[PATCH v10 04/17] x86/resctrl: Honor rdt=perf option to force enable AET perf events

[Tony Luck]

resctrl can disable ("force_off") a feature supported by the platform if,
for example, there is an erratum related to the feature or, in the case of AET
(Application Energy Telemetry), when the event group has insufficient RMIDs. The
"rdt=" kernel command line option lets the user override ("force_on") when
a feature is disabled in such case. Users may also disable ("force_off")
individual supported resctrl features they do not need.

Linux enumerates AET once on first mount and skips this enumeration if
the associated event group is supported but disabled. Since AET is only
enumerated once, this check is guaranteed to only consider disabling via
the rdt= kernel parameter.

However, when transitioning to enumerate AET on every mount, it is no longer
correct to skip enumeration simply because an event group is marked as
disabled. It is then possible for resctrl to have force-disabled the event
group during an earlier enumeration due to insufficient RMIDs, which would
improperly cause the kernel to ignore a user's explicit rdt= override on
subsequent mounts.

Ensure the user's command line choices take precedence over system-level limits.

Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
v10:
	Rewrite commit comment based on Reinette suggestion
	Dropped the change to event_group::force_off kerneldoc

 arch/x86/kernel/cpu/resctrl/intel_aet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/resctrl/intel_aet.c b/arch/x86/kernel/cpu/resctrl/intel_aet.c
index c22c3cf5167d..4ad6ad78e93e 100644
--- a/arch/x86/kernel/cpu/resctrl/intel_aet.c
+++ b/arch/x86/kernel/cpu/resctrl/intel_aet.c
@@ -228,7 +228,7 @@ static bool enable_events(struct event_group *e, struct pmt_feature_group *p)
 	struct rdt_resource *r = &rdt_resources_all[RDT_RESOURCE_PERF_PKG].r_resctrl;
 	int skipped_events = 0;
 
-	if (e->force_off)
+	if (e->force_off && !e->force_on)
 		return false;
 
 	if (!group_has_usable_regions(e, p))
-- 
2.55.0