Re: [PATCH v4 0/4] arm64: cross-CPU NMI via SDEI

[Catalin Marinas <catalin.marinas@arm.com>]

Hi Kiryl,

On Wed, Jun 17, 2026 at 08:20:01PM +0100, Kiryl Shutsemau wrote:
>   - GICv3 pseudo-NMI (interrupt priority masking). Its cost is on the
>     interrupt mask/unmask hot path: local_irq_enable() becomes an
>     ICC_PMR_EL1 write plus a synchronising barrier, and exception
>     entry/exit save and restore the PMR, paid on every CPU whether or not
>     an NMI is ever delivered. In our measurements, enabling pseudo-NMI
>     costs up to ~5% on real workloads, and ~66% on a syscall-in-a-loop
>     microbenchmark. A fleet-wide ~5% regression is not acceptable, so
>     these systems run with pseudo-NMI disabled.

Does your firmware set ICC_CTLR_EL1.PMHE? I'd be curious to see the
numbers if the DSB was omitted on the enable path.

> This series adds a third delivery backend that costs nothing on the hot
> path: SDEI. Firmware delivers an SDEI event into a CPU regardless of its
> DAIF state, so interrupt masking stays the cheap PSTATE.DAIF operation and
> the firmware round-trip is paid only at the rare moment a CPU must be
> interrupted.

The direction of travel is to deprecated SDEI. I wouldn't add more stuff
on top of this interface.

(I haven't looked at the patches yet; Marc/Mark/James are more
knowledgeable than me in this area)

-- 
Catalin