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

[Jiri Denemark]

On Tue, Oct 01, 2019 at 11:20:42 +0200, Paolo Bonzini wrote:
> On 30/09/19 18:16, Jiri Denemark wrote:
> > 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?
> 
> No, haveing osxsave and ospke was a mistake in the first place (they are
> not CPU features at all; they are more like a special way to let
> unprivileged programs read some bits of CR4).  For pconfig, it's just
> very unlikely.
> 
> >> 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.
> 
> I think I wasn't very clear, I meant "unsupported by QEMU" when I said
> "absent".  Libvirt on the destination knows that from
> query-cpu-model-expansion, so it can leave off pconfig if it is not
> supported by the destination QEMU.

Oh yeah, we should do this (and I plan to do so), but it won't really
help us in this case. Although it could potentially save us some work in
case we end up in a similar situation.

Jirka