Boot devices on s390x

Booting with bootindex parameter

For classical mainframe guests (i.e. LPAR or z/VM installations), you always have to explicitly specify the disk where you want to boot from (or “IPL” from, in s390x-speak – IPL means “Initial Program Load”).

So for booting an s390x guest in QEMU, you should always mark the device where you want to boot from with the bootindex property, for example:

qemu-system-s390x -drive if=none,id=dr1,file=guest.qcow2 \
                  -device virtio-blk,drive=dr1,bootindex=1

Multiple devices may have a bootindex. The lowest bootindex is assigned to the device to IPL first. If the IPL fails for the first, the device with the second lowest bootindex will be tried and so on until IPL is successful or there are no remaining boot devices to try.

For booting from a CD-ROM ISO image (which needs to include El-Torito boot information in order to be bootable), it is recommended to specify a scsi-cd device, for example like this:

qemu-system-s390x -blockdev file,node-name=c1,filename=... \
                  -device virtio-scsi \
                  -device scsi-cd,drive=c1,bootindex=1

Note that you really have to use the bootindex property to select the boot device. The old-fashioned -boot order=... command of QEMU (and also -boot once=...) is not supported on s390x.

Booting without bootindex parameter

The QEMU guest firmware (the so-called s390-ccw bios) has also some rudimentary support for scanning through the available block devices. So in case you did not specify a boot device with the bootindex property, there is still a chance that it finds a bootable device on its own and starts a guest operating system from it. However, this scanning algorithm is still very rough and may be incomplete, so that it might fail to detect a bootable device in many cases. It is really recommended to always specify the boot device with the bootindex property instead.

This also means that you should avoid the classical short-cut commands like -hda, -cdrom or -drive if=virtio, since it is not possible to specify the bootindex with these commands. Note that the convenience -cdrom option even does not give you a real (virtio-scsi) CD-ROM device on s390x. Due to technical limitations in the QEMU code base, you will get a virtio-blk device with this parameter instead, which might not be the right device type for installing a Linux distribution via ISO image. It is recommended to specify a CD-ROM device via -device scsi-cd (as mentioned above) instead.

Selecting kernels with the loadparm property

The s390-ccw-virtio machine supports the so-called loadparm parameter which can be used to select the kernel on the disk of the guest that the s390-ccw bios should boot. When starting QEMU, it can be specified like this:

qemu-system-s390x -machine s390-ccw-virtio,loadparm=<string>

The first way to use this parameter is to use the word PROMPT as the <string> here. In that case the s390-ccw bios will show a list of installed kernels on the disk of the guest and ask the user to enter a number to chose which kernel should be booted – similar to what can be achieved by specifying the -boot menu=on option when starting QEMU. Note that the menu list will only show the names of the installed kernels when using a DASD-like disk image with 4k byte sectors. On normal SCSI-style disks with 512-byte sectors, there is not enough space for the zipl loader on the disk to store the kernel names, so you only get a list without names here.

The second way to use this parameter is to use a number in the range from 0 to 31. The numbers that can be used here correspond to the numbers that are shown when using the PROMPT option, and the s390-ccw bios will then try to automatically boot the kernel that is associated with the given number. Note that 0 can be used to boot the default entry.

Booting from a network device

The firmware that ships with QEMU includes a small TFTP network bootloader for virtio-net-ccw devices. The bootindex property is especially important for booting via the network. If you don’t specify the bootindex property here, the network bootloader won’t be taken into consideration and the network boot will fail. For a successful network boot, try something like this:

qemu-system-s390x -netdev user,id=n1,tftp=...,bootfile=... \
                  -device virtio-net-ccw,netdev=n1,bootindex=1

The network bootloader also has basic support for pxelinux.cfg-style configuration files. See the PXELINUX Configuration page for details how to set up the configuration file on your TFTP server. The supported configuration file entries are DEFAULT, LABEL, KERNEL, INITRD and APPEND (see the Syslinux Config file syntax for more information).