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

Ján Tomko jtomko at redhat.com
Fri Aug 12 14:59:28 UTC 2016

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
>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?


>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'/>
<model compatibility='legacy/transitional/modern'/>
(which could possibly be reused for other hypervisors with a similar
concept, not just QEMU and virtio)

was what we needed all along, but I misunderstood their purpose.

That's good to know :)

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