selftests/rseq: Implement rseq mm_cid field support

Add support for the mm_cid field (per-memory-map concurrency ID) of
struct rseq to rseq selftests.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221122203932.231377-11-mathieu.desnoyers@efficios.com

tools/testing/selftests/rseq/rseq-abi.h

@@ -155,6 +155,15 @@ struct rseq_abi {
 	 */
 	__u32 node_id;
 
+	/*
+	 * Restartable sequences mm_cid field. Updated by the kernel. Read by
+	 * user-space with single-copy atomicity semantics. This field should
+	 * only be read by the thread which registered this data structure.
+	 * Aligned on 32-bit. Contains the current thread's concurrency ID
+	 * (allocated uniquely within a memory map).
+	 */
+	__u32 mm_cid;
+
 	/*
 	 * Flexible array member at end of structure, after last feature field.
 	 */

tools/testing/selftests/rseq/rseq.h

@@ -191,6 +191,16 @@ static inline uint32_t rseq_current_node_id(void)
 	return RSEQ_ACCESS_ONCE(rseq_get_abi()->node_id);
 }
 
+static inline bool rseq_mm_cid_available(void)
+{
+	return (int) rseq_feature_size >= rseq_offsetofend(struct rseq_abi, mm_cid);
+}
+
+static inline uint32_t rseq_current_mm_cid(void)
+{
+	return RSEQ_ACCESS_ONCE(rseq_get_abi()->mm_cid);
+}
+
 static inline void rseq_clear_rseq_cs(void)
 {
 	RSEQ_WRITE_ONCE(rseq_get_abi()->rseq_cs.arch.ptr, 0);