x86/cpu/topology: Rework possible CPU management

Managing possible CPUs is an unreadable and uncomprehensible maze. Aside of
that it's backwards because it applies command line limits after
registering all APICs.

Rewrite it so that it:

  - Applies the command line limits upfront so that only the allowed amount
    of APIC IDs can be registered.

  - Applies eventual late restrictions in an understandable way

  - Uses simple min_t() calculations which are trivial to follow.

  - Provides a separate function for resetting to UP mode late in the
    bringup process.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/r/20240213210252.290098853@linutronix.de

1 files changed, 4 insertions, 5 deletions

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index b1e52ac164b1..4e320d4d3898 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1131,6 +1131,8 @@ void __init setup_arch(char **cmdline_p)
 
 	early_quirks();
 
+	topology_apply_cmdline_limits_early();
+
 	/*
 	 * Parse SMP configuration. Try ACPI first and then the platform
 	 * specific parser.
@@ -1138,13 +1140,10 @@ void __init setup_arch(char **cmdline_p)
 	acpi_boot_init();
 	x86_init.mpparse.parse_smp_cfg();
 
-	/*
-	 * Systems w/o ACPI and mptables might not have it mapped the local
-	 * APIC yet, but prefill_possible_map() might need to access it.
-	 */
+	/* Last opportunity to detect and map the local APIC */
 	init_apic_mappings();
 
-	prefill_possible_map();
+	topology_init_possible_cpus();
 
 	init_cpu_to_node();
 	init_gi_nodes();