[PATCH v4 07/11] perf/cxl: Don't share the overflow interrupt, and keep it pinned

The PMU pins its overflow interrupt to info->on_cpu in the hotplug
callbacks, but requests it with only IRQF_SHARED | IRQF_NO_THREAD. Without
IRQF_NOBALANCING, irqbalance or a userspace smp_affinity write can move it
elsewhere, and cxl_pmu_irq() then runs local64_cmpxchg() and local64_add()
on hwc->prev_count and event->count at the same time as the managing CPU.
local64_t is only atomic against same-CPU access, so the counts corrupt.

The flag alone does not fix that while the line is shared. __setup_irq()
only acts on it for the first action on a line, and a co-owner has no
reason to want our affinity - it would keep taking the interrupt wherever
its own points, running our handler on the wrong CPU.

Drop IRQF_SHARED and add IRQF_NOBALANCING. The spec only recommends a
distinct Interrupt Message Number per CPMU instance (CXL r4.0 8.2.7.1.1),
so a device may put several on one vector, or share with the mailbox or
event log. Such a device now loses a PMU rather than silently miscounting -
and only the PMU, since the preceding patch adds the CPMUs after the event
configuration.

Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
Reported-by: sashiko-bot@kernel.org
Closes: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Jonathan Cameron <redacted>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
---
 drivers/perf/cxl_pmu.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
index 481d32d0c1b0..448e1da3d59f 100644
--- a/drivers/perf/cxl_pmu.c
+++ b/drivers/perf/cxl_pmu.c

@@ -783,7 +783,7 @@ static irqreturn_t cxl_pmu_irq(int irq, void *data)
 
 	overflowed = readq(base + CXL_PMU_OVERFLOW_REG);
 
-	/* Interrupt may be shared, so maybe it isn't ours */
+	/* Nothing overflowed, so the device did not raise this */
 	if (!overflowed)
 		return IRQ_NONE;
 

@@ -886,7 +886,14 @@ static int cxl_pmu_probe(struct device *dev)
 	if (!irq_name)
 		return -ENOMEM;
 
-	rc = devm_request_irq(dev, irq, cxl_pmu_irq, IRQF_SHARED | IRQF_NO_THREAD,
+	/*
+	 * The handler must run on info->on_cpu, so the interrupt cannot be
+	 * shared - IRQF_NOBALANCING is only honoured for the first action on a
+	 * line, and a co-owner would keep taking the interrupt wherever its own
+	 * affinity points.
+	 */
+	rc = devm_request_irq(dev, irq, cxl_pmu_irq,
+			      IRQF_NO_THREAD | IRQF_NOBALANCING,
 			      irq_name, info);
 	if (rc)
 		return rc;

-- 
2.54.0