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

[Paolo Bonzini]

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.

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.


Paolo