Re: [Qemu-devel] [PATCH v5 06/16] apic: Introduce backend/frontend infrastructure for KVM reuse

[Anthony Liguori]

On 12/19/2011 05:32 PM, Jan Kiszka wrote:
> > struct APICCommonInfo {
> >      DeviceInfo qdev;
> >      void (*init)(APICState *s);
> >      void (*set_base)(APICState *s, uint64_t val);
> >      void (*set_tpr)(APICState *s, uint8_t val);
> >      void (*external_nmi)(APICState *s);
> > };
> >
> > Take a look at SCSIDevice for an example of this in practice.  This is
> > nicer because as we move save/load into devices methods, it becomes
> > natural to define the state and save/load function in the base class.
> > Provided it only uses base class state, it lets save/load be compatible
> > between both in-kernel and in-qemu device model.
>
> The difference is (unless I completely miss your point) that a common
> SCSI base class is used by different derived classes.

The 'frontend' is the common code and the 'backend' are the bits that are 
different, no?

We ultimately want there to be two devices that share all of the 'frontend' code 
by providing different 'backend' implementations.

So make the 'frontend' a base class that provides a set of abstract virtual 
methods (the set you have as the 'backend' interface).  Each device instance 
then inherits from the base class and provides its own implementation of the 
virtual methods.

> Here we have a
> common frontend class but different base classes, so to say. And we have
> a mechanism to chose where to inherit from on instantiation. Precisely
> this allows to keep the compatibility between in-kernel and user space
> model in this series.

Okay, so I really think this is the problem.  The in-kernel APIC is a separate 
device, no a property of the userspace APIC device.

It should be modeled as two separate devices.

Regards,

Anthony Liguori

> Jan