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

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Date:   Sat, 27 Mar 2021 22:13:22 +0100
From:   Mateusz Jończyk <mat.jonczyk@...pl>
To:     linux-kernel@...r.kernel.org
Cc:     Mateusz Jończyk <mat.jonczyk@...pl>
Subject: Re: Testers wanted: Atom netbooks with x86_64 disabled by BIOS

W dniu 27.03.2021 o 21:32, Mateusz Jończyk 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.

Greetings,
Mateusz

Signed-off-by: Mateusz Jończyk <mat.jonczyk@...pl>

---
 arch/x86/boot/compressed/head_64.S | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index e94874f4bbc1..a9f0415da7c2 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -111,8 +111,6 @@ SYM_FUNC_START(startup_32)
 	leal	rva(boot_stack_end)(%ebp), %esp
 
 	call	verify_cpu
-	testl	%eax, %eax
-	jnz	.Lno_longmode
 
 /*
  * Compute the delta between where we were compiled to run at
-- 
2.25.1