From: Michael Ellerman <mpe@ellerman.id.au>

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: pass current SMT level to cpu_smt_set_num_threads]
[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 <redacted>
---
  Documentation/admin-guide/kernel-parameters.txt |  4 ++--
  arch/powerpc/Kconfig                            |  2 ++
  arch/powerpc/include/asm/topology.h             | 15 +++++++++++++++
  arch/powerpc/kernel/smp.c                       |  8 +++++++-
  4 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9e5bab29685f..5efb6c73a928 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -3838,10 +3838,10 @@
  	nosmp		[SMP] Tells an SMP kernel to act as a UP kernel,
  			and disable the IO APIC.  legacy for "maxcpus=0".
  
-	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
+	nosmt		[KNL,S390,PPC] 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.
  

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 8b955bc7b59f..bacabc3d7f0c 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -273,6 +273,8 @@ 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

diff --git a/arch/powerpc/include/asm/topology.h b/arch/powerpc/include/asm/topology.h
index 8a4d4f4d9749..f4e6f2dd04b7 100644
--- a/arch/powerpc/include/asm/topology.h
+++ b/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 */

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 406e6d0ffae3..eb539325dff8 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/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)