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1= Bootindex propery =
2
3Block and net devices have bootindex property. This property is used to
4determine the order in which firmware will consider devices for booting
5the guest OS. If the bootindex property is not set for a device, it gets
6lowest boot priority. There is no particular order in which devices with
7unset bootindex property will be considered for booting, but they will
8still be bootable.
9
10== Example ==
11
12Lets assume we have QEMU machine with two NICs (virtio, e1000) and two
13disks (IDE, virtio):
14
15qemu -drive file=disk1.img,if=none,id=disk1
16 -device ide-drive,drive=disk1,bootindex=4
17 -drive file=disk2.img,if=none,id=disk2
18 -device virtio-blk-pci,drive=disk2,bootindex=3
19 -netdev type=user,id=net0 -device virtio-net-pci,netdev=net0,bootindex=2
20 -netdev type=user,id=net1 -device e1000,netdev=net1,bootindex=1
21
22Given the command above, firmware should try to boot from the e1000 NIC
23first. If this fails, it should try the virtio NIC next, if this fails
24too, it should try the virtio disk, and then the IDE disk.
25
26== Limitations ==
27
281. Some firmware has limitations on which devices can be considered for
29booting. For instance, the PC BIOS boot specification allows only one
30disk to be bootable. If boot from disk fails for some reason, the BIOS
31won't retry booting from other disk. It still can try to boot from
32floppy or net, though.
33
342. Sometimes, firmware cannot map the device path QEMU wants firmware to
35boot from to a boot method. It doesn't happen for devices the firmware
36can natively boot from, but if firmware relies on an option ROM for
37booting, and the same option ROM is used for booting from more then one
38device, the firmware may not be able to ask the option ROM to boot from
39a particular device reliably. For instance with PC BIOS, if a SCSI HBA
40has three bootable devices target1, target3, target5 connected to it,
41the option ROM will have a boot method for each of them, but it is not
42possible to map from boot method back to a specific target. This is a
43shortcoming of PC BIOS boot specification.