[libvirt] [RFC] cpu_map: Remove pconfig from Icelake-Server CPU model

[Jiri Denemark]

On Mon, Sep 30, 2019 at 17:16:27 +0200, Paolo Bonzini wrote:
> On 30/09/19 16:31, Hu, Robert wrote:
> >> This might be a problem if there are plans to eventually make KVM support
> >> pconfig, though.  Paolo, Robert, are there plans to support pconfig in KVM in the
> >> future?
> > [Robert Hoo] 
> > Thanks Eduardo for efforts in resolving this issue, introduced from my Icelake CPU
> > model patch.
> > I've no idea about PCONFIG's detail and plan. Let me sync with Huang, Kai and answer
> > you soon.
> 
> It's really, really unlikely.  It's possible that some future processor
> overloads PCONFIG in such a way that it will become virtualizable, but
> not IceLake.

I guess, the likelihood of this happening would be similar to
reintroducing other features, such as osxsave or ospke, right?

> Would it make sense for libvirt to treat absent CPU flags as "default
> off" during migration, so that it can leave out the flag in the command
> line if it's off?  If it's on, libvirt would pass pconfig=on as usual.
> This is a variant of [2], but more generally applicable:
> 
> > [2] However starting a domain with Icelake-Server so that it can be
> > migrated or saved/restored on QEMU in 3.1.1 and 4.0.0 would be
> > impossible. This can be solved by a different hack, which would drop
> > pconfig=off from QEMU command line.

The domain XML does not contain a complete list of all CPU features.
Features which are implicitly included in a CPU model are not listed in
the XML. Count in the differences in libvirt's vs QEMU's definitions of
a particular CPU model and you can see feat=off cannot be mechanically
dropped from the command line as the CPU model itself could turn it on
by default and thus feat=off is not redundant.

Jirka