From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jes Sorensen <Jes.Sorensen@redhat.com>
Subject: Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest
 os statistics collection in guest os
Date: Tue, 22 Jun 2010 11:39:07 +0200
Message-ID: <4C2084BB.3040501@redhat.com>
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Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	Avi Kivity <avi@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
	Ingo Molnar <mingo@elte.hu>,
	Fr??d??ric Weisbecker <fweisbec@gmail.com>,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Lin Ming <ming.m.lin@intel.com>,
	Sheng Yang <sheng@linux.intel.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	oerg Roedel <joro@8bytes.org>, Gleb Natapov <gleb@redhat.com>,
	Zachary Amsden <zamsden@redhat.com>, zhiteng.huang@intel.com,
	tim.c.chen@intel.com
To: Peter Zijlstra <peterz@infradead.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1277199060.1875.675.camel@laptop>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On 06/22/10 11:31, Peter Zijlstra wrote:
> On Tue, 2010-06-22 at 17:29 +0800, Zhang, Yanmin wrote:
>> On Tue, 2010-06-22 at 10:00 +0200, Jes Sorensen wrote:
>>> On 06/22/10 09:55, Peter Zijlstra wrote:
>>>> On Tue, 2010-06-22 at 15:47 +0800, Zhang, Yanmin wrote:
>>>>> Besides the para virt perf interface, I'm also considering the direct exposition
>>>>> of PMU hardware to guest os. 
>>>>
>>>> NAK NAK NAK NAK, we've been over that, its not going to happen, full
>>>> stop!
>>>>
>>>> Use MSR read/write traps and host perf to emulate the hardware. In some
>>>> cases we could allow the reads without trap but that's a later
>>>> optimization.
>>>
>>> I believe whats meant here is a PMU compatible interface which is
>>> partially emulated. Not a handover of the PMU.
>> Right. We need capture all write to PMU MSR and allows guest os to read MSR directly.
> 
> That latter is not possible, only in a subset of cases can you allow
> that read.

Avi's suggestion of using virtual MSRs makes a ton of sense for this
though, and it makes it possible to switch direct access on/off for the
cases where direct access is possible, and go emulated when it isn't.

Cheers,
Jes