Re: [Qemu-devel] [PATCH] full introspection support for QMP

[Anthony Liguori]

Kevin Wolf <address@hidden> writes:

> Am 02.07.2013 um 19:06 hat Anthony Liguori geschrieben:
>> Eric Blake <address@hidden> writes:
>> > On 07/02/2013 08:51 AM, Anthony Liguori wrote:
>> >> Amos Kong <address@hidden> writes:
>> >> 
>> >>> Introduces new monitor command to query QMP schema information,
>> >>> the return data is a nested dict/list, it contains the useful
>> >>> metadata.
>> >>>
>> >>> we can add events definations to qapi-schema.json, then it can
>> >>> also be queried.
>> >>>
>> >>> Signed-off-by: Amos Kong <address@hidden>
>> >> 
>> >> Maybe I'm being too meta here, but why not just return qapi-schema.json
>> >> as a string and call it as day?
>
> I know you don't agree with this, but as I mentioned several times
> before, I think the schema as returned by the introspection functions
> shouldn't contain what a qemu of this version _could_ in theory provide,
> but what this specific build actually _does_ provide. It shouldn't
> include things that are compiled out.

I really don't disagree with you here.  I just don't like having two
formats for the schema.

>> > I've also been the one arguing that the additional complexity (an array of
>> > {"name":"str","type":"str","optional":bool"}) is better for libvirt in
>> > that the JSON is then well-suited for scanning (it is easier to scan
>> > through an array where the key is a constant "name", and looking for the
>> > value that we are interested in, than it is to scan through a dictionary
>> > where the keys of the dictionary are the names we are interested in).
>> > That is, the JSON in qapi-schema.json is a nice compact representation
>> > that works for humans, but may be a bit TOO compact for handling via
>> > machines.
>> 
>> But adding a bunch of code to do JSON translation just adds a bunch of
>> additional complexity.
>> 
>> One reasonable compromise would be:
>> 
>> { "command": "foo", "arguments": { "name": "str", "id": "int" },
>>                     "optional": { "bar": "bool" } }
>
> This assumes that optional vs. mandatory is the only property we ever
> want to describe for fields. Eric's approach is much more future-proof.
> Let's keep the format of qapi-schema.json an implementation detail that
> we can change and extend when necessary.

It's always possible to add another argument that describes additional
information.

For instance:

{ "command": "foo",
  "arguments": { "name": "str", "id": "int" },
  "optional": { "bar": "bool" },
  "defaults": { "bar": false } }

That doesn't mean I think exposing defaults is good, but rather that
it's still possible to do this in a compact form.

Regards,

Anthony Liguori

>
> Kevin