Re: [RFC PATCH 18/25] hw/cxl/device: Add a memory device (8.2.8.5)

[Ben Widawsky]

On 20-11-13 08:47:59, Markus Armbruster wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
> > On 11/10/20 11:47 PM, Ben Widawsky wrote:
> >> A CXL memory device (AKA Type 3) is a CXL component that contains some
> >> combination of volatile and persistent memory. It also implements the
> >> previously defined mailbox interface as well as the memory device
> >> firmware interface.
> >> 
> >> The following example will create a 256M device in a 512M window:
> >> 
> >> -object 
> >> "memory-backend-file,id=cxl-mem1,share,mem-path=cxl-type3,size=512M"
> >> -device "cxl-type3,bus=rp0,memdev=cxl-mem1,id=cxl-pmem0,size=256M"
> >> 
> >> Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
> >> ---
> >
> >> +++ b/qapi/machine.json
> >> @@ -1394,6 +1394,7 @@
> >>  { 'union': 'MemoryDeviceInfo',
> >>    'data': { 'dimm': 'PCDIMMDeviceInfo',
> >>              'nvdimm': 'PCDIMMDeviceInfo',
> >> +            'cxl': 'PCDIMMDeviceInfo',
> >>              'virtio-pmem': 'VirtioPMEMDeviceInfo',
> >>              'virtio-mem': 'VirtioMEMDeviceInfo'
> >>            }
> >
> > Missing documentation of the new data type, and the fact that it will be
> > introduced in 6.0.
> 
> Old wish list item: improve the QAPI schema frontend to flag this.
> 

"Introduced in 6.0" - quite the optimist. Kidding aside, this is the area where
I could use some feedback. CXL Type 3 memory devices can contain both volatile
and persistent memory at the same time. As such, I think I'll need a new type to
represent that, but I'd love to know how best to accomplish that.

> >                     Also, Markus has been trying to get rid of so-called
> > "simple unions" in favor of "flat unions" - every time we modify an
> > existing simple union, it is worth asking if it is time to first flatten
> > this.
> 
> 0. Simple unions can be transformed into flat unions.  The
> transformation can either preserve the nested wire format, or flatten
> it.  See docs/devel/qapi-code-gen.txt "A simple union can always be
> re-written as a flat union ..."
> 
> 1. No new simple unions.
> 
> 2. Existing simple unions that can be flattened without breaking
> backward compatibility have long been flattened.
> 
> 3. The remaining simple unions are part of QMP, where we need to
> preserve the wire format.  We could turn them into flat union preserving
> the wire format.  Only worthwhile if we kill simple unions and simplify
> scripts/qapi/.  Opportunity to make the flat union syntax less
> cumbersome.  Not done due to lack of time.
> 
> 4. Kevin and I have been experimenting with ways to provide both flat
> and nested wire format.  This would pave the way for orderly deprecation
> of the nested wire format.  May not be practical for QMP output.
> 

So is there anything for me to do here?