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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Date:	Tue, 22 Jun 2010 11:46:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jes Sorensen <Jes.Sorensen@...hat.com>
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 Tue, 2010-06-22 at 11:39 +0200, Jes Sorensen wrote:
> 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.

/me has no clue what virtual MSRs are, but yeah, that sounds about
right. Anyway, the generic case is full trap and emulate get that
working first, then try and be smart and avoid some traps.
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