linux-kernel - Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest os statistics collection in guest os

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Message-ID: <4C2084BB.3040501@redhat.com>
Date:	Tue, 22 Jun 2010 11:39:07 +0200
From:	Jes Sorensen <Jes.Sorensen@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Avi Kivity <avi@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Lin Ming <ming.m.lin@...el.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	oerg Roedel <joro@...tes.org>, Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, zhiteng.huang@...el.com,
	tim.c.chen@...el.com
Subject: Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest
 os statistics collection in guest os

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

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