[PATCH V2 4/6] x86/intel_rdt: Add helper to obtain performance counter index

[Reinette Chatre <reinette.chatre@intel.com>]

In support of the most accurate measurements rdpmcl() is used instead of
the more elaborate perf_event_read_local(). rdpmcl() requires the index
of the performance counter used so a helper is introduced to determine
the index used by a provided performance event.

The index used by a performance event may change when interrupts are
enabled. A check is added to ensure that the index is only accessed
with interrupts disabled. Even with this check the use of this counter
needs to be done with care to ensure it is queried and used within the
same disabled interrupts section.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
 arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c b/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
index ab93079e9e5b..20b76024701d 100644
--- a/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
+++ b/arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c
@@ -942,6 +942,27 @@ static struct perf_event_attr __attribute__((unused)) perf_hit_attr = {
 	.exclude_user	= 1,
 };
 
+/**
+ * x86_perf_rdpmc_ctr_get - Return PMC counter used for event
+ * @event: the perf_event to which the PMC counter was assigned
+ *
+ * The counter assigned to this performance event may change if interrupts
+ * are enabled. This counter should thus never be used while interrupts are
+ * enabled. Before this function is used to obtain the assigned counter the
+ * event could be checked for validity using, for example,
+ * perf_event_read_local(), within the same interrupt disabled section in
+ * which this counter is planned to be used.
+ *
+ * Return: The index of the performance monitoring counter assigned to
+ * @perf_event, -1 if event is not valid.
+ */
+static inline int x86_perf_rdpmc_ctr_get(struct perf_event *event)
+{
+	lockdep_assert_irqs_disabled();
+
+	return IS_ERR_OR_NULL(event) ? -1 : event->hw.event_base_rdpmc;
+}
+
 static int measure_cycles_perf_fn(void *_plr)
 {
 	unsigned long long l3_hits = 0, l3_miss = 0;
-- 
2.17.0