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

[Daniel P. Berrange]

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>


Regards,
Daniel
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