Re: Handling multiple inheritance [for CXL]

[Ben Widawsky]

On 21-01-27 10:06:48, Daniel P. Berrangé wrote:
> On Tue, Jan 26, 2021 at 01:33:52PM -0800, Ben Widawsky wrote:
> > I'm working on CXL 2.0 type 3 memory devices [1]. In short, these are PCIe 
> > devices
> > that have persistent memory on them. As such, it would be nice to inherit 
> > from
> > both a PCI_DEVICE class as well as an NVDIMM device class.
> > 
> > Truth be told, using TYPE_MEMORY_DEVICE as the interface does provide most 
> > of
> > what I need. I'm wondering what the best way to handle this is. Currently, 
> > the
> > only thing NVDIMM class provides is write/read_label_data, this is driven by
> > _DSM. For CXL, the mechanism to read/write the equivalent area is not done 
> > via
> > _DSM, but done directly via a mailbox interface. However, the intent is the
> > same, and so utilizing similar code seems ideal.
> > 
> > If there's a desire to unify these code paths, I'd need something like 
> > multiple
> > inheritance. I'm looking for some feedback here on how to do it.
> 
> We don't have a direct concept of multiple inheritance in QOM.
> 
> The closest you can get is to turn the NVDIMM class into an
> interface. You can inherit from PCI_DEVICE and then implement
> the NVDIMM interface.
> 
> Regards,
> Daniel

Is there a concise summary of what the tradeoffs would be of moving NVDIMM to an
interface? AFAICT, there's a lot of things done through subclassing that can
just as easily be done as an interface, but I don't understand the reason for
that.