Hi Valentin,
Yep, this is a very good question. I'd like change the code to:

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c
index 7cb31d959f33..fc0836f460fb 100644
--- a/drivers/base/arch_topology.c
+++ b/drivers/base/arch_topology.c

@@ -622,7 +622,8 @@ void update_siblings_masks(unsigned int cpuid)
                if (cpuid_topo->package_id != cpu_topo->package_id)
                        continue;

-               if (cpuid_topo->cluster_id == cpu_topo->cluster_id) {
+               if (cpuid_topo->cluster_id == cpu_topo->cluster_id &&
+                   cpuid_topo->cluster_id != -1) {
                        cpumask_set_cpu(cpu, &cpuid_topo->cluster_sibling);
                        cpumask_set_cpu(cpuid, &cpu_topo->cluster_sibling);
                }

This should be consistent with Tim's patch3/3 for x86 in case
id is BAD_APICID:
static bool match_l2c(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
{
        ...
        /* Do not match if we do not have a valid APICID for cpu: */
        if (per_cpu(cpu_l2c_id, cpu1) == BAD_APICID)
                return false;
        ...
}