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Date:   Sun, 28 Mar 2021 00:25:51 +0100
From:   Willy Tarreau <w@1wt.eu>
To:     Mateusz Jonczyk <mat.jonczyk@o2.pl>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: Testers wanted: Atom netbooks with x86_64 disabled by BIOS
Message-ID: <20210327232551.GA20783@1wt.eu>
References: <20210327203218.119372-1-mat.jonczyk@o2.pl>
 <20210327211322.121708-1-mat.jonczyk@o2.pl>
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Hi,

On Sat, Mar 27, 2021 at 10:13:22PM +0100, Mateusz Jonczyk wrote:
> W dniu 27.03.2021 o 21:32, Mateusz Jonczyk pisze:
> > Hello,
> >
> > There are some netbooks with Intel Atom processors that have 64-bit
> > support disabled by BIOS. Theoretically, the processor supports 64-bit
> > operation, but BIOS allows only 32-bit code to run.
> >
> > I wonder whether the 64-bit mode is really disabled in the CPU or only
> > hidden in the CPUID flags. If the latter, the computer could be made to
> > run a 64-bit kernel.
> >
> > Similarly, there are some Pentium M processors that support PAE
> > (Physical Address Extensions), but do not show this in CPUID. They could
> > be made to run distributions that require PAE with the "forcepae" kernel
> > command line parameter.
> >
> > I would like to ask people with such netbooks to try to run a 64-bit kernel
> > with this patch applied.
> >
> > When a patched 64-bit kernel is run in `qemu-system-i386`, the virtual
> > machine restarts instantly. Without this patch in such a case a 64-bit
> > kernel hangs indefinitely (inside .Lno_longmode in head_64.S).
> 
> I have made two mistakes:
> - I left commented out code,
> - I have commented out lines with '#'. The code compiled though.
> 
> Attaching corrected patch, please excuse me.

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.

Given these machines were really cheap, I've always suspected that they
employ cheaper, low-grade CPUs, possibly having been subject to reduced
tests where x86_64-specific parts were not even verified and might be
defective. This may explain why they forcefully disable long mode there,
but that's just speculation.

Cheers,
Willy