[PATCH v13 2/5] perf: Add a counter for number of user access events in context

Rob Herring
2021-12-08T20:13:29Z

On arm64, user space counter access will be controlled differently
compared to x86. On x86, access in the strictest mode is enabled for all
tasks in an MM when any event is mmap'ed. For arm64, access is
explicitly requested for an event and only enabled when the event's
context is active. This avoids hooks into the arch context switch code
and gives better control of when access is enabled.

In order to configure user space access when the PMU is enabled, it is
necessary to know if any event (currently active or not) in the current
context has user space accessed enabled. Add a counter similar to other
counters in the context to avoid walking the event list every time.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>
---
v13:
- Rewrote commit message to give more background/reasoning
v10:
- Re-added.
- Maintain the count in the perf core
v9:
- Dropped
v8:
- new patch
---
include/linux/perf_event.h | 1 +
kernel/events/core.c | 4 ++++
2 files changed, 5 insertions(+)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index ba9467972c09..411e34210fbf 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -831,6 +831,7 @@ struct perf_event_context {

int nr_events;
int nr_active;
+ int nr_user;
int is_active;
int nr_stat;
int nr_freq;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 523106a506ee..68952c134968 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -1808,6 +1808,8 @@ list_add_event(struct perf_event *event, struct perf_event_context *ctx)

list_add_rcu(&event->event_entry, &ctx->event_list);
ctx->nr_events++;
+ if (event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT)
+ ctx->nr_user++;
if (event->attr.inherit_stat)
ctx->nr_stat++;

@@ -1999,6 +2001,8 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
event->attach_state &= ~PERF_ATTACH_CONTEXT;

ctx->nr_events--;
+ if (event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT)
+ ctx->nr_user--;
if (event->attr.inherit_stat)
ctx->nr_stat--;