the only difference is that a virtio bus is
> > available if not using -nodefaults.
> 
> Hrm.. that's odd.  Well, it makes sense for the s390 which has special
> virtio arrangements.  

I don't think it makes much sense for s390 either... is this a 'virtio'
bus or a 'virtio-{pci,ccw}' bus? The transport bus should be present
with -nodefaults; the virtio bus is basically a glue bus for virtio
devices...

> However, the others are all embedded ppc
> machines, whose virtio should be bog-standard virtio-pci.  I'm
> wondering if the addition of the virtio "bus" is a side-effect of the
> NIC or storage device created without -nodefaults being virtio.

I'd suspect the default NICs (which are virtio at least in the s390
case).