[RFC PATCH for 5.8 4/4] rseq: selftests: Expect reliable cpu_id field

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

The rseq selftests should discover whether the kernel implements the
RSEQ_FLAG_RELIABLE_CPU_ID flag, which indicates that the
__rseq_abi.cpu_id field is reliable.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Paul Turner <pjt@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Neel Natu <neelnatu@google.com>
Cc: linux-api@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
---
 tools/testing/selftests/rseq/rseq.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/rseq/rseq.c b/tools/testing/selftests/rseq/rseq.c
index 7159eb777fd3..55f1edb0649c 100644
--- a/tools/testing/selftests/rseq/rseq.c
+++ b/tools/testing/selftests/rseq/rseq.c
@@ -73,6 +73,11 @@ static int sys_rseq(volatile struct rseq *rseq_abi, uint32_t rseq_len,
 	return syscall(__NR_rseq, rseq_abi, rseq_len, flags, sig);
 }
 
+static bool rseq_reliable_cpu_id(void)
+{
+	return sys_rseq(NULL, 0, RSEQ_FLAG_RELIABLE_CPU_ID, 0) == 0;
+}
+
 int rseq_register_current_thread(void)
 {
 	int rc, ret = 0;
@@ -87,7 +92,8 @@ int rseq_register_current_thread(void)
 	}
 	if (__rseq_refcount++)
 		goto end;
-	rc = sys_rseq(&__rseq_abi, sizeof(struct rseq), 0, RSEQ_SIG);
+	rc = sys_rseq(&__rseq_abi, sizeof(struct rseq),
+		      RSEQ_FLAG_REGISTER | RSEQ_FLAG_RELIABLE_CPU_ID, RSEQ_SIG);
 	if (!rc) {
 		assert(rseq_current_cpu_raw() >= 0);
 		goto end;
@@ -96,6 +102,8 @@ int rseq_register_current_thread(void)
 		__rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
 	ret = -1;
 	__rseq_refcount--;
+	if (errno == EINVAL && !rseq_reliable_cpu_id())
+		fprintf(stderr, "Error: rseq does not provide a reliable cpu_id field.\n");
 end:
 	signal_restore(oldset);
 	return ret;
-- 
2.17.1