cpu/SMT: create and export cpu_smt_possible()

KVM needs to know if SMT is theoretically possible, this means it is
supported and not forcefully disabled ('nosmt=force'). Create and
export cpu_smt_possible() answering this question.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/linux/cpu.h

@@ -201,12 +201,14 @@ enum cpuhp_smt_control {
 extern enum cpuhp_smt_control cpu_smt_control;
 extern void cpu_smt_disable(bool force);
 extern void cpu_smt_check_topology(void);
+extern bool cpu_smt_possible(void);
 extern int cpuhp_smt_enable(void);
 extern int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval);
 #else
 # define cpu_smt_control		(CPU_SMT_NOT_IMPLEMENTED)
 static inline void cpu_smt_disable(bool force) { }
 static inline void cpu_smt_check_topology(void) { }
+static inline bool cpu_smt_possible(void) { return false; }
 static inline int cpuhp_smt_enable(void) { return 0; }
 static inline int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) { return 0; }
 #endif

kernel/cpu.c

@@ -392,8 +392,7 @@ enum cpuhp_smt_control cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
 
 void __init cpu_smt_disable(bool force)
 {
-	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED ||
-		cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
+	if (!cpu_smt_possible())
 		return;
 
 	if (force) {
@@ -438,6 +437,14 @@ static inline bool cpu_smt_allowed(unsigned int cpu)
 	 */
 	return !cpumask_test_cpu(cpu, &cpus_booted_once_mask);
 }
+
+/* Returns true if SMT is not supported of forcefully (irreversibly) disabled */
+bool cpu_smt_possible(void)
+{
+	return cpu_smt_control != CPU_SMT_FORCE_DISABLED &&
+		cpu_smt_control != CPU_SMT_NOT_SUPPORTED;
+}
+EXPORT_SYMBOL_GPL(cpu_smt_possible);
 #else
 static inline bool cpu_smt_allowed(unsigned int cpu) { return true; }
 #endif