[libvirt] [PATCH 2/4] PowerPC : Add support for launching VMs in 'compat' mode.

[Daniel P. Berrange]

On Fri, Oct 10, 2014 at 04:12:26PM +0530, Prerna Saxena wrote:
> 
> On Friday 10 October 2014 03:46 PM, Daniel P. Berrange wrote:
> > On Fri, Oct 10, 2014 at 03:42:41PM +0530, Prerna Saxena wrote:
> >> >From 7228aacaa4b24907b1cbf33838ada8fb81890419 Mon Sep 17 00:00:00 2001
> >> From: Prerna Saxena <prerna at linux.vnet.ibm.com>
> >> Date: Tue, 7 Oct 2014 10:53:08 +0530
> >>
> >> PowerISA allows processors to run VMs in binary compatibility ("compat")
> >> mode supporting an older version of ISA. QEMU has recently added support to
> >> explicitly denote a VM running in compatibility mode through commit 6d9412ea
> >> & 8dfa3a5e85.
> >> Now, a "compat" mode VM can be run by invoking this qemu commandline on a
> >> POWER8 host:  -cpu host,compat=power7.
> > If this is using 'host' 
> >
> >> This patch allows libvirt to extend the "fallback" semantics of cpu model to
> >> describe this new mode for PowerKVM guests.
> >> As an example:
> >> When a user wants to request a power7 vm to run in compatibility mode on
> >> a Power8 host, this can be described in XML as follows :
> >>   <cpu mode='custom' match='exact'>
> >>     <model fallback='compat'>power7</model>
> >>   </cpu>
> > Then I don't think this should be using mode='custom', but rather it
> > should be 'host-passthrough'. We would use the <model> to denote the
> > compatibility mode for the host passthrough. IOW, it feels like this
> > is better described as
> >
> >     <cpu mode='host-passthrough' match='exact'>
> >       <model>power7</model>
> >     </cpu>
> Hi Daniel,
> Thanks for taking a look.
> It appears that my example didnt bring out the use case very clearly.
> 
> On PowerKVM, we can run VMs in 2 possible vCPU configurations:
> 1) Host native mode:
>     Where host is power7; guest VMs also see power7 vcpus..
>      Required qemu command line : "-cpu host"
> You correctly pointed out that this would use <cpu mode='host-passthrough'.>
> 
> 2) Compat mode:
>     In this case, the host CPU is power7, but it can run in binary compatibility mode
> with an older version of ISA(such as power6). So it will exhibit only those features that were available in an older generation of Power ISA. The guest will see a "power6" vCPU even though it is
> running on a physical "power7" chip.
>     Required QEMU command line for PowerKVM : "-cpu host,compat=power6"
> 
> My patch attempts to define semantics for this mode of execution.
> 
>   <cpu mode='custom' match='exact'>
>     <model fallback='compat'>power7</model>
>   </cpu>
> 
> I came up with this ^^^ XML schema to denote a compat mode guest.

The CPU mode 'host' is a form of host CPU passthrough.
ie Running '-cpu host,compat=power6' on a Power7 host is
equivalent to running '-cpu host' on a Power6 host.

So IMHO that should still use mode='host-passthrough' in
libvirt not mode=custom

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|