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

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Message-ID: <4C3E3F77.1090109@goop.org>
Date:	Wed, 14 Jul 2010 15:51:35 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Linus Torvalds <torvalds@...ux-foundation.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>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On 07/14/2010 03:37 PM, Linus Torvalds wrote:
> I think the %rip check should be pretty simple - exactly because there
> is only a single point where the race is open between that 'mov' and
> the 'iret'. So it's simpler than the (similar) thing we do for
> debug/nmi stack fixup for sysenter that has to check a range.
>
> The only worry is if that crazy paravirt code wants to paravirtualize
> the iretq. Afaik, paravirt does that exactly because they screw up
> iret handling themselves. Maybe we could stop doing that stupid iretq
> paravirtualization, and just tell the paravirt people to do the same
> thing I propose, and just allow nesting.
>   

We screw around with iret because there's a separate interrupt mask flag
which can't be set atomically with respect to a stack/ring change (well,
there's more to it, but I won't confuse matters).

But it only really matters if the PV guest can also get NMI-like
interrupts.  While Xen supports NMI for PV guests, we don't use it much
and I haven't looked into implementing support for it yet.

    J
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