Managing device boot order with bootindex properties
QEMU can tell QEMU-aware guest firmware (like the x86 PC BIOS)
which order it should look for a bootable OS on which devices.
A simple way to set this order is to use the
-boot order= option,
but you can also do this more flexibly, by setting a
property on the individual block or net devices you specify
on the QEMU command line.
bootindex properties are used to determine the order in which
firmware will consider devices for booting the guest OS. If the
bootindex property is not set for a device, it gets the lowest
boot priority. There is no particular order in which devices with no
bootindex property set will be considered for booting, but they
will still be bootable.
Some guest machine types (for instance the s390x machines) do
-boot order=; on those machines you must always
There is no way to set a
bootindex property if you are using
a short-form option like
-cdrom, so to use
bootindex properties you will need to expand out those options
-device option pairs.
Let’s assume we have a QEMU machine with two NICs (virtio, e1000) and two disks (IDE, virtio):
qemu-system-x86_64 -drive file=disk1.img,if=none,id=disk1 \ -device ide-hd,drive=disk1,bootindex=4 \ -drive file=disk2.img,if=none,id=disk2 \ -device virtio-blk-pci,drive=disk2,bootindex=3 \ -netdev type=user,id=net0 \ -device virtio-net-pci,netdev=net0,bootindex=2 \ -netdev type=user,id=net1 \ -device e1000,netdev=net1,bootindex=1
Given the command above, firmware should try to boot from the e1000 NIC first. If this fails, it should try the virtio NIC next; if this fails too, it should try the virtio disk, and then the IDE disk.
Some firmware has limitations on which devices can be considered for booting. For instance, the PC BIOS boot specification allows only one disk to be bootable. If boot from disk fails for some reason, the BIOS won’t retry booting from other disk. It can still try to boot from floppy or net, though.
Sometimes, firmware cannot map the device path QEMU wants firmware to boot from to a boot method. It doesn’t happen for devices the firmware can natively boot from, but if firmware relies on an option ROM for booting, and the same option ROM is used for booting from more then one device, the firmware may not be able to ask the option ROM to boot from a particular device reliably. For instance with the PC BIOS, if a SCSI HBA has three bootable devices target1, target3, target5 connected to it, the option ROM will have a boot method for each of them, but it is not possible to map from boot method back to a specific target. This is a shortcoming of the PC BIOS boot specification.
Mixing bootindex and boot order parameters
Note that it does not make sense to use the bootindex property together
-boot order=... (or
-boot once=...) parameter. The guest
firmware implementations normally either support the one or the other,
but not both parameters at the same time. Mixing them will result in
undefined behavior, and thus the guest firmware will likely not boot
from the expected devices.