From: Rob Herring <robh@kernel.org>
To: Will Deacon <will@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Raphael Gault <raphael.gault@arm.com>,
Jonathan Cameron <Jonathan.Cameron@huawei.com>
Subject: [PATCH 5/5] Documentation: arm64: Document PMU counters access from userspace
Date: Tue, 7 Jul 2020 14:53:33 -0600 [thread overview]
Message-ID: <20200707205333.624938-6-robh@kernel.org> (raw)
In-Reply-To: <20200707205333.624938-1-robh@kernel.org>
From: Raphael Gault <raphael.gault@arm.com>
Add a documentation file to describe the access to the pmu hardware
counters from userspace
Signed-off-by: Raphael Gault <raphael.gault@arm.com>
---
Changes:
- Convert to rSt
- Update chained event status
- Add section for heterogeneous systems
---
Documentation/arm64/index.rst | 1 +
.../arm64/perf_counter_user_access.rst | 52 +++++++++++++++++++
2 files changed, 53 insertions(+)
create mode 100644 Documentation/arm64/perf_counter_user_access.rst
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index 09cbb4ed2237..62f45d620180 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -14,6 +14,7 @@ ARM64 Architecture
hugetlbpage
legacy_instructions
memory
+ perf_counter_user_access
pointer-authentication
silicon-errata
sve
diff --git a/Documentation/arm64/perf_counter_user_access.rst b/Documentation/arm64/perf_counter_user_access.rst
new file mode 100644
index 000000000000..afbc7acaae66
--- /dev/null
+++ b/Documentation/arm64/perf_counter_user_access.rst
@@ -0,0 +1,52 @@
+=============================================
+Access to PMU hardware counter from userspace
+=============================================
+
+Overview
+--------
+The perf userspace tool relies on the PMU to monitor events. It offers an
+abstraction layer over the hardware counters since the underlying
+implementation is cpu-dependent.
+Arm64 allows userspace tools to have access to the registers storing the
+hardware counters' values directly.
+
+This targets specifically self-monitoring tasks in order to reduce the overhead
+by directly accessing the registers without having to go through the kernel.
+
+How-to
+------
+The focus is set on the armv8 pmuv3 which makes sure that the access to the pmu
+registers is enabled and that the userspace has access to the relevant
+information in order to use them.
+
+In order to have access to the hardware counter it is necessary to open the event
+using the perf tool interface: the sys_perf_event_open syscall returns a fd which
+can subsequently be used with the mmap syscall in order to retrieve a page of
+memory containing information about the event.
+The PMU driver uses this page to expose to the user the hardware counter's
+index and other necessary data. Using this index enables the user to access the
+PMU registers using the `mrs` instruction.
+
+Have a look at `tools/perf/arch/arm64/tests/user-events.c`_ for an example. It
+can be run using the perf tool to check that the access to the registers works
+correctly from userspace:
+
+.. code-block:: sh
+
+ perf test -v user
+
+About heterogeneous systems
+---------------------------
+On heterogeneous systems such as big.LITTLE, userspace PMU counter access can
+only be enabled when the tasks are pinned to a homogeneous subset of cores and
+the corresponding PMU instance is opened by specifying the 'type' attribute.
+The use of generic event types is not supported in this case.
+
+About chained events
+--------------------
+Chained events are not supported in userspace. If a 64-bit counter is requested,
+userspace access will only be enabled if the underlying counter is 64-bit.
+
+.. Links
+.. _tools/perf/arch/arm64/tests/user-events.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c
--
2.25.1
prev parent reply other threads:[~2020-07-07 20:53 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-07 20:53 [PATCH 0/5] arm64: Enable access to pmu registers by user-space Rob Herring
2020-07-07 20:53 ` [PATCH 1/5] perf: arm64: Add tests to check userspace access to hardware counters Rob Herring
2020-07-07 20:53 ` [PATCH 2/5] arm64: pmu: Add hook to handle pmu-related undefined instructions Rob Herring
2020-07-07 20:53 ` [PATCH 3/5] arm64: pmu: Add function implementation to update event index in userpage Rob Herring
2020-07-07 20:53 ` [PATCH 4/5] arm64: perf: Enable pmu counter direct access for perf event on armv8 Rob Herring
2020-07-07 20:53 ` Rob Herring [this message]
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