Re: [libvirt] [PATCHv3 04/11] Add virtio revision attribute to controllers
On Thu, 11 Aug 2016 16:17:10 +0200
Ján Tomko <jtomko(a)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?
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.