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

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.