From: Raphael Gault <raphael.ga...@arm.com> Add a documentation file to describe the access to the pmu hardware counters from userspace
Signed-off-by: Raphael Gault <raphael.ga...@arm.com> Signed-off-by: Rob Herring <r...@kernel.org> --- v2: - Update links to test examples Changes from Raphael's v4: - Convert to rSt - Update chained event status - Add section for heterogeneous systems --- Documentation/arm64/index.rst | 1 + .../arm64/perf_counter_user_access.rst | 56 +++++++++++++++++++ 2 files changed, 57 insertions(+) create mode 100644 Documentation/arm64/perf_counter_user_access.rst diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst index d9665d83c53a..c712a08e7627 100644 --- a/Documentation/arm64/index.rst +++ b/Documentation/arm64/index.rst @@ -15,6 +15,7 @@ ARM64 Architecture legacy_instructions memory perf + 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..e49e141f10cc --- /dev/null +++ b/Documentation/arm64/perf_counter_user_access.rst @@ -0,0 +1,56 @@ +============================================= +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. + +The userspace access is supported in libperf using the perf_evsel__mmap() +and perf_evsel__read() functions. See `tools/lib/perf/tests/test-evsel.c`_ for +an example. + +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. + +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 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