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

[Markus Armbruster]

Avi Kivity <address@hidden> 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.

>> 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.

>>>>> To pretend you're a media changer, blockdev_add all your cds at once
>>>>> and just change the guest/host association when you want to hear a new
>>>>> band.
>>>>>
>>>>> For a fake a multipath setup, blockdev_add one device, associate it
>>>>> with multiple guest interfaces.
>>>>>
>>>>> Otherwise, looks good.
>>>>>
>>>>>          
>>>> Any preference on the command line option syntax?
>>>>
>>>>        
>>> Something incredibly long and complicated?
>>>
>>> We might keep the existing stuff which is already complicated enough
>>> for users, and ask machines to build guests using QMP instead of the
>>> command line.
>>>      
>> I sketched three ways to do -blockdev.  They attempt to make the simple
>> simple, and the complex possible.  Any preference among them?
>>
>>    
>
> I'll look again.

Thanks!

>> We can't simply keep -drive, because of the flaws I listed in the
>> rationale.
>>    
>
> Ok.