linux-kernel - Re: 64bit x86: NMI nesting still buggy?

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Message-ID: <20140429100345.3f76a5bd@gandalf.local.home>
Date:	Tue, 29 Apr 2014 10:03:45 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Salman Qazi <sqazi@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Michal Hocko <mhocko@...e.cz>, Borislav Petkov <bp@...en8.de>,
	Vojtech Pavlik <vojtech@...e.cz>,
	Petr Tesarik <ptesarik@...e.cz>, Petr Mladek <pmladek@...e.cz>
Subject: Re: 64bit x86: NMI nesting still buggy?

On Tue, 29 Apr 2014 15:05:55 +0200 (CEST)
Jiri Kosina <jkosina@...e.cz> wrote:

> According to 38.4 of [1], when SMM mode is entered while the CPU is 
> handling NMI, the end result might be that upon exit from SMM, NMIs will 
> be re-enabled and latched NMI delivered as nested [2].

Note, if this were true, then the x86_64 hardware would be extremely
buggy. That's because NMIs are not made to be nested. If SMM's come in
during an NMI and re-enables the NMI, then *all* software would break.
That would basically make NMIs useless.

The only time I've ever witness problems (and I stress NMIs all the
time), is when the NMI itself does a fault. Which my patch set handles
properly. I've also stressed this on boxes that do have SMIs and SMMs.

Now, you can have a bad BIOS that does re-enable NMIs from SMMs or
SMIs, but then you need to take that up with your vendor.

-- Steve

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