Re: [Xen-devel] [PATCH 1 of 2] Add the support for Xen to include OVMF UEFI support and directly use it

[Jordan Justen <jljusten@xxxxxxxxx>]

On Thu, Feb 23, 2012 at 07:37, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Thu, 2012-02-23 at 14:48 +0000, Jan Beulich wrote:
>> >>> Attilio Rao <attilio.rao@xxxxxxxxxx> 02/23/12 11:18 AM >>>
>> >On 23/02/12 10:07, Ian Campbell wrote:
>> >> On Wed, 2012-02-22 at 19:17 +0000, Attilio Rao wrote:
>> >> Can you confirm that you need an OVMF which matches the OS bit-width you
>> >> are installing. i..e that there is no support for booting a 32 bit EFI
>> >> OS (or bootloader, shell, whatever it is called) on a 64 bit OVMF?
>> >
>> >I didn't test this case, really, but I would think OVMF-64 / OS-32 could
>> >possibly work.
>>
>> Native EFI requires bit-matched OS loaders,
>
> Is that a shortcoming of EFI generally or just this implementation?

This is basically built into the design of UEFI.  The issue is that
the firmware boot/runtime calls are only available for the native
architecture.

If the OS or loader had support for changing to the native
architecture before calling these services, it can run in another
mode.

> Surely people don't reflash their "BIOS" to be able to run a 32 vs 64
> bit OS? Or do most OSes (even 32 bit ones) have a 64 bit loader capable
> of loading a 32 bit OS?

I don't think people often update their firmware to switch modes.  In
fact, I think in most cases they don't have that option.  For example,
most desktop/server systems with UEFI support will likely only support
UEFI X64.

The loader or OS could have special support for running under this
environment.  It would just need to change to the proper CPU mode
before calling the firmware runtime services.

In the case of UEFI Linux and Windows x86, I think they only support
calling UEFI IA32.  In the case of UEFI Linux and Windows x86-64, I
think they only support calling UEFI X64.

-Jordan

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