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Date:   Sun, 28 Mar 2021 17:55:27 +0200
From:   Willy Tarreau <w@1wt.eu>
To:     Mateusz Jonczyk <mat.jonczyk@o2.pl>
Cc:     Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org
Subject: Re: Testers wanted: Atom netbooks with x86_64 disabled by BIOS
Message-ID: <20210328155527.GA24931@1wt.eu>
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On Sun, Mar 28, 2021 at 03:30:29PM +0200, Willy Tarreau wrote:
> On Sun, Mar 28, 2021 at 02:37:55PM +0200, Mateusz Jonczyk wrote:
> > W dniu 28.03.2021 o?00:25, Willy Tarreau pisze:
> > > FWIW I tested on my ASUS 1025C which runs on an Atom N2600 forced to
> > > 32-bit. I had already tried in the past but wanted to give it a try
> > > again in case I'd have missed anything. Sadly it didn't work, I'm
> > > still getting the "requires an x86-64 CPU" message.
> > 
> > Thank you. It looks like your bootloader uses the 16-bit kernel boot
> > protocol. The 16-bit kernel boot code checks for x86_64 presence with a
> > similar message ( inside arch/x86/boot/cpu.c ), which I did not patch out.
> > If you would like to test again, use the same patched kernel, but change in
> > GRUB: "linux16" to "linux" and "initrd16" to "initrd" to use the 32-bit boot
> > protocol. Which distribution and bootloader do you use?
> 
> I'm using Lilo on an old Slackware. I can patch the 16-bit code myself,
> it's no big deal.
> 
> > Of course, force enabling x86_64 would require passing a kernel command line
> > parameter with a prominent warning in documentation, just like with
> > "forcepae".
> 
> Sure, but I mean, I suspect that the risk could be higher with very low
> priced laptops were crappy chips are to be expected by definition based
> on contracts neither you nor me have seen. 
> 
> I'll try again after patching the 16-bit code, thanks for the suggestion.

So I added this at the end of get_cpuflags():

     set_bit(X86_FEATURE_LM, cpu.flags);

But now it goes further, the screen turns black and after 2 seconds or so
it reboots, it looks like a triple fault late in the init process. No need
to go further on this machine!

Willy