[libvirt] [Patch v6 0/5] Libvirt CPU enhancements for Power KVM

Prerna Saxena prerna at linux.vnet.ibm.com
Tue Nov 4 17:19:03 UTC 2014

This patch series is a collection of enhancements for PowerPC CPUs on PowerKVM.
The v5 of this series has been acked. I have just rebased the patches on top of 
latest master and added a testcase.

 Series Summary:
Patch 1/5 : Introduce a new architecture 'ppc64le' for libvirt.
Patch 2/5 : Add libvirt support for VMs running in 'compat' mode on Power KVM.
Patch 3/5 : Improve PVR handling to fall back to cpu generation.
Patch 4/5 : Add documentation describing compat mode usage for PowerPC guests.
Patch 5/5 : Add a test case for compat mode.

* 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. Existing xml semantics of 'host-model' are interpreted differently on PowerPc architecture 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.

* Patch 4 : Documentation is added to explain functionality introduced by Patch 2.
* Patch 5 : Added a test case for patch 2.

v1 : https://www.redhat.com/archives/libvir-list/2014-June/msg01338.html
v2 : http://www.redhat.com/archives/libvir-list/2014-October/msg00351.html
v3 : http://www.mail-archive.com/libvir-list@redhat.com/msg104010.html
v4 : http://www.mail-archive.com/libvir-list@redhat.com/msg104067.html
v5 : http://www.mail-archive.com/libvir-list@redhat.com/msg104311.html 

Changes since v5:
* Added a new patch which introduces a test case for compat mode.
* Fixed a whitespace in documentation patch #4.
* Added listing for Power8e cpu model in cpu_map.xml

Prerna Saxena

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

More information about the libvir-list mailing list