[libvirt] RFC: turn cpu 'host-model' into 'custom' for active config on domain start

[Jiri Denemark]

On Wed, Jun 01, 2016 at 14:23:54 +0300, Nikolay Shirokovskiy wrote:
> Hi, everyone.
>   
>   Cpu mode 'host-model' is not convinient for migration in current form.
> Consider migration from less capable host to more capable one - it is possible.
> But then migration backwards is impossible despite the fact that qemu process
> on destination is running with the same model and features as on source because
> migration does not update 'host-model' cpu upon migration on destination. The
> problem is that on migrating back instead of taking current cpu config
> 'host-model' is expanded again and gain extra features of destination and
> migration failed by source (new destination) side.
>   
>   What if we change cpu for active config from 'host-model' into 'custom' in
> the starting process? Then this issue will be fixed. It is quite reasonable 
> from my POV - 'host-model' is effectively about taking active domain cpu from host on
> start, after that active domain cpu is on its own. Persistent config stays
> 'host-model' so migrating of active domain and restarting it gains extra
> features of new host as before.

Yeah, your thoughts go in the right direction, although it's not enough.
We also need to start creating better CPU specifications for host-model
and we need to start checking QEMU gives us what we asked for. For all
these reasons host-model is currently completely broken, although it
usually just works (until you start looking closer at it) :-)

Anyway, I'm working on all these (and other) issues with our CPU
configuration code and you can expect patches coming to libvirt-list
this month.

Jirka