From: Raphael Gault <raphael.ga...@arm.com>

In order to be able to access the counter directly for userspace,
we need to provide the index of the counter using the userpage.
We thus need to override the event_idx function to retrieve and
convert the perf_event index to armv8 hardware index.

Since the arm_pmu driver can be used by any implementation, even
if not armv8, two components play a role into making sure the
behaviour is correct and consistent with the PMU capabilities:

* the ARMPMU_EL0_RD_CNTR flag which denotes the capability to access
counter from userspace.
* the event_idx call back, which is implemented and initialized by
the PMU implementation: if no callback is provided, the default
behaviour applies, returning 0 as index value.

Signed-off-by: Raphael Gault <raphael.ga...@arm.com>
Signed-off-by: Rob Herring <r...@kernel.org>
---
v7:
 - Add define ARMV8_IDX_CYCLE_COUNTER_USER for userspace index
   of cycle counter
---
 arch/arm64/kernel/perf_event.c | 20 +++++++++++++++++++-
 include/linux/perf/arm_pmu.h   |  2 ++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index 4658fcf88c2b..40cf59455ce8 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -332,7 +332,7 @@ static const struct attribute_group 
armv8_pmuv3_caps_attr_group = {
  */
 #define        ARMV8_IDX_CYCLE_COUNTER 0
 #define        ARMV8_IDX_COUNTER0      1
-
+#define        ARMV8_IDX_CYCLE_COUNTER_USER    32
 
 /*
  * We unconditionally enable ARMv8.5-PMU long event counter support
@@ -871,6 +871,22 @@ static void armv8pmu_clear_event_idx(struct pmu_hw_events 
*cpuc,
                clear_bit(idx - 1, cpuc->used_mask);
 }
 
+static int armv8pmu_access_event_idx(struct perf_event *event)
+{
+       if (!(event->hw.flags & ARMPMU_EL0_RD_CNTR))
+               return 0;
+
+       /*
+        * We remap the cycle counter index to 32 to
+        * match the offset applied to the rest of
+        * the counter indices.
+        */
+       if (event->hw.idx == ARMV8_IDX_CYCLE_COUNTER)
+               return ARMV8_IDX_CYCLE_COUNTER_USER;
+
+       return event->hw.idx;
+}
+
 /*
  * Add an event filter to a given event.
  */
@@ -1098,6 +1114,8 @@ static int armv8_pmu_init(struct arm_pmu *cpu_pmu, char 
*name,
        cpu_pmu->set_event_filter       = armv8pmu_set_event_filter;
        cpu_pmu->filter_match           = armv8pmu_filter_match;
 
+       cpu_pmu->pmu.event_idx          = armv8pmu_access_event_idx;
+
        cpu_pmu->name                   = name;
        cpu_pmu->map_event              = map_event;
        cpu_pmu->attr_groups[ARMPMU_ATTR_GROUP_EVENTS] = events ?
diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
index 505480217cf1..d29aa981d989 100644
--- a/include/linux/perf/arm_pmu.h
+++ b/include/linux/perf/arm_pmu.h
@@ -26,6 +26,8 @@
  */
 /* Event uses a 64bit counter */
 #define ARMPMU_EVT_64BIT               1
+/* Allow access to hardware counter from userspace */
+#define ARMPMU_EL0_RD_CNTR             2
 
 #define HW_OP_UNSUPPORTED              0xFFFF
 #define C(_x)                          PERF_COUNT_HW_CACHE_##_x
-- 
2.27.0

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