From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: KVM PMU virtualization
Date: Fri, 26 Feb 2010 14:31:49 +0100
Message-ID: <20100226133149.GA23422@elte.hu>
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>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>,
	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: Avi Kivity <avi@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
Received: from mx3.mail.elte.hu ([157.181.1.138]:38362 "EHLO mx3.mail.elte.hu"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S936179Ab0BZNcW (ORCPT <rfc822;kvm@vger.kernel.org>);
	Fri, 26 Feb 2010 08:32:22 -0500
Content-Disposition: inline
In-Reply-To: <4B87BC74.7050207@redhat.com>
Sender: kvm-owner@vger.kernel.org
List-ID: <kvm.vger.kernel.org>


* Avi Kivity <avi@redhat.com> wrote:

> Or do you mean to define a new, kvm-specific pmu model and feed it off the 
> host pmu?  In this case all the guests will need to be taught about it, 
> which raises the compatibility problem.

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.

With the 'steal the PMU' messy approach the guest OS has to be upgraded to the 
new CPU type all the time. Ad infinitum.

	Ingo