linux-kernel - Re: [PATCH] efi: Clarify Kconfig help for EFI

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Date:	Mon, 14 Apr 2014 10:33:32 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	Thomas Bächler <thomas@...hlinux.org>
Cc:	Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
	Matt Fleming <matt.fleming@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] efi: Clarify Kconfig help for EFI_MIXED

On Mon, 14 Apr, at 10:28:02AM, Thomas Bächler wrote:
> Am 14.04.2014 09:15, schrieb Ingo Molnar:
> >>> Right, it is really that it is not possible to boot a mixed-mode kernel
> >>> on its non-native firmware using the stub, since the stub itself can
> >>> only be one way or the other.
> >>
> >> Yeah, my help text was a bit... unhelpful.
> >>
> >> CONFIG_EFI_MIXED does not introduce a regression of any sort, you can
> >> enable it without worrying about that.
> >>
> >> All the boot configurations that used to work will continue to work.
> >>
> >> What I meant to say was that it isn't possible to use the EFI mixed 
> >> *feature* (booting a 64-bit kernel with 32-bit firmware) with the 
> >> EFI boot stub - so you can build support into your kernel but 
> >> there's no way to make the CPU actually execute those code paths.
> > 
> > Same goes for booting a 32-bit kernel on 64-bit firmware, right?
> > 
> 
> It seems that isn't implemented:
> 
> config EFI_MIXED
>         bool "EFI mixed-mode support"
>         depends on EFI_STUB && X86_64
> 

Correct, since that would require translating 64-bit pointers from the
firmware into 32-bit pointers for the kernel, which isn't as straight
forward as the 32-bit firmware / 64-bit kernel case, i.e. ensure all
allocations are < 4GB and truncate 64-bit pointers when they're passed
to the firmware.

That and the fact that no one has said "We really need support".

-- 
Matt Fleming, Intel Open Source Technology Center
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