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

From: Anthony Liguori
Subject: Re: [Qemu-devel] OVMF, SeaBIOS & non-CSM based legacy boot
Date: Mon, 21 Mar 2011 13:27:41 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110223 Lightning/1.0b2 Thunderbird/3.1.8

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.


Anthony Liguori



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