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

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Date:	Wed, 14 Jul 2010 16:54:07 -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 04:02 PM, Linus Torvalds wrote:
> Umm, I know. It's what this whole discussion (non-paravirtualized) is
> all about. And I have a suggestion that should fix the
> non-paravirtualized case _without_ actually touching anything but the
> NMI code itself.
>
> What I tried to say is that the paravirtualized people should take a
> look at my suggestion, and see if they can apply the logic to their
> NMI handling too.

My point is that it's moot (for now) because there is no NMI handing.

>  And in the process totally remove the need for
> paravirtualizing iret, exactly because the approach handles the magic
> NMI lock logic entirely in the NMI handler itself.
>   

Nothing in this thread is ringing any alarm bells from that perspective,
so I don't much mind either way.  When I get around to dealing with
paravirtualized NMI, I'll look at the state of things and see how to go
from there.  (Xen's iret hypercall takes a flag to say whether to unmask
NMIs, which will probably come in handy.)

I don't think any of the other pure PV implementations have NMI either,
so I don't think it affects them.

> Wouldn't it be nice to be able to remove the need to paravirtualize iret?
>   

Of course.  But we also need to do an iret in a hypercall to handle ring
switching in some cases, so we still need it aside from the interrupt issue.

    J

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