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:	Wed, 23 Jun 2010 10:15:23 +0200
From:	Jes Sorensen <Jes.Sorensen@...hat.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
CC:	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, Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest
 os statistics collection in guest os

On 06/23/10 03:13, Zhang, Yanmin wrote:
> On Tue, 2010-06-22 at 09:58 +0200, Jes Sorensen wrote:
>> Exposing the counters read-only would save a lot of overhead for sure.
>>>> Question is if it is safe to drop overflow support?
>>> Not safe. One of PMU hardware design objectives is to use interrupt or NMI to notify
>>> software when event counter overflows. Without overflow support, software need poll
>>> the PMU registers looply. That is not good and consumes more cpu resources.
>>
>> Here is an idea, how about having the overflow NMI in the host trigger a
>> flag that causes the PMU register read to trap and get special handling?
>> That way you could propagate the overflow back down to the guest.
> That doesn't resolve the issue that guest os software has to poll register.

That is true, but it could set a flag through the para virt interface.

Cheers,
Jes
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