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

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Message-ID: <a3aae375-5582-46e4-866b-6a81641998af@redhat.com>
Date: Sun, 24 Mar 2024 15:49:23 +0100
From: Hans de Goede <hdegoede@...hat.com>
To: Ard Biesheuvel <ardb@...nel.org>, Clayton Craft <clayton@...ftyguy.net>
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

Hi,

On 3/21/24 11:48 PM, Ard Biesheuvel wrote:
> (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.

I've been offline for most of the week and I see that in the mean time
you seem to have found a fix, great.

FWIW I have been booting everything up to 6.8.0 on my own mixed-mode
Bay Trail tablets without issues, so the problem seems to be specific to
certain BIOS-es.

Please Cc me on the final fix, then I can test that early and double check
that things don't regress on other mixed-mode Bay Trail devices.

Regards,

Hans