[tip: x86/cache] fs/resctrl: Pass error reading event through to user space

[tip-bot2 for Reinette Chatre]

The following commit has been merged into the x86/cache branch of tip:

Commit-ID:     cc2ca4296f72519686a9f949e725275f497f90ae
Gitweb:        https://git.kernel.org/tip/cc2ca4296f72519686a9f949e725275f497f90ae
Author:        Reinette Chatre <reinette.chatre@xxxxxxxxx>
AuthorDate:    Tue, 30 Jun 2026 21:27:05 -07:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Thu, 23 Jul 2026 13:13:49 -07:00

fs/resctrl: Pass error reading event through to user space

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@xxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Tested-by: Babu Moger <babu.moger@xxxxxxx>
Link: https://patch.msgid.link/6b3c66a49788828bd8c04a6911bd74c91ccd56f3.1782857711.git.reinette.chatre@xxxxxxxxx
---
 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 29b13a6..7f3fe05 100644
--- a/fs/resctrl/monitor.c
+++ b/fs/resctrl/monitor.c
@@ -458,8 +458,10 @@ 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);
@@ -496,8 +498,10 @@ 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