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

Stefan Hajnoczi stefanha at gmail.com
Tue Oct 12 10:44:39 CEST 2010

On Mon, Oct 11, 2010 at 6:04 PM, Gleb Natapov <gleb at redhat.com> wrote:
> On Mon, Oct 11, 2010 at 12:01:58PM -0500, Anthony Liguori wrote:
>> On 10/11/2010 10:52 AM, Stefan Hajnoczi wrote:
>> >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.
>> It still registers a BEV entry though, no?


>> Does it at least try to boot from the PCI bus/addr/fn of the
>> selected BEV entry?

Not directly.  It probes all PCI devices and tries them in bus/addr/fn
order.  If you have two identical NICs and only have the boot ROM on
the second NIC, the first NIC will still try to network boot first.

Changing this behavior requires stashing away the bus/addr/fn and then
using it later in gPXE's startup.  It's possible but not implemented


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