Re: [Qemu-devel] Should we auto-generate IDs?

[Programmingkid]

On Aug 27, 2015, at 12:02 PM, Daniel P. Berrange wrote:

> On Thu, Aug 27, 2015 at 11:58:22AM -0400, Programmingkid wrote:
>> 
>> On Aug 27, 2015, at 11:40 AM, Jeff Cody wrote:
>> 
>>> On Thu, Aug 27, 2015 at 11:20:20AM -0400, Programmingkid wrote:
>>>> 
>>>> On Aug 27, 2015, at 10:42 AM, Daniel P. Berrange wrote:
>>>> 
>>>>> On Thu, Aug 27, 2015 at 10:34:05AM -0400, Programmingkid wrote:
>>>>>> 
>>>>>> On Aug 27, 2015, at 10:02 AM, Eric Blake wrote:
>>>>>> 
>>>>>>> On 08/27/2015 07:56 AM, Programmingkid wrote:
>>>>>>> 
>>>>>>>>> If we did have auto-generated names, we would need to come up with a
>>>>>>>>> scheme that is not going to clash with any existing naming that users
>>>>>>>>> of QEMU may already be doing, otherwise we risk causing a regression.
>>>>>>>>> Something as simple as what you suggest has non-trivial chance of
>>>>>>>>> clashing.
>>>>>>>> 
>>>>>>>> Actually there is a way to prevent clashing. When QEMU auto-generates a
>>>>>>>> name, it could scan all the ID's to see if there is a clash. If the ID 
>>>>>>>> is already
>>>>>>>> taken, just increment the ID until it is detected to be unique. The 
>>>>>>>> previous
>>>>>>>> threads on this subject has patches that did just that. This means 
>>>>>>>> that a
>>>>>>>> ID scheme that is just a single number would work without clashes. 
>>>>>>> 
>>>>>>> No, because you cannot predict what FUTURE names the user will request.
>>>>>>> The name generated by qemu must be IMPOSSIBLE to request manually, and
>>>>>>> not just one that happens not to clash at the current moment.
>>>>>> 
>>>>>> If I add a device with an ID that is taken, QEMU can just say sorry that 
>>>>>> ID is already
>>>>>> taken. I could just increment the ID myself until I find one that is 
>>>>>> unique. It is a
>>>>>> simple algorithm. Maybe you are talking about some program that has hard 
>>>>>> coded
>>>>>> ID's it depends on. If that is the case, perhaps this management program 
>>>>>> could be
>>>>>> made to be a little flexible. Or use a 160-bit SHA-1 generated ID that 
>>>>>> is virtually
>>>>>> guaranteed to always be unique.
>>>>> 
>>>>> If breaking existing apps was OK, we would just mandate that users always
>>>>> provide an ID which trivially avoids the problem of not having an ID to
>>>>> use when deleting the object. We want to /avoid/ breaking existing apps
>>>>> though, so saying an app should change the way it works in order to cope
>>>>> with QEMU's ID generation is not appropriate. Hence any use of 
>>>>> auto-generated
>>>>> IDs, must use a separate namespace to avoid such clashes.
>>>> 
>>>> This is making the assumption that an auto-generated ID system would break 
>>>> any existing
>>>> application. We don't know this. In fact, I predict a future patch would 
>>>> allow existing
>>>> applications to continue running without modification. The patch would be 
>>>> a win-win
>>>> for both users and any management application.
>>>> 
>>> 
>>> Daniel's assumption is spot on.  The idea of "QEMU can just say sorry
>>> that ID is already taken" will break existing applications.
>>> 
>>> But we are all striving to make your prediction true, with this very
>>> conversation.
>> 
>> Ok, it sounds like some are concerned that Libvirt would not be able to work 
>> with this
>> feature. Let me ask you this: does Libvirt interact with QEMU before the 
>> user has a
>> chance to do so? If the answer is yes, then that means Libvirt will have 
>> finished 
>> creating all its devices with their ID's long before the user has a chance 
>> to enter
>> his own devices.
> 
> Just to be clear - libvirt will *never* use an auto-generated device
> IDs feature. It is way more complicated to let QEMU assign device IDs
> and then auto-detect them from some 'info device-list' output, than
> to just specify IDs upfront at device/object creation time which
> it already does[1]. There is simply no benefit to auto-generating device
> IDs for a mgmt app like libvirt, and plenty of downside.  Auto-generated
> IDs will only be of interest to humans talking to the monitor directly
> without a mgmt app involved.

I've haven't used Libvirt but I do believe in the saying "never say never". The
rest of what you said does sound accurate.