[libvirt] [PATCH alt] conf: Allow user define their own alias

Michal Privoznik mprivozn at redhat.com
Tue Oct 3 10:53:47 UTC 2017

On 10/02/2017 05:52 PM, Roman Mohr wrote:
> On Fri, Sep 29, 2017 at 3:49 PM, Michal Privoznik <mprivozn at redhat.com>
> wrote:
>> On 09/29/2017 01:16 PM, Peter Krempa wrote:
>>> On Fri, Sep 29, 2017 at 12:57:29 +0200, Michal Privoznik wrote:
>>>> On 09/29/2017 09:52 AM, Peter Krempa wrote:
>>>>> On Fri, Sep 29, 2017 at 09:06:01 +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
>>>>>> tag 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'/>
>>>>>>       <alias user='myDisk0'/>
>>>>>>       <address type='drive' controller='0' bus='0' target='0'
>> unit='0'/>
>>>>>>     </disk>
>>>>>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>>>>>> ---
>>>>>> An alternative approach to:
>>>>>> https://www.redhat.com/archives/libvir-list/2017-
>> September/msg00765.html
>>>>>> Honestly, I prefer this one as it's simpler and we don't have to care
>> about
>>>>>> devices changing their aliases on cold plug. I mean, on cold
>> (un-)plug we'd
>>>>>> have to regenerate the aliases so it might be hard to track certain
>> device.
>>>>>> But with this approach, it's no problem.
>>>>>> Also, I'm not completely convinced about the name of @user attribute.
>> So I'm
>>>>>> open for suggestions.
>>>>> With this proposed design you need to make sure to document that the
>>>>> user alias is _not_ guaranteed to be unique and also that it can't be
>>>>> used to match the device in any way.
>>>> Sure. I'll just add it at the end of the paragraph that describes
>>>> <alias/>. Err, hold on. There's none! But okay, I think I can find a
>>>> place to add it there.
>>>> Even though my patch doesn't implement it (simply because I wanted to
>>>> have ACK on the design so I've saved it for follow up patches), I don't
>>>> quite see why we can't match user supplied aliases?
>>> I think it will introduce more problems than solve. You will need to
>>> make sure that it's unique even across hotplug and add lookup code for
>>> everything. Additionally you can't add that as a mandatory field when
>>> looking up, since some device classes allow lookup only with partial XML
>>> descriptions (for unplug/update).
>> It can't be mandatory, that's the whole point. Sure. Well, in
>> DomainPostParse I can check for uniqueness pretty easily: just create an
>> empty list and start adding all strings provided by user. If the string
>> we're trying to add is already in the list we just error out. Sure, I'll
>> have to iterate over all devices, but that should be pretty easy since
>> we have virDomainDeviceInfoIterate(). All that's needed is to write the
>> callback function (= a few lines of code). Then, on hotplug goto 10.
>> IOW, if user alias is provided just check for its uniqueness.
>>>> virsh domif-setlink fedora myNet down
>>>> This has the great advantage of being ordering agnostic. You don't have
>>>> to check for the alias of the device you want to modify (as aliases can
>>>> change across different startups, right?). True, for that we would have
>>> Well, you've used a bad example for this. 'domif-setlink' uses target and
>>> mac address, both of which are stable and don't rely on ordering on
>>> startup. Same thing applies to disks.
>> Oh right. domiftune is more like it.
>>> The matching in virsh in this particular command is done by looking for
>>> the full element and then using that with the UpdateDevice() API, so the
>>> lookup is basically syntax sugar.
>>>> to make sure that the supplied aliases are unique per domain (which is
>>>> trivial to achieve). But apart from that I don't quite see why we
>>>> shouldn't/can't do it.
>>> Well, I don't think it's trivial. It's simpler than providing the alias
>>> which would be used with qemu, but you still have a zillion hotplug code
>>> paths which would need to check this.
>> Well, it might be slightly tricky yes. But in general, the
>> virDomain*Find() would try to match the user alias first (if provided)
>> else continue with current behaviour. I'm not quite sure what you mean
>> by zillion code paths.
>>>>> I think that users which know semantics of the current alias may be
>> very
>>>>> confused by putting user data with different semantics into the same
>>>>> field.
>>>> Yep. As I say, I'm not happy with the name either. Any suggestion is
>>>> welcome. So a separate element then? Naming is hard.
>>> I'm voting for separate element in case there's consensus that this
>>> route is a good idea.
>> Yeah, it may look a bit better. Any idea on the element name?
> How about "label" or "userlabel"?

Looks like the approach that has the highest chance of merging is UUID
stored in a separate element. For instance:

<interface type="network">
  <source network="default"/>
  <alias name="net0"/>   <!-- if this is the live XML -->

I still think that we don't have to be that restrictive and just allow
users to store any string (including UUID), but whatever. I'll implement
this approach.


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