Re: [Qemu-devel] [Patch v1 00/29] s390x CPU models: exposing features

[Eduardo Habkost]

On Tue, Aug 02, 2016 at 08:12:34PM +0200, David Hildenbrand wrote:
> > On Tue, Aug 02, 2016 at 01:58:46PM +0200, David Hildenbrand wrote:
> > [...]
> > > So we have:
> > > a) "query-cpu-model-expansion" - tell us what the "host" or another CPU
> > >    model looks like. Either falling back to a static model or
> > >    completely exposing all properties.  
> > 
> > The query-cpu-model-expansion interface looks good to me. I just
> > had a few comments about the interface documentation.
> > 
> > > b) "query-cpu-model-comparison" - tell us how two CPU models compare,
> > >     indicating which properties were responsible for the decision.
> > > c) "query-cpu-model-baseline" - create a new model out of two models,
> > >     taking a requested level of stability into account.  
> > 
> > I miss a clearer specifiction of what are the actual requirements
> > and use cases of query-cpu-model-baseline. Is it related to
> > runnability? If so, how exactly?
> 
> cpu-baseline and cpu-compare are only needed to make
> - "virsh cpu-compare"
> - "virsh cpu-baseline" work
> (see libvirt usecases below)
> 
> These commands are needed to find/test runnability of a CPU model for
> a cluster in bigger installations by tooling.
> 
> As libvirt won't have details about s390x models, we have to provide
> an interface so it can carry out these tasks.
> 
> > 
> > Related to that (as mentioned in my reply to patch 25/29), I
> > would like a clearer definintion of what "superset" and "subset"
> > mean exactly, in query-cpu-model-comparison. Likewise, I would
> > like to understand the requirements and use cases that make
> > "superset" and "subset" useful.
> 
> I took these definitions from libvirt directly.
> 
> Example: core2duo against my sandybridge
> $ virsh cpu-compare test.xml
> Host CPU is a superset of CPU described in test.xml
> 
> Usually, you do a "virsh cpu-compare" against your host cpu model. Chances
> that the result is identical are very low. So depending on which
> one is the first model, you get superset or subset.
> 
> So
> if A is a subset of B, A will run where B runs
> if A is a superset of B, B will run where A runs
> 
> That means, if "cpu-compare" (against your host!) returns "identical" or
> "superset", you're good to go. If they are "incompatible" or "subset",
> you will have to use cpu-baseline to create a compatible model.
> 
> Does that answer your question?

It does, thanks! We need this to be clearly specified in the QMP
command documentation.

Proably the "if A is a superset of B, [...]" rule is enough to
unambigiously specify the semantics, while the rest of your
explanation is useful to explain when/how exactly the command is
useful.

-- 
Eduardo