x86/smp: Provide topology_is_primary_thread()

If the CPU is supporting SMT then the primary thread can be found by
checking the lower APIC ID bits for zero. smp_num_siblings is used to build
the mask for the APIC ID bits which need to be taken into account.

This uses the MPTABLE or ACPI/MADT supplied APIC ID, which can be different
than the initial APIC ID in CPUID. But according to AMD the lower bits have
to be consistent. Intel gave a tentative confirmation as well.

Preparatory patch to support disabling SMT at boot/runtime.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/apic.h

@@ -502,6 +502,12 @@ extern int default_check_phys_apicid_present(int phys_apicid);
 
 #endif /* CONFIG_X86_LOCAL_APIC */
 
+#ifdef CONFIG_SMP
+bool apic_id_is_primary_thread(unsigned int id);
+#else
+static inline bool apic_id_is_primary_thread(unsigned int id) { return false; }
+#endif
+
 extern void irq_enter(void);
 extern void irq_exit(void);
 

arch/x86/include/asm/topology.h

@@ -123,13 +123,15 @@ static inline int topology_max_smt_threads(void)
 }
 
 int topology_update_package_map(unsigned int apicid, unsigned int cpu);
-extern int topology_phys_to_logical_pkg(unsigned int pkg);
+int topology_phys_to_logical_pkg(unsigned int pkg);
+bool topology_is_primary_thread(unsigned int cpu);
 #else
 #define topology_max_packages()			(1)
 static inline int
 topology_update_package_map(unsigned int apicid, unsigned int cpu) { return 0; }
 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 bool topology_is_primary_thread(unsigned int cpu) { return true; }
 #endif
 
 static inline void arch_fix_phys_package_id(int num, u32 slot)

arch/x86/kernel/apic/apic.c

@@ -2189,6 +2189,21 @@ static int cpuid_to_apicid[] = {
 	[0 ... NR_CPUS - 1] = -1,
 };
 
+/**
+ * apic_id_is_primary_thread - Check whether APIC ID belongs to a primary thread
+ * @id:	APIC ID to check
+ */
+bool apic_id_is_primary_thread(unsigned int apicid)
+{
+	u32 mask;
+
+	if (smp_num_siblings == 1)
+		return true;
+	/* Isolate the SMT bit(s) in the APICID and check for 0 */
+	mask = (1U << (fls(smp_num_siblings) - 1)) - 1;
+	return !(apicid & mask);
+}
+
 /*
  * Should use this API to allocate logical CPU IDs to keep nr_logical_cpuids
  * and cpuid_to_apicid[] synchronized.

arch/x86/kernel/smpboot.c

@@ -265,6 +265,15 @@ static void notrace start_secondary(void *unused)
 	cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
 }
 
+/**
+ * topology_is_primary_thread - Check whether CPU is the primary SMT thread
+ * @cpu:	CPU to check
+ */
+bool topology_is_primary_thread(unsigned int cpu)
+{
+	return apic_id_is_primary_thread(per_cpu(x86_cpu_to_apicid, cpu));
+}
+
 /**
  * topology_phys_to_logical_pkg - Map a physical package id to a logical
  *