Subject: Re: [PATCH bpf-next] bpf: Add BPF-helper for accessing CLOCK_TAI

From: Kurt Kanzenbach <[email-masked]>
Date: Wed, 03 Aug 2022 08:29:29 +0200

On Tue Aug 02 2022, Alexei Starovoitov wrote:
> On Tue, Aug 2, 2022 at 12:06 AM Kurt Kanzenbach <[email-masked]> wrote:
>>
>> Hi Alexei,
>>
>> On Tue Jun 07 2022, Alexei Starovoitov wrote:
>> > Anyway I guess new helper bpf_ktime_get_tai_ns() is ok, since
>> > it's so trivial, but selftest is necessary.
>>
>> So, I did write a selftest [1] for testing bpf_ktime_get_tai_ns() and
>> verifying that the access to the clock works. It uses AF_XDP sockets and
>> timestamps the incoming packets. The timestamps are then validated in
>> user space.
>>
>> Since AF_XDP related code is migrating from libbpf to libxdp, I'm
>> wondering if that sample fits into the kernel's selftests or not. What
>> kind of selftest are you looking for?
>
> Please use selftests/bpf framework.
> There are plenty of networking tests in there.
> bpf_ktime_get_tai_ns() doesn't have to rely on af_xdp.

OK.

> It can be skb based.

Something like this?

+++ b/tools/testing/selftests/bpf/prog_tests/check_tai.c
@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022 Linutronix GmbH */
+
+#include <test_progs.h>
+#include <network_helpers.h>
+
+#include <time.h>
+#include <stdint.h>
+
+#define TAI_THRESHOLD	1000000000ULL /* 1s */
+#define NSEC_PER_SEC	1000000000ULL
+
+static __u64 ts_to_ns(const struct timespec *ts)
+{
+	return ts->tv_sec * NSEC_PER_SEC + ts->tv_nsec;
+}
+
+void test_tai(void)
+{
+	struct __sk_buff skb = {
+		.tstamp = 0,
+		.hwtstamp = 0,
+	};
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.ctx_in = &skb,
+		.ctx_size_in = sizeof(skb),
+		.ctx_out = &skb,
+		.ctx_size_out = sizeof(skb),
+	);
+	struct timespec now_tai;
+	struct bpf_object *obj;
+	int ret, prog_fd;
+
+	ret = bpf_prog_test_load("./test_tai.o",
+				 BPF_PROG_TYPE_SCHED_CLS, &obj, &prog_fd);
+	if (!ASSERT_OK(ret, "load"))
+		return;
+	ret = bpf_prog_test_run_opts(prog_fd, &topts);
+	ASSERT_OK(ret, "test_run");
+
+	/* TAI != 0 */
+	ASSERT_NEQ(skb.tstamp, 0, "tai_ts0_0");
+	ASSERT_NEQ(skb.hwtstamp, 0, "tai_ts0_1");
+
+	/* TAI is moving forward only */
+	ASSERT_GT(skb.hwtstamp, skb.tstamp, "tai_forward");
+
+	/* Check for reasoneable range */
+	ret = clock_gettime(CLOCK_TAI, &now_tai);
+	ASSERT_EQ(ret, 0, "tai_gettime");
+	ASSERT_TRUE((ts_to_ns(&now_tai) - skb.hwtstamp) < TAI_THRESHOLD,
+		    "tai_range");
+
+	bpf_object__close(obj);
+}
diff --git a/tools/testing/selftests/bpf/progs/test_tai.c b/tools/testing/selftests/bpf/progs/test_tai.c
new file mode 100644
index 000000000000..34ac4175e29d
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_tai.c
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (C) 2022 Linutronix GmbH */
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("tc")
+int save_tai(struct __sk_buff *skb)
+{
+	/* Save TAI timestamps */
+	skb->tstamp = bpf_ktime_get_tai_ns();
+	skb->hwtstamp = bpf_ktime_get_tai_ns();
+
+	return 0;
+}