s390/smp: Add support for HOTPLUG_SMT

Add support for HOTPLUG_SMT. With this the s390 specific "nosmt" kernel
command line parameter handling is replaced with common code handling.

This means that just specifying "nosmt" still enables smt from an
architectural point of view, however only the primary (base) cpu can be set
online. Enabling smt during runtime via /sys/devices/system/cpu/smt/control
allows to set secondary cpus online. This way "nosmt" works like on other
architectures where enabling and disabling smt during runtime is possible.

If "nosmt=force" is specified smt is also still enabled from an
architectural point of view, but there is no way to set secondary cpus
online during runtime, also like on other architectures.

In order to disable smt from architectural point of view, which was
previously achieved with the s390 specific "nosmt" command line option,
"smt=1" can be used.

Tested-by: Mete Durlu <meted@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: hcahca

Documentation/admin-guide/kernel-parameters.txt

@@ -4236,10 +4236,10 @@
 	nosmp		[SMP,EARLY] Tells an SMP kernel to act as a UP kernel,
 			and disable the IO APIC.  legacy for "maxcpus=0".
 
-	nosmt		[KNL,MIPS,PPC,S390,EARLY] Disable symmetric multithreading (SMT).
+	nosmt		[KNL,MIPS,PPC,EARLY] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
-			[KNL,X86,PPC] Disable symmetric multithreading (SMT).
+			[KNL,X86,PPC,S390] Disable symmetric multithreading (SMT).
 			nosmt=force: Force disable SMT, cannot be undone
 				     via the sysfs control file.
 

arch/s390/Kconfig

@@ -239,6 +239,7 @@ config S390
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HAVE_VIRT_CPU_ACCOUNTING_IDLE
+	select HOTPLUG_SMT
 	select IOMMU_HELPER		if PCI
 	select IOMMU_SUPPORT		if PCI
 	select KASAN_VMALLOC if KASAN

arch/s390/include/asm/topology.h

@@ -61,6 +61,12 @@ static inline void topology_expect_change(void) { }
 
 #endif /* CONFIG_SCHED_TOPOLOGY */
 
+static inline bool topology_is_primary_thread(unsigned int cpu)
+{
+	return smp_get_base_cpu(cpu) == cpu;
+}
+#define topology_is_primary_thread topology_is_primary_thread
+
 #define POLARIZATION_UNKNOWN	(-1)
 #define POLARIZATION_HRZ	(0)
 #define POLARIZATION_VL		(1)

arch/s390/kernel/smp.c

@@ -99,13 +99,6 @@ __vector128 __initdata boot_cpu_vector_save_area[__NUM_VXRS];
 static unsigned int smp_max_threads __initdata = -1U;
 cpumask_t cpu_setup_mask;
 
-static int __init early_nosmt(char *s)
-{
-	smp_max_threads = 1;
-	return 0;
-}
-early_param("nosmt", early_nosmt);
-
 static int __init early_smt(char *s)
 {
 	get_option(&s, &smp_max_threads);
@@ -808,6 +801,7 @@ void __init smp_detect_cpus(void)
 	mtid = boot_core_type ? sclp.mtid : sclp.mtid_cp;
 	mtid = (mtid < smp_max_threads) ? mtid : smp_max_threads - 1;
 	pcpu_set_smt(mtid);
+	cpu_smt_set_num_threads(smp_cpu_mtid + 1, smp_cpu_mtid + 1);
 
 	/* Print number of CPUs */
 	c_cpus = s_cpus = 0;