linux-kernel - Re: Testers wanted: Atom netbooks with x86

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Date:   Sun, 28 Mar 2021 14:37:55 +0200
From:   Mateusz Jończyk <mat.jonczyk@...pl>
To:     Thomas Gleixner <tglx@...utronix.de>, Willy Tarreau <w@....eu>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Testers wanted: Atom netbooks with x86_64 disabled by BIOS

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?

W dniu 28.03.2021 o 03:07, Thomas Gleixner pisze:
 > There are some of these '32bit only' CPUs out there in the wild which
 > actually support long mode. Some of them even do not have the long mode
 > CPUID bit fused out. But whether it works is a different story:
 >
 >   - If the CPUID bit is on, then the chance is high, but it runs out of
 >     spec (guarantee wise)
 >
 >   - If it's off is still might work by some definition of work as they
 >     might have fused off more or there are actual defects in some 64bit
 >     only area which are irrelevant when in 32bit mode.

Of course, force enabling x86_64 would require passing a kernel command line 
parameter with a prominent warning in documentation, just like with "forcepae".

Greetings,
Mateusz