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

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 3 Aug 2022 11:28:50 +0200

On Wed, Aug 03, 2022 at 08:29:29AM +0200, Kurt Kanzenbach wrote:
> 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.

FWIW there is xskxceiver and libbpf's xsk part in selftests/bpf framework,
so your initial work should be fine in there. Personally I found both
(AF_XDP and SKB one, below) tests valuable.

Later on, if we add a support to xskxceiver for loading external BPF progs
then your sample would just become another test case in there.

> 
> 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;
> +}