linux-kernel - Re: [RFC][PATCH 3/3] x86: Add workaround to NMI iret woes

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Date:	Fri, 09 Dec 2011 16:10:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jason Baron <jbaron@...hat.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [RFC][PATCH 3/3] x86: Add workaround to NMI iret woes

On Fri, 2011-12-09 at 10:00 -0500, Steven Rostedt wrote:
> On Fri, 2011-12-09 at 10:22 +0100, Peter Zijlstra wrote:
> 
> > Its was definitely tongue in cheek, also I did say this'll be a massive
> > pain with paravirt since I doubt paravirt calls are NMI safe. 
> 
> But does paravirt simulate NMIs? Does a guest ever take an NMI? I can
> enable paravirt to see if it breaks. 

KVM does NMIs, the write_idt thing is a paravirt call. Then again, a
vcpu is a single thread of execution, there can only ever be 1 hypercall
at the same time.

The only thing that remains is if the hypercall interface is NMI-safe,
what if the NMI comes in just as another context is also starting a
hypercall. I really don't know enough about all that crap to know.


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