[libvirt] Fwd: [v5][Patch 0/4] Libvirt CPU enhancements for Power KVM [for 1.2.10]

Prerna Saxena prerna at linux.vnet.ibm.com
Sun Nov 2 03:14:37 UTC 2014

Ping !
I understand it is a busy time for libvirt folks -- would someone still be able to take a look ?
I have addressed all concerns raised by Dan in this version.

-------- Original Message --------
Subject: 	[libvirt] [v5][Patch 0/4] Libvirt CPU enhancements for Power KVM
Date: 	Fri, 31 Oct 2014 14:45:52 +0530
From: 	Prerna Saxena <prerna at linux.vnet.ibm.com>
To: 	libvirt mailing list <libvir-list at redhat.com>
CC: 	Michal Privoznik <mprivozn at redhat.com>, jtomko at redhat.com

This patch series is a collection of enhancements for PowerPC CPUs on PowerKVM.
In this iteration, I have followed Dan's suggestion on using existing cpu mode 
format to describe powerPC compatibility mode.
Hope this can finally make it for 1.2.10 !

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

* 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.

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

Changes since v4:
* Patches 1,3 already ack'ed by Michal, and have been posted unchanged.
* In Patch 2,Discarded fallback=compat, and implemented Dan's suggestion to overload cpu mode "host-model".
Now, on PowerPc, a VM with this XML schema will automatically be a compat-mode VM:
<cpu mode='host-model'>
* Introduced a new patch '4' to add documentation that explains this.


Prerna Saxena

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

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