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

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Date:	Wed, 21 May 2014 15:42:24 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Steven Rostedt <rostedt@...dmis.org>
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, Steven Rostedt wrote:

> 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.

Alright, Andy's iret optimization efforts do immediately bring a followup 
question -- why is this not a problem with iret-based return from #MC 
possibly interrupting NMI?

-- 
Jiri Kosina
SUSE Labs
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