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Re: [libvirt] [RFC] Host CPU passthrough



On Tue, Aug 16, 2011 at 03:46:23PM -0700, Daniel P. Berrange wrote:
> On Tue, Aug 16, 2011 at 05:46:43PM -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.
> 
> The goal of the XML is that it should always reflect the canonical
> configuration of the guest. So when the guest is running it should
> not show 'host' as the CPU model, but rather the real CPU model that
> the guest is using. Although libvirt has the info internally, we want
> that to be equally visible via the XML description to the app.

Agreed.

Dave

> Regards,
> Daniel
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