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Date:   Mon, 03 Jul 2023 15:00:42 -0700
From:   John Fastabend <john.fastabend@gmail.com>
To:     Tero Kristo <tero.kristo@linux.intel.com>, shuah@kernel.org,
        tglx@linutronix.de, x86@kernel.org, bp@alien8.de,
        dave.hansen@linux.intel.com, mingo@redhat.com
Cc:     ast@kernel.org, linux-kselftest@vger.kernel.org,
        linux-kernel@vger.kernel.org, andrii@kernel.org,
        daniel@iogearbox.net, bpf@vger.kernel.org
Message-ID: <64a3450a2a062_65205208a9@john.notmuch>
In-Reply-To: <20230703105745.1314475-3-tero.kristo@linux.intel.com>
References: <20230703105745.1314475-1-tero.kristo@linux.intel.com>
 <20230703105745.1314475-3-tero.kristo@linux.intel.com>
Subject: RE: [PATCH 2/2] selftests/bpf: Add test for bpf_rdtsc
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Tero Kristo wrote:
> Add selftest for bpf_rdtsc() which reads the TSC (Time Stamp Counter) on
> x86_64 architectures. The test reads the TSC from both userspace and the
> BPF program, and verifies the TSC values are in incremental order as
> expected. The test is automatically skipped on architectures that do not
> support the feature.
> 
> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
> ---
>  .../selftests/bpf/prog_tests/test_rdtsc.c     | 67 +++++++++++++++++++
>  .../testing/selftests/bpf/progs/test_rdtsc.c  | 21 ++++++
>  2 files changed, 88 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_rdtsc.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c b/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
> new file mode 100644
> index 000000000000..2b26deb5b35a
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/test_rdtsc.c
> @@ -0,0 +1,67 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright(c) 2023 Intel Corporation */
> +
> +#include "test_progs.h"
> +#include "test_rdtsc.skel.h"
> +
> +#ifdef __x86_64__
> +
> +static inline u64 _rdtsc(void)
> +{
> +	u32 low, high;
> +
> +	__asm__ __volatile__("rdtscp" : "=a" (low), "=d" (high));

I think its ok but note this could fail if user doesn't have
access to rdtscp and iirc that can be restricted?

> +	return ((u64)high << 32) | low;
> +}