[SeaBIOS] [Qemu-devel] [RFC] Passing boot order from qemu to seabios

Stefan Hajnoczi stefanha at gmail.com
Mon Oct 11 17:52:31 CEST 2010

2010/10/11 Gleb Natapov <gleb at redhat.com>:
> On Mon, Oct 11, 2010 at 01:48:09PM +0100, Stefan Hajnoczi wrote:
>> On Mon, Oct 11, 2010 at 12:16 PM, Bernhard Kohl <bernhard.kohl at nsn.com> wrote:
>> > Am 11.10.2010 12:18, schrieb ext Gleb Natapov:
>> >>
>> >> Currently if VM is started with multiple disks it is almost impossible to
>> >> guess which one of them will be used as boot device especially if there
>> >> is a mix of ATA/virtio/SCSI devices. Essentially BIOS decides the order
>> >> and without looking into the code you can't tell what the order will
>> >> be (and in qemu-kvm if boot=on is used it brings even more havoc). We
>> >> should allow fine-grained control of boot order from qemu command line,
>> >> or as a minimum control what device will be used for booting.
>> >>
>> >> To do that along with inventing syntax to specify boot order on qemu
>> >> command line we need to communicate boot order to seabios via fw_cfg
>> >> interface. For that we need to have a way to unambiguously specify a
>> >> disk from qemu to seabios.  PCI bus address is not enough since not all
>> >> devices are PCI (do we care about them?) and since one PCI device may
>> >> control more then one disk (ATA slave/master, SCSI LUNs). We can do what
>> >> EDD specification does. Describe disk as:
>> >>     bus type (isa/pci),
>> >>     address on a bus (16 bit base address for isa, b/s/f for pci)
>> >>     device type (ATA/SCSI/VIRTIO)
>> >>     device path (slave/master for ATA, LUN for SCSI, nothing for virtio)
>> >>
>> >> Will it cover all use cased? Any other ideas?
>> >
>> > I think this also applies to network booting via gPXE. Usually our VMs
>> > have 4 NICs, mixed virtio-net and PCI pass-through. 2 of the NICs shall
>> > be used for booting, even if there are hard disks or floppy disks
>> > connected. This scenario is currently almost impossible to configure.
>> Here is a gPXE to support fw_cfg.  You can pass gPXE script files from
>> the host to gPXE inside the guest.  This means you can boot specific
>> NICs:
>> http://patchwork.ozlabs.org/patch/43777/
>> Just wanted to post the link because it is related to the gPXE side of
>> this discussion.
> Don't we load gPXE for each NIC and seabios passes PCI device to boot from
> when it invokes one of them?

SeaBIOS may do that but gPXE internally just probes all PCI devices.
It does not take advantage of the PCI bus/addr/fn that was passed to
the option ROM.  A gPXE instance will try booting from each available
NIC in sequence.


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