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

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Date:	Wed, 14 Jul 2010 14:52:24 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Maciej W. Rozycki" <macro@...ux-mips.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Frederic Weisbecker <fweisbec@...il.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

On Wed, Jul 14, 2010 at 2:45 PM, Maciej W. Rozycki <macro@...ux-mips.org> wrote:
> On Wed, 14 Jul 2010, Linus Torvalds wrote:
>
>> No. As mentioned, there is no such counter in real hardware either.
>
>  There is a 1-bit counter or actually a latch.

Well, that's what our single-word flag is too.

>> Look at what happens for the not-nested case:
>>
>>  - NMI1 triggers. The CPU takes a fault, and runs the NMI handler with
>> NMI's disabled
>
>  Correct.
>
>>  - NMI2 triggers. Nothing happens, the NMI's are disabled.
>
>  The NMI latch records the second NMI.  Note this is edge-sensitive like
> the NMI line itself.
>
>>  - NMI3 triggers. Again, nothing happens, the NMI's are still disabled
>
>  Correct.
>
>>  - the NMI handler returns.
>>
>>  - What happens now?
>
>  NMI2 latched above causes the NMI handler to be invoked as the next
> instruction after IRET.  The latch is cleared as the interrupt is taken.
>
>> How many NMI interrupts do you get? ONE. Exactly like my "emulate it
>> in software" approach. The hardware doesn't have any counters for
>> pending NMI's either. Why should the software emulation have them?
>
>  Two. :)

You just count differently. I don't count the first one (the "real"
NMI). That obviously happens. So I only count how many interrupts we
need to fake. That's my "one". That's the one that happens as a result
of the fault that we take on the iret in the emulated model.

So there is no need to count anything. We take a fault on the iret if
we got a nested NMI (regardless of how _many_ such nested NMI's we
took). That's the "latch", exactly like in the hardware. No counter.

(Yeah, yeah, you can call it a "one-bit counter", but I don't think
that's a counter. It's just a bit of information).

                      Linus
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