On 09/16/2015 01:11 AM, Wen Congyang wrote:

>>>> Possible workaround in the meantime - instead of trying to go with a
>>>> nice flat union (where all QMP keys are in the same {} level), we can
>>>> use nesting (structs that add another {} to include the unions).
>>>
>>> How to include the unions to a structs? Use 'base'?
>>
>> Conceptually, by adding a layer of nesting.  On the wire, instead of:
>>
>> { "switch1":"value", "switch2":"value", "body2":"blah" }
>>
>> you would instead have:
>>
>> { "switch1":"value", "data": { "switch2":"value", "body2":"blah" } }
>>
>> Anywhere in qapi that you try to have:
>> { 'union': ..., 'data':{'switch1':'Union'}}
>>
>> you instead create a wrapper type:
>> { 'struct':'Wrapper', 'data':{'data':'Union'}}
>> { 'union': ..., 'data':{'switch1':'Wrapper'}}
> 
> If so, the option is "data.switch1" not "switch1"
> 
>>
>>
>> What I don't know is whether the extra QMP nesting makes it easier or
>> harder to support the existing NBD command line options, and it would
> 
> Yes, it is harder to support it.

All the more reason to push the qapi improvements through, so that qapi
can expose a flat union and not make the command line have to go through
ugly nesting.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org