LoongArch: Add HOTPLUG_SMT implementation

For benchmarking or debugging purpose, we usually want to control SMT
via boot parameter and sysfs knobs. So add HOTPLUG_SMT implementation.

1. Boot parameters:

nosmt:		Disable SMT, can be enabled via sysfs knobs.
nosmt=force:	Disable SMT, cannot be enabled via sysfs knobs.

2. Runtime sysfs controls:

Write "on", "off", "forceoff" or the number of SMT threads (1, 2, ...)
to /sys/devices/system/cpu/smt/control.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Author: chenhuacai

Documentation/admin-guide/kernel-parameters.txt

@@ -4585,7 +4585,7 @@ Kernel parameters
 	nosmt		[KNL,MIPS,PPC,EARLY] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
-			[KNL,X86,PPC,S390] Disable symmetric multithreading (SMT).
+			[KNL,LOONGARCH,X86,PPC,S390] Disable symmetric multithreading (SMT).
 			nosmt=force: Force disable SMT, cannot be undone
 				     via the sysfs control file.
 

arch/loongarch/Kconfig

@@ -186,6 +186,7 @@ config LOONGARCH
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_TIF_NOHZ
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN
+	select HOTPLUG_SMT if HOTPLUG_CPU
 	select IRQ_FORCED_THREADING
 	select IRQ_LOONGARCH_CPU
 	select LOCK_MM_AND_FIND_VMA

arch/loongarch/kernel/smp.c

@@ -365,16 +365,29 @@ void __init loongson_smp_setup(void)
 void __init loongson_prepare_cpus(unsigned int max_cpus)
 {
 	int i = 0;
+	int threads_per_core = 0;
 
 	parse_acpi_topology();
 	cpu_data[0].global_id = cpu_logical_map(0);
 
+	if (!pptt_enabled)
+		threads_per_core = 1;
+	else {
+		for_each_possible_cpu(i) {
+			if (cpu_to_node(i) != 0)
+				continue;
+			if (cpus_are_siblings(0, i))
+				threads_per_core++;
+		}
+	}
+
 	for (i = 0; i < loongson_sysconf.nr_cpus; i++) {
 		set_cpu_present(i, true);
 		csr_mail_send(0, __cpu_logical_map[i], 0);
 	}
 
 	per_cpu(cpu_state, smp_processor_id()) = CPU_ONLINE;
+	cpu_smt_set_num_threads(threads_per_core, threads_per_core);
 }
 
 /*