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

[Gleb Natapov <gleb@redhat.com>]

On Mon, Oct 11, 2010 at 04:52:31PM +0100, Stefan Hajnoczi wrote:
> 2010/10/11 Gleb Natapov <gleb@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@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.

Ah, thanks for clarification. Looks like gPXE does the wrong thing here.
Can this behaviour be changed by compile time option?

--
			Gleb.