linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Message-ID: <20100714201753.GB22096@Krystal>
Date:	Wed, 14 Jul 2010 16:17:53 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> On Wed, Jul 14, 2010 at 12:36 PM, Frederic Weisbecker
> <fweisbec@...il.com> wrote:
> >
> > There is also the fact we need to handle the lost NMI, by defering its
> > treatment or so. That adds even more complexity.
> 
> I don't think your read my proposal very deeply. It already handles
> them by taking a fault on the iret of the first one (that's why we
> point to the stack frame - so that we can corrupt it and force a
> fault).

It only handles the case of a single NMI coming in. What happens in this
scenario?

- NMI (1) comes in.
- takes a fault
    - iret
- NMI (2) comes in.
  - nesting detected, popf/ret
- takes another fault
- NMI (3) comes in.
  - nesting detected, popf/ret
- iret faults
  - executes only one extra NMI handler

We miss NMI (3) here. I think this is an important change from a semantic where,
AFAIK, the hardware should be allowed to assume that the CPU will execute as
many nmi handlers are there are NMIs acknowledged.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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