Support KVM/libvirt on System z (S/390) as a hypervisor platform

Support KVM/libvirt on System z (S/390) as a hypervisor platform

https://blueprints.launchpad.net/nova/+spec/libvirt-kvm-systemz

Add support for KVM/libvirt in Linux on System z as a Nova hypervisor platform. The existing Nova driver for KVM/libvirt will be used. There are some platform-specific changes needed in the Nova driver to get the platform going.

Additional OpenStack functionality beyond initial Nova support is not part of this blueprint; we will have specific additional blueprints for that, as needed.

A 3rd party Continuous Integration environment for OpenStack for KVM/libvirt on System z will be established and maintained.

Problem description

The existing Nova driver for KVM/libvirt does not work unchanged for KVM on System z.

The issues currently known, are:

  1. The System z machine type ('s390-ccw-virtio') needs to be returned by LibvirtDriver._get_machine_type(), if the host architecture is System z. This then causes the os/type/@machine attribute in the libvirt domain.xml to be set up accordingly.

  2. The CPU capabilities returned by libvirt on System z do not include the model element. This is intended to be added to libvirt, but until then we need a temporary circumvention in the Nova driver (adding the model information from /proc/cpuinfo if not present and if on S390x).

  3. For the interactive console and the log of OpenStack instances, console devices need to be generated (instead of serial devices), if the host architecture is System z (see [2]). These console devices need to have a target type "sclp" for the interactive console of OpenStack, and "sclplm" for the log of OpenStack.

    Background: Linux on System z writes its messages during its early boot phases to an interface called “SCLP”. SCLP is available in QEMU as a console device. As a result, it is also a console device in libvirt, with type=”pty” and target type=”sclp” for the interactive console of OpenStack, and with type=”file” and target type=”sclplm” (“lm” for “line mode”) for the log of OpenStack. If we use the default target type (“virtio”), we will not capture the messages written by Linux in its early boot phases, and we do want to provide them.

    Changing the default target type for console devices in libvirt to “sclp” or “sclplm” (e.g. dependent on the console/@type attribute) would be a backwards incompatible change of libvirt.

    If commonality in the Nova driver across the libvirt platforms is important: A console device could also be used for other platforms including x86, and is already used in one of the code paths in the current Nova driver. Serial devices in libvirt are meant to represent serial ports (aka RS-232), so they are not an ideal choice anyway, for interactive console and log.

  4. The FCP support in the libvirt volume driver needs to be adjusted to the System z specific format of the device file paths.

In the hypervisor support matrix (https://wiki.openstack.org/wiki/HypervisorSupportMatrix), the (new) column for KVM/libvirt on System z is intended to look like the column for KVM/libvirt on x86, except that the following functions will not be supported (in addition to those marked as not supported for KVM/libvirt on x86):

  • VNC Console

  • SPICE Console

  • Inject Networking

For block storage, Cinder will be supported.

For networking, Neutron will be supported. Nova networking should work but will not be a focus.

As a result, all features marked as required on https://wiki.openstack.org/wiki/HypervisorSupportMatrix/Requirements will be supported for KVM/libvirt on System z.

Use Cases

Use OpenStack with KVM on System z

Project Priority

None

Proposed change

Change code in the libvirt Nova driver to address these issues, dependent on the host capabilities indicating a CPU architecture of arch.S390X. For details, see section Work Items.

Alternatives

None

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None

Other deployer impact

None (no need for platform-specific parameters in nova.conf as part of this blueprint)

Developer impact

None (changes should not affect other libvirt platforms)

Implementation

Assignee(s)

Primary assignee:

mzoeller

Other contributors:

maiera

Work Items

In nova/virt/libvirt/driver.py:

  • For issue 1: In LibvirtDriver._get_machine_type(), return the System z machine type ('s390-ccw-virtio'), if the host architecture is System z.

  • For issue 2: In `` LibvirtDriver._get_host_capabilities()``, add the model variable to the capabilities, if not present and if the host architecture is System z; the model is based on the machine type from /proc/cpuinfo.

  • For issue 3: In LibvirtDriver._get_guest_config(), create console devices instead of serial devices for the interactive console and for the log, with target type “sclplm” (for the log) and “sclp” (for the interactive console), if the host architecture is System z.

In nova/virt/libvirt/config.py:

  • For issue 3: In LibvirtConfigGuestChar.__init__() and format_dom(), add support for specifying a target type.

In nova/virt/libvirt/volume.py:

  • For issue 4: The FCP support in the libvirt volume driver needs to be adjusted to the System z specific format of the device file paths.

In nova/virt/libvirt/utils.py:

  • For issue 4: Possibly, supporting utility functions for the FCP support issue are needed.

Doc changes (see section Documentation Impact).

Dependencies

  • Software versions:

    • Linux kernel: 3.10 or higher

    • libvirt: 1.0.4 or higher

    • qemu: 1.4.0 or higher

  • 3rd party CI environment for KVM/libvirt on System z set up

  • Future replacements for temporary circumventions:

    • libvirt patch to add model information to CPU capabilities.

Testing

Unit test:

  • Existing Nova unit tests should suffice for the generic bits of Nova.

  • Additional Nova unit tests for any s390-specific behaviors will be written.

3rd party CI:

  • A 3rd party CI environment for KVM/libvirt on Systemz will be set up and run by by IBM, to run full tempest tests.

Documentation Impact

  • No changes needed in config docs.

  • Doc changes for the platform will be made as needed (details are to be determined).

References

Creative Commons Attribution 3.0 License

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.

nova-specs