Re: [PATCH for-6.0 1/6] qapi: Add query-accel command

[Markus Armbruster]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Wed, Nov 18, 2020 at 09:56:28AM -0600, Eric Blake wrote:
>> On 11/18/20 9:45 AM, Eduardo Habkost wrote:
>> > On Wed, Nov 18, 2020 at 02:53:26PM +0100, Markus Armbruster wrote:
>> > [...]
>> >> Another way to skin this cat:
>> >>
>> >>       {"available": {"kvm": { extra properties... },
>> >>                      "tcg": ...,
>> >>                      "xen": ...},
>> >>        "active": "kvm"}
>> > 
>> > How would this structure be represented in the QAPI schema?
>> > 
>> > In other words, how do I say "Dict[str, AccelInfo]" in QAPIese?
>> 
>> {'type':'AvailAccel', 'data': {
>>   '*kvm': 'KvmExtraProps',
>>   '*tcg': 'TcgExtraProps',
>>   '*xen': 'XenExtraProps',
>>   '*hax': 'HaxExtraProps' } }
>> {'command':'query-accel', 'returns': {
>>    'available': 'AvailAccel', 'active': 'strOrEnum' } }
>> 
>> where adding a new accelerator then adds a new optional member to
>> AvailAccel as well as possibly a new enum member if 'active' is driving
>> by an enum instead of 'str'.
>
> Is it possible to represent this if we don't enumerate all
> possible dictionary keys in advance?  (I'm not suggesting we
> should/shouldn't do that, just wondering if it's possible)

Mostly no.

The definition of a complex type (struct or union) specifies all
members.  There is no way to say "and whatever else may be there".

We actually have such types anyway.  Consider command device_add: it
takes arguments 'driver', 'bus', 'str', and properties.  Its arguments
type is "struct of driver, bus, str, and whatever else may be there".

Since the schema language can't express this, we cheat:

    { 'command': 'device_add',
      'data': {'driver': 'str', '*bus': 'str', '*id': 'str'},
      'gen': false } # so we can get the additional arguments

With 'gen': false, 'data' is at best a statement of intent.  In this
case, it's correct, just incomplete[*].

Introspection takes 'data' at face value.  It's exactly as accurate as
'data' is.

We could extend the schema language so we can say

    { 'command': 'device_add',
      'data': {'driver': 'str', '*bus': 'str', '*id': 'str',
               '**props': 'dict'}

where 'props' receives any remaining arguments.  Fairly common language
feature, e.g. &rest in Lisp, ** in Python, ...

Removed the need for 'gen': false, and enables more accurate
introspection.

Type 'dict' doesn't exist, yet.  I think it could.  We got 'any'
already.


[*] There have been uses of 'gen': false where 'data' was actually
wrong.  For an example, see commit b8a98326d5 "qapi-schema: Fix up
misleading specification of netdev_add".