x86/cpu/topology: Make primary thread mask available with SMP=n

cpu_primary_thread_mask is only defined when CONFIG_SMP=y. However, even
in UP kernels there is always exactly one CPU, which can reasonably be
treated as the primary thread.

Historically, topology_is_primary_thread() always returned true with
CONFIG_SMP=n. A recent commit:

  4b455f5 ("cpu/SMT: Provide a default topology_is_primary_thread()")

replaced it with a generic implementation with the note:

  "When disabling SMT, the primary thread of the SMT will remain
   enabled/active. Architectures that have a special primary thread (e.g.
   x86) need to override this function. ..."

For consistency and clarity, make the primary thread mask available
regardless of SMP, similar to cpu_possible_mask and cpu_present_mask.

Move __cpu_primary_thread_mask into common code to prevent build issues.
Let cpu_mark_primary_thread() configure the mask even for UP kernels,
alongside other masks. Then, topology_is_primary_thread() can
consistently reference it.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20250320234104.8288-1-chang.seok.bae@intel.com

Author: ChangSeokBae

arch/x86/include/asm/topology.h

@@ -218,6 +218,12 @@ static inline unsigned int topology_amd_nodes_per_pkg(void)
 	return __amd_nodes_per_pkg;
 }
 
+#else /* CONFIG_SMP */
+static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
+static inline int topology_max_smt_threads(void) { return 1; }
+static inline unsigned int topology_amd_nodes_per_pkg(void) { return 1; }
+#endif /* !CONFIG_SMP */
+
 extern struct cpumask __cpu_primary_thread_mask;
 #define cpu_primary_thread_mask ((const struct cpumask *)&__cpu_primary_thread_mask)
 
@@ -241,12 +247,6 @@ static inline bool topology_is_core_online(unsigned int cpu)
 }
 #define topology_is_core_online topology_is_core_online
 
-#else /* CONFIG_SMP */
-static inline int topology_phys_to_logical_pkg(unsigned int pkg) { return 0; }
-static inline int topology_max_smt_threads(void) { return 1; }
-static inline unsigned int topology_amd_nodes_per_pkg(void) { return 1; }
-#endif /* !CONFIG_SMP */
-
 static inline void arch_fix_phys_package_id(int num, u32 slot)
 {
 }

arch/x86/kernel/cpu/topology.c

@@ -75,15 +75,11 @@ bool arch_match_cpu_phys_id(int cpu, u64 phys_id)
 	return phys_id == (u64)cpuid_to_apicid[cpu];
 }
 
-#ifdef CONFIG_SMP
 static void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid)
 {
 	if (!(apicid & (__max_threads_per_core - 1)))
 		cpumask_set_cpu(cpu, &__cpu_primary_thread_mask);
 }
-#else
-static inline void cpu_mark_primary_thread(unsigned int cpu, unsigned int apicid) { }
-#endif
 
 /*
  * Convert the APIC ID to a domain level ID by masking out the low bits

arch/x86/kernel/cpu/topology_common.c

@@ -16,6 +16,9 @@ EXPORT_SYMBOL_GPL(x86_topo_system);
 unsigned int __amd_nodes_per_pkg __ro_after_init;
 EXPORT_SYMBOL_GPL(__amd_nodes_per_pkg);
 
+/* CPUs which are the primary SMT threads */
+struct cpumask __cpu_primary_thread_mask __read_mostly;
+
 void topology_set_dom(struct topo_scan *tscan, enum x86_topology_domains dom,
 		      unsigned int shift, unsigned int ncpus)
 {

arch/x86/kernel/smpboot.c

@@ -103,9 +103,6 @@ EXPORT_PER_CPU_SYMBOL(cpu_core_map);
 DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_die_map);
 EXPORT_PER_CPU_SYMBOL(cpu_die_map);
 
-/* CPUs which are the primary SMT threads */
-struct cpumask __cpu_primary_thread_mask __read_mostly;
-
 /* Representing CPUs for which sibling maps can be computed */
 static cpumask_var_t cpu_sibling_setup_mask;