powerpc: Add HOTPLUG_SMT support

Add support for HOTPLUG_SMT, which enables the generic sysfs SMT support
files in /sys/devices/system/cpu/smt, as well as the "nosmt" boot
parameter.

Implement the recently added hooks to allow partial SMT states, allow
any number of threads per core.

Tie the config symbol to HOTPLUG_CPU, which enables it on the major
platforms that support SMT. If there are other platforms that want the
SMT support that can be tweaked in future.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
[ldufour: remove topology_smt_supported]
[ldufour: remove topology_smt_threads_supported]
[ldufour: select CONFIG_SMT_NUM_THREADS_DYNAMIC]
[ldufour: update kernel-parameters.txt]
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Link: https://msgid.link/20230705145143.40545-10-ldufour@linux.ibm.com

Documentation/admin-guide/kernel-parameters.txt

@@ -3853,10 +3853,10 @@
 	nosmp		[SMP] Tells an SMP kernel to act as a UP kernel,
 			and disable the IO APIC.  legacy for "maxcpus=0".
 
-	nosmt		[KNL,MIPS,S390] Disable symmetric multithreading (SMT).
+	nosmt		[KNL,MIPS,PPC,S390] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
-			[KNL,X86] Disable symmetric multithreading (SMT).
+			[KNL,X86,PPC] Disable symmetric multithreading (SMT).
 			nosmt=force: Force disable SMT, cannot be undone
 				     via the sysfs control file.
 

arch/powerpc/Kconfig

@@ -272,6 +272,8 @@ config PPC
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_VIRT_CPU_ACCOUNTING
 	select HAVE_VIRT_CPU_ACCOUNTING_GEN
+	select HOTPLUG_SMT			if HOTPLUG_CPU
+	select SMT_NUM_THREADS_DYNAMIC
 	select HUGETLB_PAGE_SIZE_VARIABLE	if PPC_BOOK3S_64 && HUGETLB_PAGE
 	select IOMMU_HELPER			if PPC64
 	select IRQ_DOMAIN

arch/powerpc/include/asm/topology.h

@@ -143,5 +143,20 @@ static inline int cpu_to_coregroup_id(int cpu)
 #endif
 #endif
 
+#ifdef CONFIG_HOTPLUG_SMT
+#include <linux/cpu_smt.h>
+#include <asm/cputhreads.h>
+
+static inline bool topology_is_primary_thread(unsigned int cpu)
+{
+	return cpu == cpu_first_thread_sibling(cpu);
+}
+
+static inline bool topology_smt_thread_allowed(unsigned int cpu)
+{
+	return cpu_thread_in_core(cpu) < cpu_smt_num_threads;
+}
+#endif
+
 #endif /* __KERNEL__ */
 #endif	/* _ASM_POWERPC_TOPOLOGY_H */

arch/powerpc/kernel/smp.c

@@ -1087,7 +1087,7 @@ static int __init init_big_cores(void)
 
 void __init smp_prepare_cpus(unsigned int max_cpus)
 {
-	unsigned int cpu;
+	unsigned int cpu, num_threads;
 
 	DBG("smp_prepare_cpus\n");
 
@@ -1154,6 +1154,12 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 
 	if (smp_ops && smp_ops->probe)
 		smp_ops->probe();
+
+	// Initalise the generic SMT topology support
+	num_threads = 1;
+	if (smt_enabled_at_boot)
+		num_threads = smt_enabled_at_boot;
+	cpu_smt_set_num_threads(num_threads, threads_per_core);
 }
 
 void smp_prepare_boot_cpu(void)