path: root/Documentation/arm64
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authorRaphael Gault <raphael.gault@arm.com>2021-12-08 14:11:24 -0600
committerWill Deacon <will@kernel.org>2021-12-14 11:41:19 +0000
commitaa1005d15d2aee10e5b93a25db076c47e05c4efa (patch)
tree357cac762f0f31d4fa8438724141ff2209c6f08f /Documentation/arm64
parentarm64: perf: Enable PMU counter userspace access for perf event (diff)
Documentation: arm64: Document PMU counters access from userspace
Add documentation to describe the access to the pmu hardware counters from userspace. Signed-off-by: Raphael Gault <raphael.gault@arm.com> Signed-off-by: Rob Herring <robh@kernel.org> Link: https://lore.kernel.org/r/20211208201124.310740-6-robh@kernel.org Signed-off-by: Will Deacon <will@kernel.org>
Diffstat (limited to 'Documentation/arm64')
1 files changed, 77 insertions, 1 deletions
diff --git a/Documentation/arm64/perf.rst b/Documentation/arm64/perf.rst
index b567f177d385..1f87b57c2332 100644
--- a/Documentation/arm64/perf.rst
+++ b/Documentation/arm64/perf.rst
@@ -2,7 +2,10 @@
.. _perf_index:
Perf Event Attributes
@@ -88,3 +91,76 @@ exclude_host. However when using !exclude_hv there is a small blackout
window at the guest entry/exit where host events are not captured.
On VHE systems there are no blackout windows.
+Perf Userspace PMU Hardware Counter Access
+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.
+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 counters, the global sysctl
+kernel/perf_user_access must first be enabled:
+.. code-block:: sh
+ echo 1 > /proc/sys/kernel/perf_user_access
+It is necessary to open the event using the perf tool interface with config1:1
+attr bit set: 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.
+Access to the PMU registers is only valid while the sequence lock is unchanged.
+In particular, the PMSELR_EL0 register is zeroed each time the sequence lock is
+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 and counter sizes
+The user can request either a 32-bit (config1:0 == 0) or 64-bit (config1:0 == 1)
+counter along with userspace access. The sys_perf_event_open syscall will fail
+if a 64-bit counter is requested and the hardware doesn't support 64-bit
+counters. Chained events are not supported in conjunction with userspace counter
+access. If a 32-bit counter is requested on hardware with 64-bit counters, then
+userspace must treat the upper 32-bits read from the counter as UNKNOWN. The
+'pmc_width' field in the user page will indicate the valid width of the counter
+and should be used to mask the upper bits as needed.
+.. Links
+.. _tools/perf/arch/arm64/tests/user-events.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c
+.. _tools/lib/perf/tests/test-evsel.c:
+ https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c