Re: [Qemu-devel] OVMF, SeaBIOS & non-CSM based legacy boot

[Anthony Liguori]

On 03/21/2011 01:14 PM, Jordan Justen wrote:
> This weekend I spent some time working on loading SeaBIOS from OVMF to
> start a legacy boot.  I was able to get x86&  x86-64 Linux to legacy
> boot using this method.
>
> Unfortunately, (I think) it is not nearly as nice a having a true CSM.
>   Basically, you have to decide at some point in the OVMF boot that you
> want to legacy boot, and once you start SeaBIOS running, OVMF/UEFI

Interesting.  How much time does OVMF add to the total boot time when 
taking this approach?

> will never be in the picture again (until system reset).
>
> Contrast this to using a CSM where you can:
> * Load a legacy option ROM (vbios, or disk rom),
>    and use it during a UEFI boot

Is there gPXE for UEFI yet?

Other than that, this probably matters most of with device passthrough.  
I think this is a limitation we could live with in the not-to-short term 
though.

> * Fail a legacy boot, and potentially return back to UEFI if it fails.
>    (Not in all cases, if the failed boot alters the system state significantly)

I think the most typical use-case here is -boot cd or -boot dc.  From 
what I can tell, EFI support El Torito so it wouldn't be necessary to 
involve BIOS to handle booting from CDROMs.  Is that correct?

> * Specify via UEFI the boot process which disk to legacy boot in the CSM

I'm sure this is workable.

> So, would this be valuable (in the short term) to help move forward
> QEMU's usage of OVMF and add UEFI support?  Or would QEMU require true
> CSM support?

I think this certainly helps make the discussion more concrete.  I don't 
see any show stoppers here.

Regards,

Anthony Liguori

> Thanks,
>
> -Jordan