Re: [Qemu-devel] RFC v2: blockdev_add & friends, brief rationale, QMP docs

[Avi Kivity <avi@redhat.com>]

On 06/15/2010 05:54 PM, Markus Armbruster wrote:
> Avi Kivity<avi@redhat.com>  writes:
>
>    
>> On 06/15/2010 04:27 PM, Markus Armbruster wrote:
>>      
>>>        
>>>> I'm only talking about the interface, not the implementation.
>>>> Internal design details shouldn't be exposed.
>>>>
>>>> For the implementation, I imagine you can create an empty blockdev
>>>> during guest device creation and treat blockdev_add/blockdev_del as
>>>> media change/eject.
>>>>
>>>>          
>>> If blockdev_del only ejects media, then we need another command to get
>>> rid of a blockdev.
>>>
>>>        
>> No.  If you have a blockdev just to satisfy the guest device's need
>> for a blockdev pointer, you can delete it automatically when the last
>> reference goes.
>>      
> That's not how netdev and chardev behave.
>    

Ok.  To me duplicate argument lists suggest a lack of orthogonality, though.

How about

   blockdev_add id=...
   media_attach blockdev=..., media-parameters
   media_detach blockdev=...
   blockdev_del id=...

So blockdev_add/del define/remove a slot for the media, 
media_attach/detach connect it to media.

>>> Unless you propose that blockdev_del merely ejects media if the blockdev
>>> is being used by a device, but destroys the blockdev outright if not.
>>> But that would be sick, wouldn't it?
>>>
>>>        
>> Create a blockdev implicitly with guest device creation, and use
>> blockdev_add (or media_attach) to attach the media.
>>
>> (but that creates a window where the guest device is visible but media
>> is not yet inserted?)
>>      
> Think so, for hot plug.
>
> Actually, the device model driver would reject an empty blockdev, unless
> it's a device with removable media, such as a CD-ROM.
>
> Having a device with fixed media go through a "no media yet" state
> during initialization just complicates things.  Defining the media
> *before* creating the device is much simpler.
>    

That is true.  Maybe we should just ignore the duplication.

-- 
error compiling committee.c: too many arguments to function