[libvirt] Is possible that cpu_maps.xml changed during different releases?

[Jiang, Yunhong]

One question come to my mind for "but such changes will only be additive", 
which may be an issue in cloud environment:

Considering following assumed situation:
	In release 1.0's cpu_maps.xml, core2duo include feature "a, b, c"
	In release 1.1's cpu_maps.xml, core2duo include "a, b, c, d"
	Two hosts, with different CPU features:
	Host A with feature "a, b, c, d, e" and 1.1 release, capabilities will be:
		Model: Core2duo (assume core2duo is the only match one)
		Features: e
	Host B with feature "a, b, c, e", and 1.0 release, capabilities will be:
		Models: Core2Duo (again, assume core2duo is the only match one)
		Features: e

	When we compare the capabilities in Host A, I think comparCPU will pass,
because host A will use 1.1 release definition to check Host B's capabilities, this
is sure to be wrong, since host B does not have feature 'd'.

	I think the key point is, capabilities describe features with information from cpu_maps.xml, which is a per-host file.

Thanks
--jyh


Will depends on 
> -----Original Message-----
> From: Jiang, Yunhong
> Sent: Friday, November 02, 2012 9:39 AM
> To: Eric Blake
> Cc: libvir-list at redhat.com
> Subject: RE: [libvirt] Is possible that cpu_maps.xml changed during different
> releases?
> 
> Eric, thanks for the answer very .much, that's really helpful.
> Sorry for the wrong format, I'm using outlook and possibly some setting wrong.
> 
> Thanks
> --jyh
> 
> > -----Original Message-----
> > From: Eric Blake [mailto:eblake at redhat.com]
> > Sent: Thursday, November 01, 2012 11:23 PM
> > To: Jiang, Yunhong
> > Cc: libvir-list at redhat.com
> > Subject: Re: [libvirt] Is possible that cpu_maps.xml changed during
> > different releases?
> >
> > On 10/31/2012 02:20 AM, Jiang, Yunhong wrote:
> > > Hi, all
> >
> > [your mailer doesn't know how to wrap long lines, which makes it
> > awkward to read your message]
> >
> > > 	I have two questions to the cpu_maps.xml in different releases,
> > > hope
> > someone can give me some hints:
> > >
> > > 	a) Will it be possible that the features defined in cpu_maps.xml
> > > for one
> > specific CPU model (like Nehalem) will be different? For example, one
> > feature is not listed for Nehalem in release x.y, and added in release x.y+1?
> >
> > XML does have the possibility to change between releases, but such
> > changes will only be additive (we will never remove support for a
> > feature once it is listed).
> >
> > >
> > > 	2) Is the format of the cpu_maps.xml fine defined or will be it
> > > changed
> > during releases? I asked this because currently the features defined
> > in the capabilities only list features not included in the definition
> > in cpu_maps.xml for the corresponding model. So if I want to get the
> > full features supported by the host, I have to parse the capabilities
> > and the cpu_maps.xml. I didn't find the definition for cpu_maps.xml
> > format, although the capabilities format is well defined in
> > http://libvirt.org/guide/html/Application_Development_Guide-Connection
> > s-Ca
> > pability_Info.html.
> >
> > The format should be well-defined; again, our requirement is that
> > upgrades might add new xml attributes or elements, but will never
> > remove elements that have been valid in previous releases.  To some
> > extent, docs/schemas/capability.rng in libvirt.git gives an RNG
> > grammar for what capabilities will look like, although someone else
> > may be able to point to a better documentation of what will be in
> cpu_maps.xml.
> >
> > --
> > Eric Blake   eblake at redhat.com    +1-919-301-3266
> > Libvirt virtualization library http://libvirt.org