Re: [PATCH v5 0/4] arm64: cross-CPU NMI via SDEI

[Will Deacon <will@kernel.org>]

On Thu, Jul 09, 2026 at 05:16:37PM +0100, Kiryl Shutsemau wrote:
> On Mon, Jun 29, 2026 at 04:07:14PM +0100, Kiryl Shutsemau wrote:
> > From: "Kiryl Shutsemau (Meta)" <kas@kernel.org>
> > 
> > A class of debug/observability features needs to interrupt a CPU that has
> > its interrupts locally masked: the all-CPU backtrace behind sysrq-l /
> > RCU-stall / hung-task / hard-lockup dumps, and crash_smp_send_stop()
> > capturing a stuck CPU's state into the vmcore. On arm64 these need a
> > mechanism that reaches a CPU spinning with DAIF masked, which a normal IPI
> > cannot.
> 
> Gentle ping. Any feedback?
> 
> I'm looking forward to finding an upstreamable solution to the problem.

This is all pretty small, self-contained and it's useful to you, so I'm
inclined to merge it.

However, the one vague concern I have is about the direction of SDEI in
the future. AFAIK, the TF-A implementation is known to have issues, it's
not supported at all by R-FA and I worry that the spec is going to fall
behind the architecture, particularly as FEAT_NMI becomes available.

So it would be good to understand what we're signing up to maintain here.
Kiryl, do you see an (eventual) migration over to FEAT_NMI, giving us a
path to deprecating SDEI altogether, or do you think the two will live
alongside each other for the forseeable future?

Cheers,

Will