Re: [REGRESSION] x86/smpboot/64: System will not boot on v6.5, v6.7-rc4

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Dec 13 2023 at 23:04, Chris Lindee wrote:
> On Wed, Dec 13, 2023 at 7:35 PM Chris Lindee <chris.lindee@gmail.com> wrote:
>
>> The following are the processor options made available by the BIOS, with
>> the currently selected value.  If there are any interesting combinations
>> that you'd like tested, I can try a handful of them.
>>
>
>> x2APIC Mode: Enabled (requires Virtualization Technology to enable)
>>
>
> I disabled x2APIC and the system would boot, both without "cpuhp.parallel"
> and with "cpuhp.parallel=1" on the command line.

I would have expected that.

With the completely untested patch below it should also boot in X2APIC
mode even with the broken AP configuration in the BIOS.

Thanks,

        tglx
---
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -255,6 +255,22 @@ SYM_INNER_LABEL(secondary_startup_64_no_
 	testl	$X2APIC_ENABLE, %eax
 	jnz	.Lread_apicid_msr
 
+#ifdef CONFIG_X86_X2APIC
+	/*
+	 * If system is in X2APIC mode then MMIO base mignt not be
+	 * mapped causing the MMIO read below to fault. Faults can't
+	 * be handled at that point.
+	 */
+	cmpl	$0, x2apic_mode(%rip)
+	jz	.Lread_apicid_mmio
+
+	/* Force the AP into X2APIC mode. */
+	orl	$X2APIC_ENABLE, %eax
+	wrmsr
+	jmp	.Lread_apicid_msr
+#endif
+
+.Lread_apicid_mmio:
 	/* Read the APIC ID from the fix-mapped MMIO space. */
 	movq	apic_mmio_base(%rip), %rcx
 	addq	$APIC_ID, %rcx