[Qemu-devel] Re: [PATCH v2 12/15] monitor: Add basic device state visualization

[Jan Kiszka <jan.kiszka@web.de>]

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Avi Kivity wrote:
> On 05/23/2010 10:57 AM, Jan Kiszka wrote:
>> Avi Kivity wrote:
>>   
>>> On 05/22/2010 11:18 AM, Jan Kiszka wrote:
>>>     
>>>> From: Jan Kiszka<jan.kiszka@siemens.com>
>>>>
>>>> This introduces device_show, a monitor command that saves the
>>>> vmstate of
>>>> a qdev device and visualizes it. QMP is also supported. Buffers are cut
>>>> after 16 byte by default, but the full content can be requested via
>>>> '-f'. To pretty-print sub-arrays, vmstate is extended to store the
>>>> start
>>>> index name. A new qerror is introduced to signal a missing vmstate. And
>>>> it comes with documentation.
>>>>
>>>> +
>>>> +Dump a snapshot of the device state. Buffers are cut after 16 bytes
>>>> unless
>>>> +a full dump is requested.
>>>> +
>>>> +Arguments:
>>>> +
>>>> +- "path": the device's qtree path or unique ID (json-string)
>>>>
>>>>        
>>> This may be ambiguous.
>>>      
>> Can your elaborate what precisely is ambiguous?
>>    
> 
> Can't the user choose the unique ID so that it aliases an unrelated
> qtree path?

True. I'll swap the search order and document this. Qtree paths should
always rule.

> 
> I prefer having mutually exclusive 'path' and 'ref' arguments.

That would be unhandy.

> 
>>>> +- "full": report full state (json-bool, optional)
>>>>
>>>>        
>>> Is this needed for QMP?  The client can always truncate it to any
>>> length.
>>>      
>> The effect may not be needed for QMP, but I do need this channel from
>> the command line to the monitor pretty-printer. I could just stick
>> "full": json-bool back into the return dict, but that would look somehow
>> strange IMO.
>>    
> 
> So we could disallow it as a QMP input, but allow it as an HMP input.
> 
>>>> +
>>>> +Schema of returned object:
>>>> +
>>>> +{ "device": json-string, "id": json-string, "fields" : [
>>>> field-objects ] }
>>>> +
>>>> +The field object array may be empty, otherwise it consists of
>>>> +
>>>> +{ "name": json-string, "size": json-int, "elems": [ element-objects
>>>> ] }
>>>> +
>>>> +"size" describes the real number of bytes required for a binary
>>>> representation
>>>> +of a single field element in the array. The actually transfered
>>>> amount may be
>>>> +smaller unless a full dump was requested.
>>>>
>>>>        
>>> This converts the entire qdev tree into an undocumented stable protocol
>>> (the qdev paths were already in this state I believe).  This really
>>> worries me.
>>>      
>> Being primarily a debugging tool, device_show exports the entire
>> (qdev'ified) vmstates via QMP. Unlike the migration protocol, it does
>> not provide something like backward compatibility.
> 
> Should be explicitly documented.  All QMP commands should be backwards
> and forwards compatible unless noted.
> 
>> This would be
>> overkill for the intended purpose (though someone may find a different
>> use case one day).
>>    
> 
> Even for simply showing things, a GUI may depend on the presence of
> certain fields.  If we document that the fields may change, a correctly
> written GUI can fall back to a simpler display.
> 
>> I think we have the following options:
>>   - disable device_show via QMP, limit it to the monitor console
>>   - declare its output inherently unstable, maybe at least adding the
>>     vmstate version to each device so that potential QMP consumers notice
>>     that they may have to update their tools or switch to a different
>>     processing function
>>
>> Given that vmstate annotations will most probably require some work on
>> the output structure (and I don't have a QMP use case ATM anyway), I
>> would be fine with the first option for now. Still, I don't think we
>> will ever get beyond the second option because this service is tight to
>> some internals of QEMU we don't want to freeze.
>>    
> 
> I agree.  This feature is very useful as a debugging aid, and as I don't
> think we'll have debugging GUIs any time soon, it's better to defer the
> problem until we really need to solve it.

I introduced .user_only as a monitor command tag and applied it on
device_show. But I also added the vmstate version to the device output,
maybe already helpful for users. All this will come with v3.

Jan


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