Re: [Qemu-devel] [PATCH 4/5] monitor: add object-add (QMP) and object_add (HMP) command

[Markus Armbruster]

[Cc: Anthony, Mike for QAPI schema expertise]

Luiz Capitulino <address@hidden> writes:

> On Tue, 10 Dec 2013 19:15:05 +0100
> Paolo Bonzini <address@hidden> wrote:
>
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>> Il 10/12/2013 19:00, Eric Blake ha scritto:
>> >>> +  'data': {'qom-type': 'str', 'id': 'str', '*props': 'dict'}, 
>> >>> +  'gen': 'no' }
>> > 
>> > This feels VERY open-coded.  No where else in qapi-schema do we 
>> > have 'dict' as a type
>> 
>> Yes, in fact the "data" field is entirely skipped by the code
>> generator (that's 'gen':'no').
>> 
>> > ; using it violates all sorts of type-safety (which, I guess, is 
>> > the point), making it impossible to introspect what keys are valid
>> >  for use in the "props":{...} dictionary.  Do we really want to 
>> > play this fast and loose with the type system, or should we try 
>> > harder to make this a robust self-describing union of types?
>> > 
>> > That is, why can't we have object-add use a discriminated union, 
>> > where qom-type is the discriminator, and where props is an 
>> > appropriate JSON struct type that corresponds to the branch of the
>> >  union, so that we get full introspection on the set of valid keys
>> >  to put in props for any given qom-type?
>> 
>> The point of "props" is passing arbitrary data to a QOM object.  We
>> should indeed have introspection for QOM objects, where each QOM class
>> name can be introspected separately.  However, the union of all
>> possible QOM objects need not have a "C struct" representation.
>
> The "props" key was added to represent the "O" argument type of
> early QMP (which is used by commands like device_add), so that
> we could convert them to the QAPI. IIRC, we didn't plan for it
> to be used by new commands... But I don't have anything better
> to suggest, so I won't object to its usage here.

We created monitor argument type "O" to have name=val,... arguments in
the human monitor exactly like command line option arguments.  Currently
used by device_add and netdev_add.

We shoehorned type "O" into QMP in a bout of QMP feature-completeness
desperation.  This was before QAPI.

device_add still isn't in qapi-schema.json, but netdev_add is:

{ 'command': 'netdev_add',
  'data': {'type': 'str', 'id': 'str', '*props': '**'},
  'gen': 'no' }

Note the magic "'*props': '**'" (I'll be hanged if I know what that
means[*]), and "'gen': 'no'".

Yes, a proper schema for netdev_add and device_add is desirable.  In
both cases (but especially for device_add), the arguments are the
obligatory id plus a union discriminated by the device type, contraining
that device's properties.

Unless we move device properties definition to qapi-schema.json (bad),
or duplicate them there (worse), we need to derive that part of the
schema dynamically from device information available in QOM.


[*] Can we have a definition of QAPI schema semantics other than code?
Pretty-please?