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

Gleb Natapov gleb at redhat.com
Mon Oct 11 19:04:25 CEST 2010

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?
I think so. Kevin will know for sure.


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