[libvirt] [RFC 00/28] s390x CPU models: exposing features

[Jiri Denemark]

On Tue, Jun 21, 2016 at 17:33:09 -0300, Eduardo Habkost wrote:
> On Tue, Jun 21, 2016 at 07:01:44PM +0200, David Hildenbrand wrote:
> > > (CCing libvirt people)
> > > 
> > > On Tue, Jun 21, 2016 at 03:02:05PM +0200, David Hildenbrand wrote:
> > > > This is our second attempt to implement CPU models for s390x. We realized
> > > > that we also want to have features exposed via the CPU model. While doing
> > > > that we realized that we want to have a better interface for libvirt.  
> > > 
> > > Before getting into the details, I would like to clarify how the
> > > following could be accomplished using the new commands:
> > > 
> > > Example:
> > > 
> > > 1) User configures libvirt with:
> > >    <cpu match='exact'>
> > >        <model fallback='forbid'>Westmere</model>
> > >        <feature policy='require' name='aes'/>
> > >    </cpu>
> > > 2) libvirt will translate that to:
> > >    "-cpu Westmere,+aes" or "-cpu Westmere,aes=on"
> > > 3) libvirt wants to know if "-cpu Westmere,aes=on" is usable in
> > >    the current host, before trying to start the VM.
> > > 
> > > How exactly would this be done using the new commands?
> > 
> > Hi Eduardo,
> > 
> > thanks for having a look - highly appreciated that you actually map this
> > to libvirt requirements!
> > 
> > That would map to a compare operation between "host" and "Westmere,aes=on".
> > 
> > Host could at that point already be expanded by libvirt. Doesn't matter at that
> > point.
> > 
> > If the result is "identica"l or "superset", it is runnable. If the result is
> > "subset" or "incompatible", details about the responsible properties is
> > indicated. (I actually took that idea from your patch for indicating
> > runnability).
> 
> So, I have two worries about the proposal:
> 
> 
> 1) "query-cpu-model-expansion model=host" vs "query-host-cpu":
> 
> I still don't think we want to set in stone that "the result the
> guest sees when using -cpu host" is always the same as "what the
> host supports running".
> 
> For example: let's assume a given architecture have two features
> (A and B) that are both supported by the host but can never be
> enabled together. For actual "-cpu host" usage, QEMU would have
> to choose between enabling A and B. For querying host
> capabilities, we still want to let management software know that
> either A or B are supported.

What libvirt is really interested in is the guest CPU which would be
used with -cpu host. This is actually what I thought query-host-cpu was
all about. Perhaps because there's no difference for x86.

> 2) Requiring a running QEMU instance to run
>    query-cpu-model-comparison
> 
> With my previous query-host-cpu proposal, the task of comparing
> the configuration requested by the user with host capabilities
> can be done directly by libvirt. This way, no extra QEMU instance
> needs to be run before starting a VM.

I think we can just easily get around this by not comparing a guest CPU
to host (except for the explicit virConnectCompareCPU, which is not very
useful in its current form anyway).

Jirka