Re: [PATCH 2/2] selftests/bpf: Add test for bpf_rdtsc

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