[Qemu-devel] Re: [PATCHv2] Support for booting from virtio disks

[Avi Kivity]

On 05/11/2010 03:31 PM, Gleb Natapov wrote:
> On Tue, May 11, 2010 at 11:19:07AM +0300, Avi Kivity wrote:
>    
> > On 05/10/2010 06:48 PM, Anthony Liguori wrote:
> >      
> > > On 05/10/2010 03:11 AM, Gleb Natapov wrote:
> > >        
> > > > This patch adds native support for booting from virtio disks to Seabios.
> > > >
> > > > Signed-off-by: Gleb Natapov<address@hidden>
> > > >          
> > > A related problem that I think we need to think about how we solve
> > > is indicating to Seabios which device we want to boot from
> > >
> > > With your patch, a user can select a virtio device explicitly or
> > > if they use only one virtio device, it will Just Work.
> > >
> > > However, if a user uses IDE and virtio, or a user has multiple
> > > disks, they cannot select a device via -boot.
> > >
> > > Is this something we need to address?  I don't think we'd break
> > > libvirt if we didn't.
> > >        
> > BIOSes traditionally address this by storing the boot order in RTC
> > non-volatile memory, and allow the user to configure the order via a
> > menu.  We could do the same (storing the RTC memory in a small disk
> > image).
> >
> >      
> Real BIOS can do that because it enumerates all bootable devices,
> attach name for each one of them and then asks user to configure
> boot order using names it attached to devices. In our case we
> want to provide boot order on qemu command line before BIOS
> enumerated devices, so qemu should be able to pass enough information
> about boot device so that BIOS can uniquely identify it after it will
> discover all bootable devices. bus/device pair can be such thing.
>    

Having a BIOS menu is also useful, you don't have to drop to the 
management tool, instead you do everything from the console.

>    
> > Alternatively we can seed the order from the command line (-boot
> > id1,id2,id3 where id* are some qdev property attached to disks, this
> > is more flexible than the current syntax I think).
> >
> >      
> The problem is how to communicate this order to Seabios.
>    

Topology (bus/device/lun).

--
Do not meddle in the internals of kernels, for they are subtle and quick to 
panic.