linux-kernel - [PATCH] x86/xen/smp_pv: Register the boot CPU APIC properly

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Date: Thu, 02 May 2024 16:39:47 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Jürgen Groß <jgross@...e.com>,
 linux-kernel@...r.kernel.org, x86@...nel.org
Cc: Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, Dave
 Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>
Subject: [PATCH] x86/xen/smp_pv: Register the boot CPU APIC properly

The topology core expects the boot APIC to be registered from earhy APIC
detection first and then again when the firmware tables are evaluated. This
is used for detecting the real BSP CPU on a kexec kernel.

The recent conversion of XEN/PV to register fake APIC IDs failed to
register the boot CPU APIC correctly as it only registers it once. This
causes the BSP detection mechanism to trigger wrongly:

   CPU topo: Boot CPU APIC ID not the first enumerated APIC ID: 0 > 1

Additionally this results in one CPU being ignored.

Register the boot CPU APIC twice so that the XEN/PV fake enumeration
behaves like real firmware.

Reported-by: Juergen Gross <jgross@...e.com>
Fixes: e75307023466 ("x86/xen/smp_pv: Register fake APICs")
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Tested-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/xen/smp_pv.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -154,9 +154,9 @@ static void __init xen_pv_smp_config(voi
 	u32 apicid = 0;
 	int i;

-	topology_register_boot_apic(apicid++);
+	topology_register_boot_apic(apicid);

-	for (i = 1; i < nr_cpu_ids; i++)
+	for (i = 0; i < nr_cpu_ids; i++)
 		topology_register_apic(apicid++, CPU_ACPIID_INVALID, true);

 	/* Pretend to be a proper enumerated system */