Thursday, October 6, 2016

Linux kernel 4.8 released

Linux kernel 4.8 (available here) has been released, here are the highlights in support of KVM on z:

Nested Virtualization
This feature allows to start further KVM hosts within KVM guests, also called second level virtualization. As a prerequisite, it requires a recent, post-2.7 version of QEMU including the s390 CPU models (assumed to be included in the forthcoming QEMU 2.8 release).
Nested virtualization is currently turned off per default, and has to be enabled when loading the kvm module:

   [root ~]# modprobe kvm nested=1

or by appending kvm.nested=1 to the kernel command line.
When starting QEMU, make sure to chose the right machine ("s390-ccw-virtio-2.8" or higher) in the domain xml's os element for the guest:

      <type arch='s390x' machine='s390-ccw-virtio-2.8'>hvm</type>
      <boot dev='hd'/>

Finally, verify in a KVM guest that hosting further KVM guests is possible, as indicated by flag "sie" in /proc/cpuinfo as follows:

   [root ~]# cat /proc/cpuinfo | grep features
   features        : esan3 zarch stfle msa ldisp eimm dfp
edat etf3eh
                     highgprs te vx sie

STHYI Instruction available for KVM Guests
Linux kernel 4.8 saw the inclusion of an implementation of the STHYI (Store Hypervisor Information) instruction. Using Linux kernel 4.8 for the host will make the STHYI instruction available to all KVM guests of that host.
Previously available on z/VM only, this instruction provides detailed information on CPU resources on various levels (machine, LPAR, hypervisor, guest, etc.).

Use qclib to access the information. Also see this blog post for further details regarding qclib and KVM on z.

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