Re: [PATCH 1/2] genirq/manage: Provide irq_set_percpu_irqchip_state()

[Amir Noohi]

Hi Marc,

On Sat, 24 Oct 2026 13:21:00 +0100, Marc Zyngier <maz@xxxxxxxxxx> wrote:
> It really feels like a leap of faith.
>
> There is nothing that says that it is safe to call this without
> holding the global lock. For all we know, drivers have been relying on
> it for a good decade, and I don't feel confident letting things rip
> just because it (mostly) works with the GIC.
>
> At the very least, something exposed by the irqchip (a per-IRQ flag?)
> should tell you whether it is safe to elide the lock.

Agreed, and v2 does that. There's a new IRQCHIP_PERCPU_STATE_LOCKLESS
flag, and irq_set_percpu_irqchip_state() only skips the descriptor lock
if the chip implementing irq_set_irqchip_state() has it set. Otherwise,
or if there's a bus lock, it just calls irq_set_irqchip_state(), so
drivers that don't opt in see no change.

Only GICv3 sets it, in a separate patch. For SGIs/PPIs/EPPIs its
callback only touches the local redistributor (gic_data_rdist_sgi_base()
and the RWP poll), nothing shared. FWIW, irq-gic.c, irq-gic-v3.c and
irq-gic-v5.c are the only drivers with both per-CPU devid interrupts and
an irq_set_irqchip_state() callback. KVM's own timer chip (GICv5, no HW
deactivation) doesn't set it either, so those keep the lock.

On GICv2 in QEMU it still takes the lock like mainline does (333k vs
321k acquisitions in lock_stat, within 4%), and on GICv3 it doesn't
take it at all. arch_timer, arch_timer_edge_cases, vgic_irq and
vgic_init pass on the Altra. Under QEMU with lockdep, DEBUG_PREEMPT and
DEBUG_ATOMIC_SLEEP, the selftests pass on GICv3 (VHE and nVHE) and
GICv2 (the vGICv3 ones skip there). No splats.

I made it a chip flag since it's about the callback, but I can make it
per-IRQ if you prefer.

Thanks,
Amir