linux-kernel - Re: Perhaps a side effect regarding NMI returns

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Message-ID: <CA+55aFwehof2kZCENTOFFobWqVvm04BZer6Cc-fQfVvcCE1NVQ@mail.gmail.com>
Date:	Mon, 28 Nov 2011 20:53:55 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Paul Turner <pjt@...gle.com>
Subject: Re: Perhaps a side effect regarding NMI returns

On Mon, Nov 28, 2011 at 8:07 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> If enabling interrupts also enables NMIs, then there's no side effect.

The only thing that enables NMI's again is 'iret', afaik. Of course,
it can be *any* iret, so once you enable interrupts, you can get a
timer interrupt, and the timer interrupt returning with iret will
re-enable NMI's then too.

I would suggest that the actual NMI handler itself should probably
never use that paranoid exit at all, and just always use a regular
iret. Screw scheduling and TIF checks.

Probably only the exceptions that can happen *during* NMI (eg debug,
stack exception, double-fault etc) should use the paranoid versions
that try to avoid using iret.

Because I think you're right - we shouldn't call schedule() from
within the NMI handler, even if we do enable interrupts and switch to
the normal stack. Even if it probably does happen to work normally.

But I have to admit to not necessarily thinking it through a lot.

                            Linus
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