Re: [Qemu-devel] [PATCH 28/35] kvm: x86: Introduce kvmclock device to save/restore its state

[Daniel P. Berrange]

On Wed, Jan 19, 2011 at 10:54:10AM -0600, Anthony Liguori wrote:
> On 01/19/2011 07:11 AM, Markus Armbruster wrote:
> >Gerd Hoffmann<address@hidden>  writes:
> >
> >>On 01/18/11 18:09, Anthony Liguori wrote:
> >>>On 01/18/2011 10:56 AM, Jan Kiszka wrote:
> >>>>>The device model topology is 100% a hidden architectural detail.
> >>>>This is true for the sysbus, it is obviously not the case for PCI and
> >>>>similarly discoverable buses. There we have a guest-explorable topology
> >>>>that is currently equivalent to the the qdev layout.
> >>>But we also don't do PCI passthrough so we really haven't even explored
> >>>how that maps in qdev. I don't know if qemu-kvm has attempted to
> >>>qdev-ify it.
> >>It is qdev-ified.  It is a normal pci device from qdev's point of view.
> >>
> >>BTW: is there any reason why (vfio-based) pci passthrough couldn't
> >>work with tcg?
> >>
> >>>The -device interface is a stable interface. Right now, you don't
> >>>specify any type of identifier of the pci bus when you create a PCI
> >>>device. It's implied in the interface.
> >>Wrong.  You can specify the bus you want attach the device to via
> >>bus=<name>.  This is true for *every* device, including all pci
> >>devices. If unspecified qdev uses the first bus it finds.
> >>
> >>As long as there is a single pci bus only there is simply no need to
> >>specify it, thats why nobody does that today.  Once q35 finally
> >>arrives this will change of course.
> >As far as I know, libvirt does it already.
> 
> I think that's a bad idea from a forward compatibility perspective.

In our past experiance though, *not* specifying attributes like
these has also been pretty bad from a forward compatibility
perspective too. We're kind of damned either way, so on balance
we decided we'd specify every attribute in qdev that's related
to unique identification of devices & their inter-relationships.
By strictly locking down the topology we were defining, we ought
to have a more stable ABI in face of future changes. I accept
this might not always work out, so we may have to adjust things
over time still. Predicting the future is hard :-)

Daniel