[libvirt] [PATCHv3 04/11] Add virtio revision attribute to controllers

[Boris Fiuczynski]

On 08/12/2016 04:59 PM, Ján Tomko wrote:
> On Fri, Aug 12, 2016 at 10:47:13AM +0200, Cornelia Huck wrote:
>> On Thu, 11 Aug 2016 16:17:10 +0200
>> Ján Tomko <jtomko at redhat.com> wrote:
>>
>>> On Thu, Aug 11, 2016 at 02:31:55PM +0100, Daniel P. Berrange wrote:
>>> >On Thu, Aug 11, 2016 at 03:25:53PM +0200, Ján Tomko wrote:
>>> >> On Thu, Aug 11, 2016 at 01:00:08PM +0100, Daniel P. Berrange wrote:
>>> >> > On Wed, Aug 10, 2016 at 03:27:15PM +0200, Ján Tomko wrote:
>>> >> > > <controller type='scsi' index='0' model='virtio-scsi'>
>>> >> > >     <virtio revision='0.9'/>
>>> >> >
>>> >> > I'm wondering about generalizing this. eg what if there are
>>> >> > other device models where we want the ability to set a
>>> >> > revision. We don't really want to invent a new sub-elment
>>> >> > named after each device model
>>> >>
>>> >> Not even a new attribute? :)
>>> >>  <revision virtio='0.9'/>
>>> >>
>>> >> How about:
>>> >>  <revision type='virtio' version='0.9'/>
>>> >
>>> >Both of those are quite repetative - we already know its virtio.
>>> >
>>>
>>> I guess one device having <revision>s of different types is unlikely.
>>>
>>> >Most devices we have alrady include a <driver> or <model> sub-element,
>>> >so we should really just add a revision= attrbute to those existing
>>>
>>> What I liked about having it as a separate element is that it can be
>>> repeated, e.g.:
>>>   <revision type='virtio' version='0.9'/>
>>>   <revision type='virtio' version='1.0'/>
>>>
>>> for a device with both 0.9 and 1.0.
>>>
>>> I could not come up with a nice way to represent that in a single
>>> attribute:
>>> * '0.9+1.0' feels like the two values should rather be separated
>>>   at the XML level
>>> * 'all' will not be true if there happens to be another virtio
>>>   revision in the future
>>
>> [not a libvirt developer, but let me comment from the qemu virtio
>> perspective]
>>
>> I don't think you are expressing the concept of virtio (standard)
>> revisions (more like releases!) here correctly. Let me elaborate:
>>
>> - The disable-legacy/disable-modern attributes are virtio-pci only.
>> Moreover, they don't express 'compliant to virtio-1.0' or so: They do
>> exactly What It Says On The Tin. A device with both disable attributes
>> off is in fact virtio-1.0 compliant (transitional devices are
>> compliant), as is a device with disable-legacy off. But it might also
>> be virtio-1.1 compliant! (That's the most likely release of the
>> standard in the near future.)
>>
>> - virtio-ccw does not have the concept of these disable switches.
>> Instead, there are virtio-ccw specific 'revisions' which count upwards
>> and may be limited by the 'max_revision' attribute. However, this is
>> not an attribute that is supposed to be set by the user, but for
>> backwards compatibility only. Unlike pci, ccw has nothing to gain by
>> disabling legacy support.
>>
>> - We may actually want to add some kind of versioning scheme to virtio
>> devices in future versions of the standard. But that's just a very
>> vague idea right now.
>>
>> Am I right in assuming that you simply want to be able to control
>> whether your virtio-pci devices are legacy, transitional or modern?
>
> Yes.
>
>> Then I think you'd be best off adding these as virtio-pci attributes
>> only and leave the concept of a 'virtio revision' for the future when
>> we might introduce it in the standard.
>>
>
> So XML like this:
> <model legacy='on/off' modern='on/off'/>
> or
> <model compatibility='legacy/transitional/modern'/>
> (which could possibly be reused for other hypervisors with a similar
> concept, not just QEMU and virtio)
Sorry to be a pain in the bud... but... both above proposals are 
virtio-PCI only.


>
> was what we needed all along, but I misunderstood their purpose.
>
>
> That's good to know :)
>
> Jan
>
>
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-- 
Mit freundlichen Grüßen/Kind regards
    Boris Fiuczynski

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