Message-ID: <4C3E3F77.1090109@goop.org>
Date: Wed, 14 Jul 2010 15:51:35 -0700
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Steven Rostedt <rostedt@rostedt.homelinux.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        Johannes Berg <johannes.berg@intel.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Arnaldo Carvalho de Melo <acme@infradead.org>,
        Tom Zanussi <tzanussi@gmail.com>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Andi Kleen <andi@firstfloor.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "Frank Ch. Eigler" <fche@redhat.com>, Tejun Heo <htejun@gmail.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
References: <20100714154923.947138065@efficios.com>	<20100714155804.049012415@efficios.com>	<AANLkTiml2uwYqQayTKjMN2gI3LnjVFpwxXkv8GN3McEE@mail.gmail.com>	<20100714170617.GB4955@Krystal>	<AANLkTinLB3gQNKFk9QRfBS8YEfxL3qxZDFw7vWHDOnmL@mail.gmail.com>	<20100714203940.GC22096@Krystal>	<AANLkTinM0eFnreNB5l_6f20oSGBchqAtYPb57tM01kUB@mail.gmail.com>	<20100714222115.GA30122@Krystal> <AANLkTikLG4xVYfvuMtbuzsqcnzo1ut4dFAm6piqy2Lh7@mail.gmail.com>
In-Reply-To: <AANLkTikLG4xVYfvuMtbuzsqcnzo1ut4dFAm6piqy2Lh7@mail.gmail.com>
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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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