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

[Anthony Liguori]

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.

> > > Management and analysis tools must be able to traverse the system buses
> > > and find guest devices this way.
> > >        
> > We need to provide a compatible interface to the guest.  If you agree
> > with my above statements, then you'll also agree that we can do this
> > without keeping the device model topology stable.
> >
> > But we also need to provide a compatible interface to management tools.
> > Exposing the device model topology as a compatible interface
> > artificially limits us.  It's far better to provide higher level
> > supported interfaces to give us the flexibility to change the device
> > model as we need to.
> >      
> How do you want to change qdev to keep the guest and management tool
> view stable while branching off kvm sub-buses?

The qdev device model is not a stable interface.  I think that's been 
clear from the very beginning.

>   Please propose such
> extensions so that they can be discussed. IIUC, that would be second
> relation between qdev and qbus instances besides the physical topology.
> What further use cases (besides passing kvm_state around) do you have in
> mind?
>    

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.

>    
> >      
> > >    If they create a device on bus X, it
> > > must never end up on bus Y just because it happens to be KVM-assisted or
> > > has some other property.
> > >        
> > Nope.  This is exactly what should happen.
> >
> > 90% of the devices in the device model are not created by management
> > tools.  They're part of a chipset.  The chipset has well defined
> > extension points and we provide management interfaces to create devices
> > on those extension points.  That is, interfaces to create PCI devices.
> >
> >      
> Creating kvm irqchips via the management tool would be one simple way
> (not the only one, though) to enable/disable kvm assistance for those
> devices.
>    

It's automatically created as part of the CPUs or as part of the 
chipset.  How to enable/disable kvm assistance is a property of the CPU 
and/or chipset.

Regards,

Anthony Liguori


> Jan
>
>