1 files changed, 47 insertions, 0 deletions
diff --git a/arch/powerpc/include/asm/kvm_book3s.h b/arch/powerpc/include/asm/kvm_book3s.h
index 1f345a0b6ba2..83a9aa3cf689 100644
--- a/arch/powerpc/include/asm/kvm_book3s.h
+++ b/arch/powerpc/include/asm/kvm_book3s.h
@@ -390,4 +390,51 @@ extern int kvmppc_h_logical_ci_store(struct kvm_vcpu *vcpu);
 #define SPLIT_HACK_MASK			0xff000000
 #define SPLIT_HACK_OFFS			0xfb000000
 
+/*
+ * This packs a VCPU ID from the [0..KVM_MAX_VCPU_ID) space down to the
+ * [0..KVM_MAX_VCPUS) space, using knowledge of the guest's core stride
+ * (but not its actual threading mode, which is not available) to avoid
+ * collisions.
+ *
+ * The implementation leaves VCPU IDs from the range [0..KVM_MAX_VCPUS) (block
+ * 0) unchanged: if the guest is filling each VCORE completely then it will be
+ * using consecutive IDs and it will fill the space without any packing.
+ *
+ * For higher VCPU IDs, the packed ID is based on the VCPU ID modulo
+ * KVM_MAX_VCPUS (effectively masking off the top bits) and then an offset is
+ * added to avoid collisions.
+ *
+ * VCPU IDs in the range [KVM_MAX_VCPUS..(KVM_MAX_VCPUS*2)) (block 1) are only
+ * possible if the guest is leaving at least 1/2 of each VCORE empty, so IDs
+ * can be safely packed into the second half of each VCORE by adding an offset
+ * of (stride / 2).
+ *
+ * Similarly, if VCPU IDs in the range [(KVM_MAX_VCPUS*2)..(KVM_MAX_VCPUS*4))
+ * (blocks 2 and 3) are seen, the guest must be leaving at least 3/4 of each
+ * VCORE empty so packed IDs can be offset by (stride / 4) and (stride * 3 / 4).
+ *
+ * Finally, VCPU IDs from blocks 5..7 will only be seen if the guest is using a
+ * stride of 8 and 1 thread per core so the remaining offsets of 1, 5, 3 and 7
+ * must be free to use.
+ *
+ * (The offsets for each block are stored in block_offsets[], indexed by the
+ * block number if the stride is 8. For cases where the guest's stride is less
+ * than 8, we can re-use the block_offsets array by multiplying the block
+ * number by (MAX_SMT_THREADS / stride) to reach the correct entry.)
+ */
+static inline u32 kvmppc_pack_vcpu_id(struct kvm *kvm, u32 id)
+{
+	const int block_offsets[MAX_SMT_THREADS] = {0, 4, 2, 6, 1, 5, 3, 7};
+	int stride = kvm->arch.emul_smt_mode;
+	int block = (id / KVM_MAX_VCPUS) * (MAX_SMT_THREADS / stride);
+	u32 packed_id;
+
+	if (WARN_ONCE(block >= MAX_SMT_THREADS, "VCPU ID too large to pack"))
+		return 0;
+	packed_id = (id % KVM_MAX_VCPUS) + block_offsets[block];
+	if (WARN_ONCE(packed_id >= KVM_MAX_VCPUS, "VCPU ID packing failed"))
+		return 0;
+	return packed_id;
+}
+
 #endif /* __ASM_KVM_BOOK3S_H__ */