[libvirt] [RFC] Host CPU passthrough

[Jiri Denemark]

On Tue, Aug 16, 2011 at 17:46:43 -0400, Dave Allan wrote:
> On Tue, Aug 16, 2011 at 10:55:32PM +0200, Jiri Denemark wrote:
> > On Mon, Aug 08, 2011 at 13:48:59 +0200, Jiri Denemark wrote:
> > > Hi,
> > > 
> > > AFAIK this topic is not new but I think we still do not have a good solution
> > > for it. Libvirt already supports specifying what CPU and its features a guest
> > > should see but imagine one wants to run a guest on the best possible CPU. The
> > > current way is to copy the <cpu> element from capabilities XML into domain
> > > XML. This approach is fine since it provides reproducible environment and such
> > > domain can even be migrated to a different host. But the CPU shown provided to
> > > a guest is not the same as the real host CPU. It's based on a model which
> > > doesn't reflect all aspects of real CPUs. Ideally, we would model everything
> > > but that's quite complicated and may need updating anytime a new CPU is
> > > introduced.
> > 
> > There have been no comments on this so far. Perhaps the topic is not so
> > controversial as I thought it was. But more likely it's just that people are
> > busy with other things. IIRC, Daniel used to have a strong opinion on this
> > matter, is that right?
> 
> I'm thinking that this boils down essentially to syntactic sugar. 
> 
> Would it not be possible to create a <cpu>host</cpu> that simply
> automates the process of copying the host capabilities into the
> running guest XML?  That would allow libvirt to do pre-migration
> validation that the destination host was suitable, but also permit
> users to specify one value of <cpu> that should in theory run with the
> maximum capabilities of the particular host where the domain was
> started and not have to go through the work of copying the host
> capabilities every time before booting the guest.

Dave and I agreed privately on IRC that in fact both use cases are valid and
it would be nice if the XML configuration could allow both of them.

Jirka