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From: Avi Kivity <avi@redhat.com>
Subject: Re: KVM PMU virtualization
Date: Fri, 26 Feb 2010 15:55:54 +0200
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References: <4B86917C.4070102@redhat.com> <20100225173423.GB4246@8bytes.org> <20100226084241.GF15885@elte.hu> <4B87987A.2020302@redhat.com> <20100226104437.GB7463@elte.hu> <4B87AF44.9090702@redhat.com> <20100226114217.GI7463@elte.hu> <4B87B5DE.30503@redhat.com> <20100226120750.GA11578@elte.hu> <4B87BC74.7050207@redhat.com> <20100226133149.GA23422@elte.hu> <4B87CE93.1070906@redhat.com>
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Cc: Ingo Molnar <mingo@elte.hu>, Joerg Roedel <joro@8bytes.org>,
	KVM General <kvm@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Zachary Amsden <zamsden@redhat.com>,
	Gleb Natapov <gleb@redhat.com>, ming.m.lin@intel.com,
	"Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Arjan van de Ven <arjan@infradead.org>,
	Fr??d??ric Weisbecker <fweisbec@gmail.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
To: Jes Sorensen <Jes.Sorensen@redhat.com>
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On 02/26/2010 03:37 PM, Jes Sorensen wrote:
> On 02/26/10 14:31, Ingo Molnar wrote:
>> You are missing two big things wrt. compatibility here:
>>
>>   1) The first upgrade overhead a one time overhead only.
>>
>>   2) Once a Linux guest has upgraded, it will work in the future, 
>> with _any_
>>      future CPU - _without_ having to upgrade the guest!
>>
>> Dont you see the advantage of that? You can instrument an old system 
>> on new
>> hardware, without having to upgrade that guest for the new CPU support.
>
> That would only work if you are guaranteed to be able to emulate old
> hardware on new hardware. Not going to be feasible, so then we are in a
> real mess.
>

That actually works on the Intel-only architectural pmu.  I'm beginning 
to like it more and more.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.