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Re: [Qemu-devel] Re: [PATCH 3/8] Add QBuffer


From: Markus Armbruster
Subject: Re: [Qemu-devel] Re: [PATCH 3/8] Add QBuffer
Date: Tue, 18 May 2010 14:27:15 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.1 (gnu/linux)

Jan Kiszka <address@hidden> writes:

> Avi Kivity wrote:
>> On 05/17/2010 10:57 AM, Jan Kiszka wrote:
>>> Avi Kivity wrote:
>>>    
>>>> On 05/17/2010 10:40 AM, Jan Kiszka wrote:
>>>>      
>>>>>> The alternative is to have a schema.  Sun RPC/XDR doesn't carry any type
>>>>>> information (you can't even distinguish between a number and text) yet C
>>>>>> clients have to problem extracting typed information from it.
>>>>>>
>>>>>> Having __class__ everywhere means we're carrying the schema in every
>>>>>> message instead of just once.
>>>>>>
>>>>>>          
>>>>> The device_show command is already an example where you don't have a
>>>>> predefined schema. It is derived from the data stream the encodes the
>>>>> vmstate fields. So far we have no collision between base64-encoded
>>>>> buffers and real strings, but this may actually change when we start
>>>>> annotating the fields with symbolic constants.
>>>>>
>>>>>        
>>>> What is the receiver to do with it?
>>>>
>>>> If it doesn't know the schema (and there is no schema), then all it can
>>>> do is display the key/values.  If it does know the schema, then
>>>> __class__ is unnecessary.
>>>>      
>>> There is a schema describing the fields (name, size, number of
>>> elements),
>> 
>> Surely the schema has to describe the type as well?  If it does, you can 
>> use the schema to generate a classes at compile time.

Doesn't that tie you to a specific version of QMP at compile-time?

>>>   but their types (int, buffer, sub-field, array of X) are
>>> derived from the JSON objects (ie. the JSON parser does this job).
>>>    
>> 
>> The names of fields are also type information.
>
> Not in the case of device_show. The clients have no idea of the vmstate
> structures before they were transfered. Granted, that will likely remain
> a special case in the QMP command set.

qdev device properties are similar.  Right now, they occur only as
arguments of device_add.  When do_info_qtree() gets converted, they'll
appear in results.

[...]



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