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

Gleb Natapov gleb at redhat.com
Mon Oct 11 14:08:13 CEST 2010

On Mon, Oct 11, 2010 at 01:16:00PM +0200, Bernhard Kohl 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.
> I already posted a patch 'new parameter boot=on|off for "-net nic" and
> "-device" NIC devices' which should solve that problem for us. The
> patch is still under discussion. Of course passing detailed boot device
> information to SeaBIOS would be the best solution.
Yeah, forgot to specify net booting specifically. Do we emulate usb
storage? If yes it can be bootable too. Sorry, I missed your patch
initially, but now looking at it I do not think this is right approach.
It resembles to me extboot hack. We shouldn't play games with roms to
specify boot order. Bios is able to handle this by itself, we just need
to help it. NIC can be specified by its bus address and may be virtual
function (for SR-IOV devices ?). Seabios will be able to find what
option rom it should run from there.

> >Any ideas about qemu
> >command line syntax? May be somebody whats to implement it? :)
> >
> >--
> >			Gleb.
> >


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