linux-kernel - [patch 3/4] x86/topology: Handle the !APIC case gracefully

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Date: Fri, 22 Mar 2024 19:56:38 +0100 (CET)
From: Thomas Gleixner <tglx@...utronix.de>
To: LKML <linux-kernel@...r.kernel.org>
Cc: x86@...nel.org,
 Guenter Roeck <linux@...ck-us.net>,
 Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [patch 3/4] x86/topology: Handle the !APIC case gracefully

If there is no local APIC enumerated and registered then the topology
bitmaps are empty. Therefore topology_init_possible_cpus() will die with a
division by zero exception.

Prevent this by registering a fake APIC id to populate the topology
bitmap. This also allows to use all topology query interfaces
unconditionally. It does not affect the actual APIC code because either the
local APIC address was not registered or no local APIC could be detected.

Fixes: f1f758a80516 ("x86/topology: Add a mechanism to track topology via APIC IDs")
Reported-by: Guenter Roeck <linux@...ck-us.net>
Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/cpu/topology.c |   11 +++++++++++
 1 file changed, 11 insertions(+)

--- a/arch/x86/kernel/cpu/topology.c
+++ b/arch/x86/kernel/cpu/topology.c
@@ -415,6 +415,17 @@ void __init topology_init_possible_cpus(
 	unsigned int total = assigned + disabled;
 	u32 apicid, firstid;
 
+	/*
+	 * If there was no APIC registered, then fake one so that the
+	 * topology bitmap is populated. That ensures that the code below
+	 * is valid and the various query interfaces can be used
+	 * unconditionally. This does not affect the actual APIC code in
+	 * any way because either the local APIC address has not been
+	 * registered or the local APIC was disabled on the command line.
+	 */
+	if (topo_info.boot_cpu_apic_id == BAD_APICID)
+		topology_register_boot_apic(0);
+
 	if (!restrict_to_up()) {
 		if (WARN_ON_ONCE(assigned > nr_cpu_ids)) {
 			disabled += assigned - nr_cpu_ids;