linux-kernel - Re: x86_64 32-bit EFI mixed mode boot broken

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Date: Thu, 21 Mar 2024 23:48:09 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Clayton Craft <clayton@...ftyguy.net>, Hans de Goede <hdegoede@...hat.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, regressions@...ts.linux.dev
Subject: Re: x86_64 32-bit EFI mixed mode boot broken

(cc Hans)

On Thu, 21 Mar 2024 at 23:05, Clayton Craft <clayton@...ftyguy.net> wrote:
>
> I've been chasing a problem with 32-bit EFI mixed mode booting on two different
> (x86_64) Intel Bay Trail platforms, where the system reboots or hangs seemingly
> very early somewhere before or after loading the kernel. I've not been able to
> get any output from the kernel or stub over efifb when the issue happens[0], and
> do not have serial console access on these systems.
>
> v6.8 fails for me, and presumably so does everything back to v6.2. v6.1 is able
> to boot OK on these platforms with mixed mode, and it looks like there are a lot
> of changes from 6.1..6.2 for EFI/mixed mode booting.

v6.1 just received some EFI related backports, so please check the
latest v6.1.y as well.

> I did managed to bisect the
> issue to:
>
>         commit e2ab9eab324cdf240de89741e4a1aa79919f0196
>         Author: Ard Biesheuvel <ardb@...nel.org>
>         Date:   Tue Nov 22 17:10:02 2022 +0100
>
>             x86/boot/compressed: Move 32-bit entrypoint code into .text section
>
> However I'm not sure how to proceed from here, or if my bisect is all that
> useful since the commit seems to be in the middle of a bunch of changes I do not
> understand. I've been using systemd-boot to test this (both the full bootloader
> and UKI w/ the sd-boot stub). Is 32-bit mixed mode on x86_64 working for others?
>

I usually test on 32-bit OVMF built with LOAD_X64_ON_IA32_ENABLE,
which allows the use of the compat entry point. This is different from
the EFI handover protocol, and I am not sure which one you are using.

I have never had any reports, or noticed any issues myself. Last time
I tried (some weeks ago) it was working for me.
CC'ing Hans who may have more data points.