Knowledge Series: CPU Models and Migration Considerations

Another new entry in our Knowledge Series explains CPU models and its relationship to live guest migration.

QEMU v3.1 released

QEMU v3.1 is out. Besides a number of small enhancements, some items that we would like to highlight from a KVM on Z perspective:

• Huge Pages Support: KVM guests can now utilize 1MB pages. As this removes one layer of address translation for the guest backing, less page-faults need to be processed, and less translation lookaside buffer (TLB) entries are needed to hold translations. This, as well as the TLB improvements in z14, will improve KVM guest performance.
  To use:
  Create config file /etc/modprobe.d/kvmhpage.conf file with the following content to enable huge pages for KVM:
  
     options kvm hpage=1
  
  Furthermore, add the following line to /etc/sysctl.conf to reserve N huge pages:
  
     vm.nr_hugepages = N
  
  Alternatively, append the following statement to the kernel parameter line in case support is compiled into the kernel: kvm.hpage=1 hugepages=N.
  Note that means to add hugepages dynamically after boot exist, but with effects like memory fragmentation, it is preferable to define huge pages as early as possible.
  If successful, the file /proc/sys/vm/nr_hugepages should show N huge pages. See here for further documentation.
  Then, to enable huge pages for a guest, add the following element to the respective domain XML:
  
     <memoryBacking>
       <hugepages/>
     </memoryBacking>
  
  The use of huge pages in the host is orthogonal to the use of huge pages in the guest. Both will improve the performance independently by reducing the number of page faults and the number of page table walks after a TLB miss.
  The biggest performance improvement can be achieved by using huge pages in both, host and guest, e.g. with libhugetlbfs, as this will also make use of the larger 1M TLB entries in the hardware.
  Requires Linux kernel 4.19.
• virtio-ap: The Adjunct Processor (AP) facility is an IBM Z cryptographic facility comprised of three AP instructions and up to 256 cryptographic adapter cards, each of which can be group into up to 85 domains , providing cryptographic services. virtio-ap maps a subset of the AP devices/domains to one or more KVM guests, such that the host and each guest has exclusive access to a discrete set of AP devices.
  Here is a small sample script illustrating host setup:
  
     # load vfio-ap device driver
     modprobe vfio-ap
  
     # create an mdev by specifying a UUID (or use uuidgen instead)
     UUID=e926839d-a0b4-4f9c-95d0-c9b34190c4ba
     echo $UUID /sys/devices/vfio_ap/matrix/create
  
     # reserve AP queue 7 on adapter 3 for use by a KVM guest
     echo -0x3 > /sys/bus/ap/apmask
     echo -0x7 > /sys/bus/ap/aqmask
  
     # create a mediated device (mdev) to provide userspace access
     # to a device in a secure manner
     echo $UUID > /sys/devices/vfio_ap/matrix/mdev_supported_types/ \
                  vfio_ap-passthrough/create
     # assign adapter, domain and control domain
     echo +0x3 > /sys/devices/vfio_ap/matrix/${UUID}/assign_adapter
     echo +0x7 > /sys/devices/vfio_ap/matrix/${UUID}/assign_domain
     echo +0x7 > /sys/devices/vfio_ap/matrix/${UUID}/assign_control_domain
  
  To make use of the AP device in a KVM guest, add the following element to the respective domain XML:
  
     <hostdev mode='subsystem' type='mdev' managed='no' model='vfio-ap'>
       <source>
         <address uuid='e926839d-a0b4-4f9c-95d0-c9b34190c4ba'/>
       </source>
     </hostdev>
  
  Once complete, use the passthrough device in a KVM guest just like a regular crypto adapter.
  Requires Linux kernel 4.20 and libvirt 4.9.

SLES 12 SP4 released

SLES 12 SP4 is out! See the announcement and their release note with Z-specific changes.
It ships the following code levels:

• Linux kernel 4.12 (SP3: 4.4),
• QEMU v2.11 (SP3: v2.9), and
• libvirt v4.0 (SP3: v3.3).

See previous blog entries on QEMU v2.10 and v2.11 for details on new features that become available by the QEMU package update.
See previous blog entries on Linux kernel 4.8 and 4.11 for details on new features becoming available through the kernel update, e.g. nested virtualization support.
An additional feature in this release is the availability of STHYI information in LPAR environments. Requires qclib v1.3 or later. See this blog post for general information on qclib.

Furthermore, note that these changes provide a full CPU model, which provides protection against live guest migration compatibility troubles. E.g. migrating a guest exploiting the latest features to a KVM instance running on an earlier IBM Z machine lacking said feature would be detected an prevented.
Note: With this feature, live guest migration back to a KVM instance that does not yet support CPU models (e.g. SLES 12 SP3) will not work anymore.

Ubuntu 18.10 released

Ubuntu Server 18.10 is out! Support for IBM Z is available here.
It ships

• Linux kernel 4.18,
• QEMU v2.12, and
• libvirt v4.6.

Knowledge Series: Black Box Guest Analysis Using kvm_stat

Another new entry in our Knowledge Series details how to gain insights into black box KVM guests using kvm_stat.

Knowledge Series: How to use vnc for Guest Installs

A new entry in our Knowledge Series details how to utilize vnc for graphical installs, exemplified using RHEL 7.5.

2018 Linux on IBM Z and LinuxONE Workshop, Poughkeepsie, NY

Meet us at this event, taking place November 5-6, 2018, at IBM Poughkeepsie, NY. See the full announcement here.
Naturally, KVM on IBM Z will be covered by both, presentations and workgroup sessions.

Find the agenda here.

Registration is open here till October 25.