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

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Date:	Fri, 11 Apr 2014 19:44:33 +0100
From:	Matt Fleming <matt@...sole-pimps.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Bächler <thomas@...hlinux.org>,
	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 Fri, 11 Apr, at 11:03:13AM, H. Peter Anvin wrote:
> On 04/11/2014 10:59 AM, Thomas Bächler wrote:
> > EFI stub support is only missing for a 64 bit kernel on 32-bit firmware,
> > on 64-bit kernels, EFI stub works as usual.
> > ---
> > 
> > Matt, I don't know if this help was intentionally discouraging,
> > however, out of curiosity, I tested this with ovmf, and the
> > kernel boots fine on 64-bit firmware bit with EFI stub, and on
> > 32-bit firmware using efilinux.
> > 
> > Documenting this properly is important for distribution kernels.
> > We would want to support this on Arch, but not if it means
> > introducing a regression for EFI stub users.
> > 
> >  arch/x86/Kconfig | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> > index 5b8ec0f..2bbbbb9 100644
> > --- a/arch/x86/Kconfig
> > +++ b/arch/x86/Kconfig
> > @@ -1529,8 +1529,8 @@ config EFI_MIXED
> >  	   mode.
> >  
> >  	   Note that it is not possible to boot a mixed-mode enabled
> > -	   kernel via the EFI boot stub - a bootloader that supports
> > -	   the EFI handover protocol must be used.
> > +	   kernel via the EFI boot stub on 32-bit firmware - a bootloader
> > +	   that supports the EFI handover protocol must be used.
> >  
> >  	   If unsure, say N.
> >  
> 
> 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.

-- 
Matt Fleming, Intel Open Source Technology Center
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