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

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(a)linux.vnet.ibm.com&gt; >>> 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