powerpc/cacheinfo: Print correct cache-sibling map/list for L2 cache

On POWER platforms where only some groups of threads within a core
share the L2-cache (indicated by the ibm,thread-groups device-tree
property), we currently print the incorrect shared_cpu_map/list for
L2-cache in the sysfs.

This patch reports the correct shared_cpu_map/list on such platforms.

Example:
On a platform with "ibm,thread-groups" set to
                 00000001 00000002 00000004 00000000
                 00000002 00000004 00000006 00000001
                 00000003 00000005 00000007 00000002
                 00000002 00000004 00000000 00000002
                 00000004 00000006 00000001 00000003
                 00000005 00000007

This indicates that threads {0,2,4,6} in the core share the L2-cache
and threads {1,3,5,7} in the core share the L2 cache.

However, without the patch, the shared_cpu_map/list for L2 for CPUs 0,
1 is reported in the sysfs as follows:

/sys/devices/system/cpu/cpu0/cache/index2/shared_cpu_list:0-7
/sys/devices/system/cpu/cpu0/cache/index2/shared_cpu_map:000000,000000ff

/sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_list:0-7
/sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map:000000,000000ff

With the patch, the shared_cpu_map/list for L2 cache for CPUs 0, 1 is
correctly reported as follows:

/sys/devices/system/cpu/cpu0/cache/index2/shared_cpu_list:0,2,4,6
/sys/devices/system/cpu/cpu0/cache/index2/shared_cpu_map:000000,00000055

/sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_list:1,3,5,7
/sys/devices/system/cpu/cpu1/cache/index2/shared_cpu_map:000000,000000aa

This patch also defines cpu_l2_cache_mask() for !CONFIG_SMP case.
Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1607596739-32439-6-git-send-email-ego@linux.vnet.ibm.com

arch/powerpc/include/asm/smp.h

@@ -201,6 +201,10 @@ static inline const struct cpumask *cpu_smallcore_mask(int cpu)
 	return cpumask_of(cpu);
 }
 
+static inline const struct cpumask *cpu_l2_cache_mask(int cpu)
+{
+	return cpumask_of(cpu);
+}
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_PPC64

arch/powerpc/kernel/cacheinfo.c

@@ -655,11 +655,27 @@ static unsigned int index_dir_to_cpu(struct cache_index_dir *index)
  * On big-core systems, each core has two groups of CPUs each of which
  * has its own L1-cache. The thread-siblings which share l1-cache with
  * @cpu can be obtained via cpu_smallcore_mask().
+ *
+ * On some big-core systems, the L2 cache is shared only between some
+ * groups of siblings. This is already parsed and encoded in
+ * cpu_l2_cache_mask().
+ *
+ * TODO: cache_lookup_or_instantiate() needs to be made aware of the
+ *       "ibm,thread-groups" property so that cache->shared_cpu_map
+ *       reflects the correct siblings on platforms that have this
+ *       device-tree property. This helper function is only a stop-gap
+ *       solution so that we report the correct siblings to the
+ *       userspace via sysfs.
  */
-static const struct cpumask *get_big_core_shared_cpu_map(int cpu, struct cache *cache)
+static const struct cpumask *get_shared_cpu_map(struct cache_index_dir *index, struct cache *cache)
 {
-	if (cache->level == 1)
-		return cpu_smallcore_mask(cpu);
+	if (has_big_cores) {
+		int cpu = index_dir_to_cpu(index);
+		if (cache->level == 1)
+			return cpu_smallcore_mask(cpu);
+		if (cache->level == 2 && thread_group_shares_l2)
+			return cpu_l2_cache_mask(cpu);
+	}
 
 	return &cache->shared_cpu_map;
 }
@@ -670,17 +686,11 @@ show_shared_cpumap(struct kobject *k, struct kobj_attribute *attr, char *buf, bo
 	struct cache_index_dir *index;
 	struct cache *cache;
 	const struct cpumask *mask;
-	int cpu;
 
 	index = kobj_to_cache_index_dir(k);
 	cache = index->cache;
 
-	if (has_big_cores) {
-		cpu = index_dir_to_cpu(index);
-		mask = get_big_core_shared_cpu_map(cpu, cache);
-	} else {
-		mask  = &cache->shared_cpu_map;
-	}
+	mask = get_shared_cpu_map(index, cache);
 
 	return cpumap_print_to_pagebuf(list, buf, mask);
 }