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

[Markus Armbruster]

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.

>                     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.