[RFC PATCH v1 0/2] genirq: add migratable NMI registration and show NMI counts

[Ashok Raj <ashok.raj@oss.qualcomm.com>]

request_nmi() currently models NMIs as per-CPU sources. This does not
represent a globally routed NMI source with an affinity-capable irqchip,
which needs to be rerouted when its target CPU is offlined.

This RFC adds IRQF_NMI_MIGRATABLE as a request_nmi() registration mode
for such a source. It remains excluded from normal IRQ balancing, but the
generic startup and CPU-hotplug paths may select an online target. The
second patch fixes the related /proc/interrupts display: NMI-safe flow
handling updates the per-CPU counters rather than irq_desc::tot_count.

Patch 1: Add the migratable-NMI registration mode, validate that it is
used only through request_nmi() with an affinity-capable non-per-CPU
irqchip, and add IRQ KUnit coverage for registration and CPU-offline
migration.

Patch 2: Emit the per-CPU counters for all NMI descriptors, matching
kstat_irqs_desc().

Testing:
  - ARM64 changed-object builds with WERROR=1
  - ARM64 !SMP changed-object build
  - ARM64 IRQ KUnit run under QEMU with two CPUs, including registration
    validation and CPU-offline migration
  - local downstream GHES delivery and migration validation for the intended
    consumer topology (not part of this series)

Base and dependencies:

This RFC was developed and tested on v7.4-rc2 plus the FEAT_NMI v2 series
[1]. It depends on that series' GIC NMI support and is therefore not directly
applicable to mainline or linux-next as posted. This RFC is for design
feedback; it will need rebasing onto the upstream form of those prerequisites
before submission.

[1] https://lore.kernel.org/linux-arm-kernel/20261112132042.129062-1-vladimir.murzin@arm.com/

A separate GHES Type-4 series is the intended first consumer and is not
included here.

Feedback is particularly welcome on the IRQF_NMI_MIGRATABLE API and its
placement in the generic IRQ API, as well as the NMI accounting presentation
in /proc/interrupts.

Ashok Raj (2):
  genirq: Add migratable NMI registration mode
  genirq/proc: Show NMI interrupt counts in /proc/interrupts

 include/linux/interrupt.h |   2 +
 kernel/irq/irq_test.c     | 141 +++++++++++++++++++++++++++++++++++++-
 kernel/irq/manage.c       |  33 +++++++--
 kernel/irq/proc.c         |   7 +-
 4 files changed, 172 insertions(+), 11 deletions(-)


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2.43.0