[libvirt] [PowerPC Patch v3] Libvirt CPU enhancements for Power KVM

[Prerna Saxena]

This patch series is a collection of enhancements for PowerPC CPUs on PowerKVM.

Series Summary:
==========
Patch 1/3 : Introduce a new architecture 'ppc64le' for libvirt.
Patch 2/3 : Add libvirt support for VMs running in 'compat' mode on Power KVM.
Patch 3/3 : Optimize PVR comparison for PowerPC CPUs.

Detail:
====
* PowerPC has traditionally been a Big-endian architecture. However, with PowerPC ISA version 2.07, it can run in Little-endian mode as well. IBM Power8 processors, compliant with ISA 2.07 allow launching VMs in little-endian mode. This is signified by 'ppc64le' architecture. Patch 1 adds this support to libvirt, to allow running VMs based on ppc64le architecture.

* Patch 2,3 tweak libvirt to correctly model PowerPC CPUs based on recent PowerKVM implementation.

PowerKVM permits VMs with vcpus in the following allowed modes :
    i) Host native mode:
      where the vcpu seen in the VM belongs to the same processor generation as the host.
      Example: A POWER7 host, conforming to PowerISA version 2.06, will run VMs with "power7" vcpus.
    ii) Binary Compatibility ("compat") mode:
      PowerISA allows processors to run VMs in binary compatibility ("compat") mode supporting an older version of ISA.
      As an example: In compatibility mode, a POWER7 host can run a "power6" VM, conforming to power ISA v2.05.        
      Similarly, a POWER8 host can run a "power7" VM conforming to PowerISA v2.06.

QEMU has recently added support to explicitly denote a VM running in compatibility mode through commits 6d9412ea & 8dfa3a5e85. Henceforth, VMs of type (i) will be invoked with the QEMU invocation "-cpu host", while VMs of type (ii) will be invoked using "-cpu host, compat=power6".
Now, an explicit cpu selection using "-cpu POWER6" is not valid. Instead, the recommended practice is to use the matching compat mode, if the requested cpu type differs from the host.
Patches 2-3 address various aspects of this change.

* Patch 2 : Adds support for generating the correct command line for QEMU. New xml semantics are introduced to signify this type.
 
* Patch 3 : PowerKVM vCPUs differ uniquely across generations ( such as power6, power7, power8). Each generation signifies a new PowerISA version that exhibits features unique to that generation. The higher order 16 bits of PVR denote the processor generation and the lower order 16 bits denote the cpu chip (sub)version.
For all practical purposes of launching a VM, we care about the generation the vCPU will belong to, and not specifically the chip version. In fact, PowerKVM does not seek out specification of a unique chip version(such as POWER7_v2.3) for running a vCPU. This patch updates the libvirt PVR check to reflect this relationship.


Changelog:
=========
v1 : https://www.redhat.com/archives/libvir-list/2014-June/msg01338.html
v2 : http://www.redhat.com/archives/libvir-list/2014-October/msg00351.html

Changes since v2:
i) CPU compat mode is now introduced under CPU mode 'host-model' as against the original proposal of using cpu mode'custom'. (Thanks for the tip, Dan.)
ii) Patch 4/4 that renamed models in cpu_map.xml is dropped. Instead, CPU generations are added to cpu_map.xml as new listings. The PVR check in patch 3 of this series takes care of making the right match happen for compat mode.

Regards,

-- 
Prerna Saxena

Linux Technology Centre,
IBM Systems and Technology Lab,
Bangalore, India