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

[Daniel P. Berrange]

On Thu, Oct 05, 2017 at 11:28:35AM +0200, Michal Privoznik wrote:
> 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.

I think its beneficial to set them at define time, as it provides a unique
identifier that apps / admins can see throughout the lifetime of the guest,
avoiding the need to both about setting them manually to a significant
extent.

Regards,
Daniel
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