Signed-off-by: Gleb Natapov <g...@redhat.com> --- docs/bootindex.txt | 42 ++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 42 insertions(+), 0 deletions(-) create mode 100644 docs/bootindex.txt
diff --git a/docs/bootindex.txt b/docs/bootindex.txt new file mode 100644 index 0000000..9609c7f --- /dev/null +++ b/docs/bootindex.txt @@ -0,0 +1,42 @@ += Bootindex property = + +Block and net devices have bootindex property. This property is used to +determine the order in which firmware will consider devices for booting +guest OS. If bootindex property is not set for a device it gets lowest +boot priority. There is no particular order in which devices with unset +bootindex property will be considered for booting, but they will still be +bootable. + +== Example == + +Lets assume we have QEMU machine with two NICs (virtio, e1000) and two +disks (IDE, virtio): + +qemu -drive file=disk1.img,if=none,id=disk1 + -device ide-drive,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 command like above firmware should try to boot guest OS from e1000 +NIC if this fails it should try virtio NIC next, if this fails too it +should try virtio disk and then IDE disk. + +== Limitations == + +1. Some firmware has limitation on order in which devices can be +considered for booting. For instance PC bios boot specification allows +only one disk to be bootable. If boot from disk fails for some reason +bios cannot retry booting from other disk. It still can try to boot from +floppy or net though. + +2. Sometimes firmware cannot map device path QEMU wants firmware to boot +from to boot method. It doesn't happen for devices firmware can natively +boot from, but if firmware relies on option rom for booting, and same +option rom is used for booting from more then one device, firmware may +not be able to ask option rom to boot from particular device reliably. +For instance with PC bios if scsi HBA has three bootable devices target1, +target3, target5, connected to it option rom will create boot method for +each of them, but it is not possible to map boot method back to specific +target. This is a shortcoming of PC bios boot specification. -- 1.7.2.3
