Reading of event data is managed through populating a struct rmid_read with
properties of event needing to be read. This data is dispatched to an
appropriate CPU and upon completion any error can be found in rmid_read::err,
or on success the event data will be in rmid_read::val.

rmid_read::err is not updated in the unlikely scenario that the reading
of the event was dispatched to a wrong CPU. If this ever occurs due to
a bug in resctrl the user space read will return "success" but the data
reported will be invalid.

Ensure accurate error reporting so that if there may be an issue with
how resctrl picks a CPU it could be learned with an error to user space
instead of silent failure.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
---
 fs/resctrl/monitor.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/resctrl/monitor.c b/fs/resctrl/monitor.c
index 421c70f96426..f1a08fdbdd61 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -453,8 +453,10 @@ static int __l3_mon_event_count(struct rdtgroup *rdtgrp, struct rmid_read *rr)
 	}
 
 	/* Reading a single domain, must be on a CPU in that domain. */
-	if (!cpumask_test_cpu(cpu, &d->hdr.cpu_mask))
+	if (!cpumask_test_cpu(cpu, &d->hdr.cpu_mask)) {
+		rr->err = -EIO;
 		return -EINVAL;
+	}
 	if (rr->is_mbm_cntr)
 		rr->err = resctrl_arch_cntr_read(rr->r, d, closid, rmid, cntr_id,
 						 rr->evt->evtid, &tval);
@@ -491,8 +493,10 @@ static int __l3_mon_event_count_sum(struct rdtgroup *rdtgrp, struct rmid_read *r
 	}
 
 	/* Summing domains that share a cache, must be on a CPU for that cache. */
-	if (!cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map))
+	if (!cpumask_test_cpu(cpu, &rr->ci->shared_cpu_map)) {
+		rr->err = -EIO;
 		return -EINVAL;
+	}
 
 	/*
 	 * Legacy files must report the sum of an event across all
-- 
2.50.1