linux-kernel - Re: arm64: pseudo NMI bootconfig question

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Message-ID: <sf4iq6uyq64lbrrggwssq2hxslk2qyetm3owplw5rwzqgnemrn@o4beszh5aewh>
Date: Thu, 13 Nov 2025 01:36:48 -0800
From: Breno Leitao <leitao@...ian.org>
To: Marc Zyngier <maz@...nel.org>
Cc: catalin.marinas@....com, will@...nel.org, mark.rutland@....com, 
	paulmck@...nel.org, mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: arm64: pseudo NMI bootconfig question

Helo Marc,

On Wed, Nov 12, 2025 at 02:21:22PM +0000, Marc Zyngier wrote:
> On Wed, 12 Nov 2025 14:05:43 +0000,
> Breno Leitao <leitao@...ian.org> wrote:
...
> > Question:
> > 
> > Would it make sense to provide an option to enable pseudo NMI in certain
> > kernel configuration without requiring an extra command-line parameter?
> 
> [I wasn't asked, but I'll give my answer anyway]
> 
> The short answer is no. The long answer is that there is so much
> broken HW out there that dies a painful death when enabling pseudo-NMI
> that is isn't practical to do so.

That’s helpful to know. I hadn’t realized there were potential issues
with hardware implementation, which explains why it isn’t
straightforward to enable. Thanks for the additional context.

> It also brings a measurable overhead to some of the most frequent
> operations (interrupt masking).

For debug kernel variants, performance overhead isn’t a concern since
they’re only used in debugging scenarios—not in production. So, it’s
fine if things run slower when the debug configuration is active, at
least from my perspective.

Thanks for jumping in!
--breno