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

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.

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.