Re: Handling multiple inheritance [for CXL]

[Ben Widawsky]

On 21-01-27 22:33:37, Igor Mammedov wrote:
> On Wed, 27 Jan 2021 12:25:44 -0800
> Ben Widawsky <ben@bwidawsk.net> wrote:
> 
> > On 21-01-27 21:18:24, Igor Mammedov wrote:
> > > On Tue, 26 Jan 2021 13:33:52 -0800
> > > Ben Widawsky <ben@bwidawsk.net> 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.  
> > > could you be more specific on what you need from it?
> > >   
> > 
> > I'm trying to register my persistent memory as normal system memory. I 
> > assume
> > it's required that I implement the memory interface to do that. If it's not,
> > that's fine too.
> > 
> > For reference:
> > https://gitlab.com/bwidawsk/qemu/-/blob/cxl-2.0v3/hw/mem/cxl_type3.c
> 
> if you use TYPE_MEMORY_DEVICE machinery, then address/(max)size a device 
> takes in
> hotplug ram window, is fixed at device creation time.
> If you use PCI BAR to map memory, it should be possible to reprogram BAR
> anywhere in PCI address space at runtime.
> 

This is not part of the PCI address space. I believe there will be quite a bit
of work to support hotplug properly for CXL devices, but I believe making it a
PCI BAR is the wrong approach.

If you're not yet familiar with the spec, it might make some sense to take a
look as I think I'm not doing a good job conveying what this hardware is.

https://www.computeexpresslink.org/download-the-specification

> > > >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.
> > > > 
> > > > Thanks.
> > > > Ben
> > > > 
> > > > [1]: 
> > > > 20210105165323.783725-1-ben.widawsky@intel.com/">https://lore.kernel.org/qemu-devel/20210105165323.783725-1-ben.widawsky@intel.com/
> > > >   
> > >   
> > 
>