[PATCH i-g-t 3/6] lib/xe/xe_sriov_admin: Lower priority before clearing timeslicing

[Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>]

Zero execution quantum and zero preemption timeout both mean infinity.
A function left at a scheduling priority above LOW with infinite
timeslicing may never be switched away from by GuC, which is a risky
state that can be left via PAUSE/FLR.

The restore helpers cleared execution quantum and preemption timeout
first and only then dropped priority to LOW, transiting through exactly
that state whenever a test had raised priority. This is reachable from
xe_pmu, which restores defaults while VFs are still enabled, and from
xe_sriov_admin_exit_cleanup_restore_sched_defaults() on abort or signal.

Lower priority first, then clear execution quantum and preemption timeout.

Assisted-by: Copilot:Claude-Opus-5
Signed-off-by: Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>
Cc: Adam Miszczak <adam.miszczak@linux.intel.com>
Cc: Jakub Kolakowski <jakub1.kolakowski@intel.com>
Cc: Lukasz Laguna <lukasz.laguna@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
---
 lib/xe/xe_sriov_admin.c | 32 ++++++++++++++++++++------------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/lib/xe/xe_sriov_admin.c b/lib/xe/xe_sriov_admin.c
index b03e77b08..0433ddcfc 100644
--- a/lib/xe/xe_sriov_admin.c
+++ b/lib/xe/xe_sriov_admin.c
@@ -547,6 +547,11 @@ void xe_sriov_admin_vf_stop(int pf_fd, unsigned int vf_num)
  *
  * Resets execution quantum, preemption timeout, and priority to driver defaults.
  *
+ * Zero execution quantum and zero preemption timeout both mean infinity, so
+ * priority is lowered first. A function left at a priority above LOW with
+ * infinite timeslicing may never be switched away from by GuC, and can only be
+ * recovered by PAUSE/FLR.
+ *
  * Returns: 0 on success or negative errno on error.
  */
 int __xe_sriov_admin_restore_sched_defaults(int pf_fd, unsigned int vf_num)
@@ -554,6 +559,12 @@ int __xe_sriov_admin_restore_sched_defaults(int pf_fd, unsigned int vf_num)
 	int ret_eq, ret_pt, ret_prio;
 	int ret = 0;
 
+	ret_prio = __xe_sriov_admin_set_sched_priority(pf_fd, vf_num,
+						       XE_SRIOV_SCHED_PRIORITY_LOW);
+	igt_warn_on(ret_prio);
+	if (!ret)
+		ret = ret_prio;
+
 	ret_eq = __xe_sriov_admin_set_exec_quantum_ms(pf_fd, vf_num, 0);
 	igt_warn_on(ret_eq);
 	if (!ret)
@@ -564,12 +575,6 @@ int __xe_sriov_admin_restore_sched_defaults(int pf_fd, unsigned int vf_num)
 	if (!ret)
 		ret = ret_pt;
 
-	ret_prio = __xe_sriov_admin_set_sched_priority(pf_fd, vf_num,
-						       XE_SRIOV_SCHED_PRIORITY_LOW);
-	igt_warn_on(ret_prio);
-	if (!ret)
-		ret = ret_prio;
-
 	return ret;
 }
 
@@ -589,6 +594,9 @@ void xe_sriov_admin_restore_sched_defaults(int pf_fd, unsigned int vf_num)
  *
  * Resets PF and all VFs to driver default scheduling parameters.
  *
+ * Priority is lowered before clearing timeslicing, see
+ * __xe_sriov_admin_restore_sched_defaults().
+ *
  * Returns: 0 on success or negative errno on error.
  */
 int __xe_sriov_admin_bulk_restore_sched_defaults(int pf_fd)
@@ -596,6 +604,12 @@ int __xe_sriov_admin_bulk_restore_sched_defaults(int pf_fd)
 	int ret_eq, ret_pt, ret_prio;
 	int ret = 0;
 
+	ret_prio = __xe_sriov_admin_bulk_set_sched_priority(pf_fd,
+							    XE_SRIOV_SCHED_PRIORITY_LOW);
+	igt_warn_on(ret_prio);
+	if (!ret)
+		ret = ret_prio;
+
 	ret_eq = __xe_sriov_admin_bulk_set_exec_quantum_ms(pf_fd, 0);
 	igt_warn_on(ret_eq);
 	if (!ret)
@@ -606,12 +620,6 @@ int __xe_sriov_admin_bulk_restore_sched_defaults(int pf_fd)
 	if (!ret)
 		ret = ret_pt;
 
-	ret_prio = __xe_sriov_admin_bulk_set_sched_priority(pf_fd,
-							    XE_SRIOV_SCHED_PRIORITY_LOW);
-	igt_warn_on(ret_prio);
-	if (!ret)
-		ret = ret_prio;
-
 	return ret;
 }
 
-- 
2.43.0