x86/smp: Add a per-cpu view of SMT state

A new field smt_active in cpuinfo_x86 identifies if the current core/cpu
is in SMT mode or not.

This is helpful when the system has some of its cores with threads offlined
and can be used for cases where action is taken based on the state of SMT.

The upcoming support for paranoid L1D flush will make use of this information.
Suggested-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NBalbir Singh <sblbir@amazon.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210108121056.21940-2-sblbir@amazon.com

arch/x86/include/asm/processor.h

@@ -136,6 +136,8 @@ struct cpuinfo_x86 {
 	u16			logical_die_id;
 	/* Index into per_cpu list: */
 	u16			cpu_index;
+	/*  Is SMT active on this core? */
+	bool			smt_active;
 	u32			microcode;
 	/* Address space bits used by the cache internally */
 	u8			x86_cache_bits;

arch/x86/kernel/smpboot.c

@@ -610,6 +610,9 @@ void set_cpu_sibling_map(int cpu)
 	if (threads > __max_smt_threads)
 		__max_smt_threads = threads;
 
+	for_each_cpu(i, topology_sibling_cpumask(cpu))
+		cpu_data(i).smt_active = threads > 1;
+
 	/*
 	 * This needs a separate iteration over the cpus because we rely on all
 	 * topology_sibling_cpumask links to be set-up.
@@ -1552,8 +1555,13 @@ static void remove_siblinginfo(int cpu)
 
 	for_each_cpu(sibling, topology_die_cpumask(cpu))
 		cpumask_clear_cpu(cpu, topology_die_cpumask(sibling));
-	for_each_cpu(sibling, topology_sibling_cpumask(cpu))
+
+	for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
 		cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
+		if (cpumask_weight(topology_sibling_cpumask(sibling)) == 1)
+			cpu_data(sibling).smt_active = false;
+	}
+
 	for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
 		cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
 	cpumask_clear(cpu_llc_shared_mask(cpu));