projects / qemu.git / blame
| Commit | Line | Data |
|---|---|---|
| 280a66a6 | 1 | = Bootindex property = |
| 466b5864 GN |
2 | |
| 3 | Block and net devices have bootindex property. This property is used to | |
| 4 | determine the order in which firmware will consider devices for booting | |
| 5 | the guest OS. If the bootindex property is not set for a device, it gets | |
| 6 | lowest boot priority. There is no particular order in which devices with | |
| 7 | unset bootindex property will be considered for booting, but they will | |
| 8 | still be bootable. | |
| 9 | ||
| 10 | == Example == | |
| 11 | ||
| 12 | Lets assume we have QEMU machine with two NICs (virtio, e1000) and two | |
| 13 | disks (IDE, virtio): | |
| 14 | ||
| 15 | qemu -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 | ||
| 22 | Given the command above, firmware should try to boot from the e1000 NIC | |
| 23 | first. If this fails, it should try the virtio NIC next, if this fails | |
| 24 | too, it should try the virtio disk, and then the IDE disk. | |
| 25 | ||
| 26 | == Limitations == | |
| 27 | ||
| 28 | 1. Some firmware has limitations on which devices can be considered for | |
| 29 | booting. For instance, the PC BIOS boot specification allows only one | |
| 30 | disk to be bootable. If boot from disk fails for some reason, the BIOS | |
| 31 | won't retry booting from other disk. It still can try to boot from | |
| 32 | floppy or net, though. | |
| 33 | ||
| 34 | 2. Sometimes, firmware cannot map the device path QEMU wants firmware to | |
| 35 | boot from to a boot method. It doesn't happen for devices the firmware | |
| 36 | can natively boot from, but if firmware relies on an option ROM for | |
| 37 | booting, and the same option ROM is used for booting from more then one | |
| 38 | device, the firmware may not be able to ask the option ROM to boot from | |
| 39 | a particular device reliably. For instance with PC BIOS, if a SCSI HBA | |
| 40 | has three bootable devices target1, target3, target5 connected to it, | |
| 41 | the option ROM will have a boot method for each of them, but it is not | |
| 42 | possible to map from boot method back to a specific target. This is a | |
| 43 | shortcoming of PC BIOS boot specification. |