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

[Anthony Liguori]

On 01/19/2011 11:35 AM, Daniel P. Berrange wrote:
> 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
>    

Yeah, we can't promise that.  If you use -M pc, you aren't guaranteed a 
stable PCI bus topology even with -nodefconfig/-nodefaults.

Regards,

Anthony Liguori

> Daniel
>