22.5. How virtualization on IBM Z differs from AMD64 and Intel 64

Virtual PCI and USB devices are not supported on IBM Z. This also means that virtio-*-pci devices are unsupported, and virtio-*-ccw devices should be used instead. For example, use virtio-net-ccw instead of virtio-net-pci.

Note that direct attachment of PCI devices, also known as PCI passthrough, is supported.

IBM Z does not support the <boot dev='device'> XML configuration element. To define device boot order, use the <boot order='number'> element in the <devices> section of the XML.

In addition, you can select the required boot entry using the architecture-specific loadparm attribute in the <boot> element. For example, the following determines that the disk should be used first in the boot sequence and if a Linux distribution is available on that disk, it will select the second boot entry:

<disk type='file' device='disk'>
  <driver name='qemu' type='qcow2'/>
  <source file='/path/to/qcow2'/>
  <target dev='vda' bus='virtio'/>
  <address type='ccw' cssid='0xfe' ssid='0x0' devno='0x0000'/>
  <boot order='1' loadparm='2'/>
</disk>

If using watchdog devices in your VM on an IBM Z host, use the diag288 model. For example:

<devices>
  <watchdog model='diag288' action='poweroff'/>
</devices>

To successfully migrate to a later host model (for example from IBM z14 to z15), or to update the hypervisor, use the host-model CPU mode. The host-passthrough and maximum CPU modes are not recommended, as they are generally not migration-safe.

If you want to specify an explicit CPU model in the custom CPU mode, follow these guidelines:

To successfully migrate to an older host model (such as from z15 to z14), or to an earlier version of QEMU, KVM, or the RHEL kernel, use the CPU type of the oldest available host model without -base at the end.

When using PXE to run a VM on IBM Z, a specific configuration is required for the pxelinux.cfg/default file. For example:

# pxelinux
default linux
label linux
kernel kernel.img
initrd initrd.img
append ip=dhcp inst.repo=example.com/redhat/BaseOS/s390x/os/