On Tue, 2023-02-21 at 09:05 +0100, Oleksandr Natalenko wrote:
> 
> Please see here: http://ix.io/4oLm

Was that just for one CPU? Can we have them all please?

The interesting part is the line starting 00000001. We're looking at
the top 8 bits of EBX:

Leaf     Subleaf    EAX            EBX            ECX            EDX            
00000000 00000000:  00000010 ....  68747541 Auth  444d4163 cAMD  69746e65 enti
00000001 00000000:  00a20f12 ....  00200800 .. .  7ef8320b .2.~  178bfbff ....
                                   ↑↑

So the first CPU is CPU0. Could have told you that... what about the others? :)

If anyone can reproduce this with a serial port, can you try this?

From 98ad11d0fb88f081f49f7b1496420dbfbeff8833 Mon Sep 17 00:00:00 2001
From: David Woodhouse <dwmw@amazon.co.uk>
Date: Sat, 4 Feb 2023 15:20:24 +0000
Subject: [PATCH] parallel debug

---
 arch/x86/kernel/head_64.S | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 0e4e53d231db..da7f4d2d9951 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -281,6 +281,15 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 .Lsetup_AP:
 	/* EDX contains the APIC ID of the current CPU */
+#if 1
+	/* Test hack: Print APIC ID and then CPU# when we find it. */
+	mov	%edx, %ecx
+	mov	%edx, %eax
+	addb	$'A', %al
+	mov	$0x3f8, %dx
+	outb    %al, %dx
+	mov	%ecx, %edx
+#endif
 	xorq	%rcx, %rcx
 	leaq	cpuid_to_apicid(%rip), %rbx
 
@@ -302,6 +311,14 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 
 .Linit_cpu_data:
 	/* Get the per cpu offset for the given CPU# which is in ECX */
+#if 1
+	mov	%rcx, %rax
+	shr	$3, %rax
+	addb	$'a', %al
+
+	mov	$0x3f8, %dx
+	outb    %al, %dx
+#endif
 	leaq	__per_cpu_offset(%rip), %rbx
 	movq	(%rbx,%rcx,8), %rbx
 	/* Save it for GS BASE setup */
-- 
2.39.0