From: John Fastabend <john.fastabend@gmail.com>
To: Tero Kristo <tero.kristo@linux.intel.com>,
John Fastabend <john.fastabend@gmail.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
Subject: Re: [PATCH 2/2] selftests/bpf: Add test for bpf_rdtsc
Date: Wed, 05 Jul 2023 21:57:46 -0700 [thread overview]
Message-ID: <64a649cab3dba_b20ce2081c@john.notmuch> (raw)
In-Reply-To: <2901d37a-6b5a-9076-1423-0db95b4c12d3@linux.intel.com>
Tero Kristo wrote:
>
> On 04/07/2023 01:00, John Fastabend wrote:
> > 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?
>
> It is possible to restrict RDTSC access from userspace by enabling the
> TSD bit in CR4 register, and it will cause the userspace process to trap
> with general protection fault.
>
> However, the usage of RDTSC appears to be built-in to C standard
> libraries (probably some timer routines) and enabling the CR4 TSD makes
> the system near unusable. Things like sshd + systemd also start
> generating the same general protection faults if RDTSC is blocked. Also,
> attempting to run anything at all with the BPF selftest suite causes the
> same general protection fault; not only the rdtsc test.
>
> I tried this with couple of setups, one system running a minimalistic
> buildroot and another one running a fedora37 installation and the
> results were similar.
Thanks. Good enough for me.
>
> -Tero
next prev parent reply other threads:[~2023-07-06 4:57 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-03 10:57 [PATCH 0/2] x86/BPF: Add new BPF helper call bpf_rdtsc Tero Kristo
2023-07-03 10:57 ` [PATCH 1/2] x86/tsc: " Tero Kristo
2023-07-04 4:49 ` Yonghong Song
2023-07-06 12:00 ` Tero Kristo
2023-07-06 3:02 ` Alexei Starovoitov
2023-07-06 5:16 ` John Fastabend
2023-07-06 11:59 ` Tero Kristo
2023-07-06 19:51 ` Alexei Starovoitov
2023-07-07 5:41 ` John Fastabend
2023-07-07 8:27 ` Tero Kristo
2023-07-07 14:42 ` Alexei Starovoitov
2023-08-09 11:31 ` Tero Kristo
2023-07-03 10:57 ` [PATCH 2/2] selftests/bpf: Add test for bpf_rdtsc Tero Kristo
2023-07-03 22:00 ` John Fastabend
2023-07-04 8:55 ` Tero Kristo
2023-07-06 4:57 ` John Fastabend [this message]
2023-07-03 21:55 ` [PATCH 0/2] x86/BPF: Add new BPF helper call bpf_rdtsc John Fastabend
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