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

On Thu, 2012-02-23 at 17:33 +0000, Jordan Justen wrote:
> 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.

Thanks for confirming. I suppose in principal there could be some sort
of compat layer (such as we use for running 32 bit guests on a 64 bit
h/v) but from what you say below it doesn't sound like it would be worth
the time to implement.

> 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.

Out of interest do you know any OSes which have good support for 32 bit

Win7 and Ubuntu have support for 64 bit but the 32 bit equivalents don't
appear to.

I'm tempted to suggest that we only support 64 bit UEFI, at least
initially and defer 32 bit support until we have an actual user request
and rationale for that feature.


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