Re: Drop CSM support in OvmfPkg?

[Gerd Hoffmann <kraxel@redhat.com>]

On Tue, Jan 22, 2019 at 04:23:06PM +0000, David Woodhouse wrote:
> On Tue, 2019-01-22 at 16:13 +0000, Ni, Ray wrote:
> > David,
> > I'd like to re-start the discussion.
> > Could you please kindly explain the background/reason of adding CSM
> > support in OVMF?
> > Maybe knowing the reason can help to make further decisions of
> > whether to
> > A. keep it outside OvmfPkg
> > B. keep it inside OvmfPkg
> > C. maybe have a chance to just remove the CSM support after
> > revisiting
> 
> The idea was to make it simple to have a single firmware image for
> virtual machines which would support both UEFI and Legacy boot for
> guests simultaneously.

The idea never really took off though.

> In libvirt there has been an alternative approach, where the BIOS image
> is switched between OVMF and SeaBIOS according to the configuration of
> the guest VM.

It's not mandated by libvirt, you can easily create a VM configuration
which uses a OVMF build with CSM support.

But, yes, it is rarely seen in practice.

Microsoft is doing the same btw:  hyper-v has gen1 (bios) and gen2
(uefi) guest types.

> That's fine for libvirt, but in situations where VM hosting is provided
> as a service, it becomes quite painful to manage the 'UEFI' vs.
> 'Legacy' flags on guest images and then switch firmware images
> accordingly.

Seems people try to address this by building cloud images which support
both BIOS and UEFI.

> A one-size-fits-all BIOS using OVMF+CSM is very much
> preferable.

Building a one-size-fits-all BIOS is pretty much impossible due to CSM
being incompatible with secure boot.

cheers,
  Gerd