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-rw-r--r--arch/arm64/kernel/topology.c32
1 files changed, 17 insertions, 15 deletions
diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index fa9528dfd0ce..113903db666c 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -35,21 +35,23 @@ void store_cpu_topology(unsigned int cpuid)
if (mpidr & MPIDR_UP_BITMASK)
return;
- /* Create cpu topology mapping based on MPIDR. */
- if (mpidr & MPIDR_MT_BITMASK) {
- /* Multiprocessor system : Multi-threads per core */
- cpuid_topo->thread_id = MPIDR_AFFINITY_LEVEL(mpidr, 0);
- cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 1);
- cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 2) |
- MPIDR_AFFINITY_LEVEL(mpidr, 3) << 8;
- } else {
- /* Multiprocessor system : Single-thread per core */
- cpuid_topo->thread_id = -1;
- cpuid_topo->core_id = MPIDR_AFFINITY_LEVEL(mpidr, 0);
- cpuid_topo->package_id = MPIDR_AFFINITY_LEVEL(mpidr, 1) |
- MPIDR_AFFINITY_LEVEL(mpidr, 2) << 8 |
- MPIDR_AFFINITY_LEVEL(mpidr, 3) << 16;
- }
+ /*
+ * This would be the place to create cpu topology based on MPIDR.
+ *
+ * However, it cannot be trusted to depict the actual topology; some
+ * pieces of the architecture enforce an artificial cap on Aff0 values
+ * (e.g. GICv3's ICC_SGI1R_EL1 limits it to 15), leading to an
+ * artificial cycling of Aff1, Aff2 and Aff3 values. IOW, these end up
+ * having absolutely no relationship to the actual underlying system
+ * topology, and cannot be reasonably used as core / package ID.
+ *
+ * If the MT bit is set, Aff0 *could* be used to define a thread ID, but
+ * we still wouldn't be able to obtain a sane core ID. This means we
+ * need to entirely ignore MPIDR for any topology deduction.
+ */
+ cpuid_topo->thread_id = -1;
+ cpuid_topo->core_id = cpuid;
+ cpuid_topo->package_id = cpu_to_node(cpuid);
pr_debug("CPU%u: cluster %d core %d thread %d mpidr %#016llx\n",
cpuid, cpuid_topo->package_id, cpuid_topo->core_id,