This is a short overview about using network boot for s390x.
The network boot feature enhances the s390x ipl device and the s390-ccw bios with support for booting via a virtio-net boot device. A new address is added to the ccw ipl block providing the address of a s390-netboot.img (effectively a network boot ROM) which performs the actual load over the network. This code has been included into QEMU since 2.9.
Starting with 2.10 QEMU includes a pre-built network boot ROM based on the SLOF firmware, enabling network booting from a DHCP/TFTP server. As s390 network bootable operating system images are not very common at this point in time, this document also gives a brief overview on how to build one.
Booting from a network interface
The network boot can't be triggered using the traditional -boot n command line option, but requires to specify the bootindex attribute on the network device. E.g.,
qemu-system-s390x ... -device virtio-net-ccw,netdev=hostnet0,id=net0,bootindex=1
or, with libvirt
... <interface type='network'> <source network='default'/> <model type='virtio'/> <boot order='1'> </interface> ...
Building a s390 network bootable binary
A s390 operating system image usable for network booting can be built from an initial ramdisk and a kernel image. These can be taken from any existing s390 system.
One way to build an image is to take a kernel and an installer initial ramdisk from a distribution's DVD/ISO and concatenate them. Some fixups are necessary in the new binary. Both, concatenation and fixups can be done by using the script in . Booting the resulting image would then start the installation process as if booted from the DVD.
Another possible way is to build a binary that behaves similar to the PXELINUX boot loader. In this case an initial ramdisk with an init process triggering the PXELINUX-like processing has to be built as described in . The site also contains a script assisting in the creation of such an initial ramdisk.