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 <jonathan.cameron@oss.qualcomm.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://sashiko.dev/#/patchset/20260715191454.459673-1-dave@stgolabs.net?part=1
Signed-off-by: Will Deacon <will@kernel.org>

0 files changed, 0 insertions, 0 deletions