Re: [Qemu-devel] Re: [PATCH 00/15] QAPI Round 1 (core code generator) (v2)

[Kevin Wolf]

Am 16.03.2011 16:59, schrieb Anthony Liguori:
> On 03/16/2011 09:34 AM, Luiz Capitulino wrote:
>> On Fri, 11 Mar 2011 17:05:30 -0600
>> Anthony Liguori<address@hidden>  wrote:
>>
>>> For more information about the background of QAPI, see
>>> http://wiki.qemu.org/Features/QAPI
>>>
>>> This series depends on 'QAPI Round 0' which I posted earlier.
>>>
>>> Since v2, the major changes are:
>>>
>>>   - Switch to a multiline code emitter to ease readability
>>>   - Use named parameters for escape sequences
>>>   - Add support for proxy commands
>>>   - Add support for asynchronous commands
>>>
>>> This version still adds a -qmp2 option.  This is the only practical way I 
>>> know
>>> to have testable code while not merging 200 patches all at once.
>> I've started reviewing this and my first impression is that this seems to be
>> real good. However, there's a lot of code here and some parts of it are a bit
>> complicated, so I need more time to do a thorough review and testing.
>>
>> Having said that, my only immediate concern is weather this will have any
>> negative side effects on the wire protocol, today or in the future.
>>
>> I mean, a C library has different extensibility constraints and functionality
>> requirements than a high-level protocol and tying/mixing the two can have
>> bad side effects, like this small one (patch 12/15):
> 
> C library is not quite as important as C interface.  I want QMP to be an 
> interface that we consume internally because that will make QMP a strong 
> external interface.
> 
> A fundamental design characteristic for me is that first and foremost, 
> QMP should be a good C interface and that the wire representation should 
> be easy to support in a good C interface.
> 
> This is a shift in our direction but the good news is that the practical 
> impact is small.  But I don't think there's a lot of value of focusing 
> on non-C consumers because any non-C consumer is capable of consuming a 
> good C interface (but the inverse is not true).
> 
>> +##
>> +# @put_event:
>> +#
>> +# Disconnect a signal.  This command is used to disconnect from a signal 
>> based
>> +# on the handle returned by a signal accessor.
>> +#
>> +# @tag: the handle returned by a signal accessor.
>> +#
>> +# Returns: Nothing on success.
>> +#          If @tag is not a valid handle, InvalidParameterValue
>> +#
>> +# Since: 0.15.0
>>
>> The name 'signal' (at least today) doesn't make sense on the wire protocol,
>> 'put_event' probably doesn't make sense in the C library, nor does 'event'.
> 
> I tried very hard to make events useful without changing the wire 
> protocol significantly but I've failed there.
> 
> I've got a new proposal for handling events that introduces the concept 
> of a signal on the wire and the notion of connecting and disconnecting 
> from signals.
> 
>> Another detail is that, event extension is more important than command
>> extension, because it's probably going to happen. I think it would be very
>> bad to add new events just because we wanted to add a new field.
> 
> The way this is typically handled is that signals tend to pass 
> structures instead of lots of fields.  For instance, most of the GDK 
> events just pass a structure for the event (like GdkButtonEvent).

Can we do that with existing events or would we break the external
interface because we'd have to nest everything one level deeper?

Kevin