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

[Anthony Liguori]

On 10/11/2010 10:52 AM, Stefan Hajnoczi wrote:
> 2010/10/11 Gleb Natapov<address@hidden>:
>    
> > On Mon, Oct 11, 2010 at 01:48:09PM +0100, Stefan Hajnoczi wrote:
> >      
> > > On Mon, Oct 11, 2010 at 12:16 PM, Bernhard Kohl<address@hidden>  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?

Regards,

Anthony Liguori

> Stefan
>
>