[libvirt] [PATCH v2 3/3] conf: Allow users to define UUID for devices

Michal Privoznik mprivozn at redhat.com
Thu Oct 5 09:28:35 UTC 2017

On 10/05/2017 11:13 AM, Daniel P. Berrange wrote:
> On Thu, Oct 05, 2017 at 10:44:29AM +0200, Michal Privoznik wrote:
>> On 10/05/2017 10:10 AM, Daniel P. Berrange wrote:
>>> On Wed, Oct 04, 2017 at 08:31:36AM +0200, Martin Kletzander wrote:
>>>> On Tue, Oct 03, 2017 at 03:10:48PM +0100, Daniel P. Berrange wrote:
>>>>> On Tue, Oct 03, 2017 at 04:03:20PM +0200, Martin Kletzander wrote:
>>>>>> On Tue, Oct 03, 2017 at 02:53:46PM +0100, Daniel P. Berrange wrote:
>>>>>>> On Tue, Oct 03, 2017 at 02:11:44PM +0200, Martin Kletzander wrote:
>>>>>>>> On Tue, Oct 03, 2017 at 12:58:59PM +0200, Michal Privoznik wrote:
>>>>>>>>> https://bugzilla.redhat.com/show_bug.cgi?id=1434451
>>>>>>>>> It comes handy for management application to be able to have a
>>>>>>>>> per-device label so that it can uniquely identify devices it
>>>>>>>>> cares about. The advantage of this approach is that we don't have
>>>>>>>>> to generate aliases at define time (non trivial amount of work
>>>>>>>>> and problems). The only thing we do is parse the user supplied
>>>>>>>>> UUID and format it back. For instance:
>>>>>>>>>    <disk type='block' device='disk'>
>>>>>>>>>      <driver name='qemu' type='raw'/>
>>>>>>>>>      <source dev='/dev/HostVG/QEMUGuest1'/>
>>>>>>>>>      <target dev='hda' bus='ide'/>
>>>>>>>>>      <uuid>1efaf08b-9317-4b0f-b227-912e4bd9f483</uuid>
>>>>>>>>>      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
>>>>>>>>>    </disk>
>>>>>>>>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>>>>>>>>> ---
>>>>>>>>> This is just a very basic implementation. If I get a green light on this, I can
>>>>>>>>> implement the feature further, i.e. allow device lookup on the UUID. For
>>>>>>>>> instance:
>>>>>>>>> virsh domiftune fedora $UUID $bandwidth
>>>>>>> <snip>
>>>>>>> I'm thinking that part of the problem we're having with agreeing how to
>>>>>>> deal with this RFE is that we're over-analysing semantics, by wondering
>>>>>>> whether its a unique name or UUID, its relation to alias, whether it has
>>>>>>> bearing on APIs.
>>>>>>> How about we change tack, and do what we did when we needed application
>>>>>>> specific information at the top level - just declare a general purpose
>>>>>>> <metadata> element and say it is a completely opaque blob. Libvirt will
>>>>>>> *never* do anything with it, other than to preserve it exactly as is.
>>>>>>> No API will ever use the metadata in any way. Libvirt will never try to
>>>>>>> guarantee uniqueness of metadata for each device. It can be JSON or a
>>>>>>> gziped microsoft word document for all we care. Entirely upto the app
>>>>>>> developer to decide what metadata is saved and guarantee uniqueness if
>>>>>>> they so desired.
>>>>>> That is kind of what I was aiming for.  But in order for it to be cleaner and
>>>>>> easier to use from user as well (and not only mgmt apps) I thought the metadata
>>>>>> would just be one identifier.  If you want to store more metadata for the
>>>>>> device, then you can do all that in the domain metadata and just reference the
>>>>>> particular device using the identifier if mgmt app wants to do that.
>>>>> Yes that is certainly possible. The caveats are that we still need a unique
>>>>> identifier for the device, and the metadata update is not atomic wrt
>>>>> to device hotplug.
>>>> Yes, well, our (libvirt) unique identifier is not going anywhere, so
>>>> that's OK, we'll be using what we have been until now.
>>>>> The plus side of the global metadata is that we have APIs to update it
>>>>> on the fly already, and its fully namespaced to allow multiple independant
>>>>> data sets to be stored.
>>>> Yes, exactly.
>>>>> I don't think lack of atomicity is a big deal as you could order it so that
>>>>> you update metadata before doing the hotplug. Then worst case you have a
>>>>> device mentioned in metadata that doesn't exist, which is easy enough to
>>>>> detect.
>>>> Right, if you want metadata for device, then you'll just update
>>>> metadata, hotplug device, and if it failed you update the metadata once
>>>> more.
>>>> So are we on the same page?  By that I mean agreeing on any sane user-supplied
>>>> identifier that we'll not guarantee uniqueness for, and neither will we use it
>>>> for anything for now?  (We can deal with the issues regarding using it when
>>>> someone wants to actually implement it).
>>> Per my reply to the earlier patch series, I'm now inclined to say that we
>>> should
>>>  - Allow the mgmt app to set the aliases upfront
>>>  - Auto-fill missing aliases at XML define time
>>> it has some downsides, but all the other solutions we've discussed have
>>> their own downsides too. So on balance I think its not worth it to add
>>> a second identifier for each device, when we already have alias.
>> Question is if we are confident enough that:
>> a) apps will provide unique aliases (since we'll be putting user input
>> onto qemu cmd line)
>> b) apps will provide only allowed characters in the alias (not every
>> character can be in id=, can it?)
> We will have to validate both these points when looking at the XML.
>> But I think we still have not answered this question: what if we need to
>> change pattern by which we generate aliases in the future? On one hand,
>> an alias is just a string so the pattern should not matter. On the other
>> hand, that's not quite true. For instance, "pci.0" has a very special
>> meaning. IOW, if we now worry about users cutting off the branch they
>> are sitting on, this is like giving them flamethrower in fireworks
>> production hall.
> 'pci.0' is not an alias - 'pci' is the alias, the '0' is a bus number,
> so users only provide the first bit which has no special semantics
> other than needing to comply with a permitted set of characters and
> be unique. 
> In terms of validation I think we should permit  a-Z, 0-9 and -, upto
> a maximum of say 32 characters in length.

Okay. We can check that. But now, does it make sense to generate the
aliases at define time? I mean, users could provide alias at define
time, and we can fill in the missing ones when starting up the domain.
Just like we're doing now.


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