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 10:53:30AM -0600, Anthony Liguori wrote:
> On 01/19/2011 03:48 AM, Gerd Hoffmann wrote:
> >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.
> 
> Right.  In terms of specifying bus=, what are we promising re:
> compatibility?  Will there always be a pci.0?  If we add some
> PCI-to-PCI bridges in order to support more devices, is libvirt
> support to parse the hierarchy and figure out which bus to put the
> device on?

The answer to your questions probably differ depending on
whether '-nodefconfig' and '-nodefaults' are set on the
command line.  If they are set, then I'd expect to only
ever see one PCI bus with name pci.0 forever more, unless
i explicitly ask for more. If they are not set, then you
might expect to see multiple PCI buses by appear by magic

Daniel