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Message-ID: <1323443432.16764.2.camel@twins>
Subject: Re: [RFC][PATCH 3/3] x86: Add workaround to NMI iret woes
From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Jason Baron <jbaron@redhat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "H. Peter Anvin" <hpa@linux.intel.com>, Paul Turner <pjt@google.com>
Date: Fri, 09 Dec 2011 16:10:32 +0100
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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.