Message-ID: <4C3E4E1F.1060303@goop.org>
Date: Wed, 14 Jul 2010 16:54:07 -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>,
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        Thomas Gleixner <tglx@linutronix.de>, Christoph Hellwig <hch@lst.de>,
        Li Zefan <lizf@cn.fujitsu.com>, Lai Jiangshan <laijs@cn.fujitsu.com>,
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        "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>	<4C3E3F77.1090109@goop.org> <AANLkTinIJGHYtxWXGEN_pBxoIy0lhl_eRW0QSbuSuMQQ@mail.gmail.com>
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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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