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

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Message-ID: <AANLkTil2r3sUcoCC_ktp3TV2JNTSMxbpke8yJMx_8lmm@mail.gmail.com>
Date:	Wed, 14 Jul 2010 12:14:01 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	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>,
	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 11:46 AM, Ingo Molnar <mingo@...e.hu> wrote:
>>  NMI entry:
>
> I think at this point [NMI re-entry] we've corrupted the top of the NMI kernel
> stack already, due to entering via the IST stack mechanism, which is
> non-nesting and which enters at the same point - right?

Yeah, you're right, but we could easily fix that up. We know we don't
need any stack for the nested case, so all we would need to do is to
just subtract a small bit off %rsp, and copy the three words or so to
create a "new" stack for the non-nested case.

> We could solve that by copying that small stack frame off before entering the
> 'generic' NMI routine - but it all feels a bit pulled in by the hair.

Why? It's much cleaner than making the _real_ codepaths much worse.

                Linus
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