If using iPXE script to boot a s390x  LPAR (classic or DPM) than two additional artifacts can be downloaded which are required to boot a LPAR:

• generic.ins
• initrd.addrsize

Example of the generic.ins file : 

• minimal lpar ins file
  images/kernel.img 0x00000000
  images/initrd.img 0x02000000
  images/genericdvd.prm 0x00010480
  images/initrd.addrsize 0x00010408

To download the generic.ins file take the link to download the kernel or rootfs file from ipxe-script and replace kernel or rootfs with ins-file.
Example:
http://api.openshift.com/api/assisted-images/boot-artifacts/ins-file?arch=s390x&version=4.16

The downloaded generic.ins file is addressing an images folder so create an folder called images and put kernel.img, initrd.img, genericdvd.prm (need to be created or renamed) and initrd.addrsize into it. Alternatively adapt the generic.ins file to meet the file location of these files.

To download the initrd.addrsize file take the link to download the initrd and replace the pxe-initrd by s390x-initrd-addrsize. Due to the fact that this applies to s390x architecture only, the arch is not needed and can be removed. Example:
http://api.openshift.com/api/assisted-images/images/<infra_env_id>/s390x-initrd-addrsize?
image_token=<token_string>&version=4.10

The initrd.addrsize file contains the correct PSW for the initrd.img file and the size of the initrd appended with the ignition.