From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sheng Yang <sheng@linux.intel.com>
Subject: Re: [PATCH V3] perf & kvm: Enhance perf to collect KVM guest os statistics from host side
Date: Wed, 14 Apr 2010 18:27:21 +0800
Message-ID: <201004141827.21877.sheng@linux.intel.com>
References: <1902387910.2078.435.camel@ymzhang.sh.intel.com> <201004141814.19330.sheng@linux.intel.com> <4BC596C5.7020507@redhat.com>
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Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	Ingo Molnar <mingo@elte.hu>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	Marcelo Tosatti <mtosatti@redhat.com>,
	oerg Roedel <joro@8bytes.org>,
	Jes Sorensen <Jes.Sorensen@redhat.com>,
	Gleb Natapov <gleb@redhat.com>,
	Zachary Amsden <zamsden@redhat.com>, zhiteng.huang@intel.com,
	tim.c.chen@intel.com, Arnaldo Carvalho de Melo <acme@infradead.org>
To: Avi Kivity <avi@redhat.com>
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On Wednesday 14 April 2010 18:19:49 Avi Kivity wrote:
> On 04/14/2010 01:14 PM, Sheng Yang wrote:
> >> I wouldn't like to depend on model specific behaviour.
> >>
> >> One option is to read all the information synchronously and store it in
> >> a per-cpu area with atomic instructions, then queue the NMI.  Another
> >> option is to have another callback which tells us that the NMI is done,
> >> and have a busy loop wait until the NMI is delivered.
> >
> > Callback seems too heavy, may affect the performance badly. Maybe a short
> > queue would help, though this one is more complex.
> 
> The patch we're replying to adds callbacks (to read rip, etc.), so it's
> no big deal.  For the queue solution, a queue of size one would probably
> be sufficient even if not guaranteed by the spec.  I don't see how the
> cpu can do another guest entry without delivering the NMI.
> 
> > But I am still curious if we extend the region, how much it would help.
> > Would get a result soon...
> 
> Yes, interesting to see what the latency is.  If it's reasonably short
> (and I expect it will be so), we can do the busy wait solution.
> 
> If we have an NMI counter somewhere, we can simply wait until it changes.
 
Good idea. Of course we have one(at least on x86). There is 
irq_stat.irq__nmi_count for per cpu. :)

-- 
regards
Yang, Sheng