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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.02.12 at 09:43, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Fri, 2012-02-24 at 08:35 +0000, Jan Beulich wrote:
>> >>> On 23.02.12 at 16:37, 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?
>> 
>> I'd assume more the former.
>> 
>> > 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?
>> 
>> When I asked the same question, I was told that the assumption is that
>> on modern (read: UEFI) systems people aren't expected to run 32-bit
>> OSes.
> 
> Doesn't that apply to running those OSes under Xen to some extent as
> well? Can we get away with only supporting 64 bit EFI?

If we don't care about 32-bit OSes using EFI, yes.

>> And no, I don't think any 32-bit OS comes with a 64-bit bootloader.
>> Furthermore, to make EFI really work, such an OS would also need
>> 64-bit stubs to call the EFI services - not something that's realistically
>> going to happen I would think.
>> 
>> If anything, I would have expected that things could work out the
>> other way around: A 64-bit OS having a 32-bit loader and 32-bit
>> stubs (which is how we implemented this when, on x86, there was
>> no 64-bit EFI yet).
> 
> Yes, that does sound more plausible. Does anyone still do that though?

I don't think so (and what we did never got released in any form).

> What about in a situation where they could just as easily run a
> traditional BIOS (like in a Xen HVM guest)? Why would anyone insist on
> being able to use 32 bit EFI if they could just use SeaBIOS? I suppose
> the underlying question is: are there any OSes which _only_ support 32
> bit EFI and don't support a traditional BIOS?

I'm unaware of such OSes (and it would make pretty little sense to me
for the next couple of years). But it is clear that without legacy BIOS
support (which the ultimate goal of UEFI is), there's not going to be
a legacy (BIOS) boot method, and hence 32-bit OSes will be impossible
to run on such systems.

Jan


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