Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758465Ab0FVH7U (ORCPT ); Tue, 22 Jun 2010 03:59:20 -0400 Received: from mx1.redhat.com ([209.132.183.28]:35519 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754885Ab0FVH7S (ORCPT ); Tue, 22 Jun 2010 03:59:18 -0400 Message-ID: <4C206D3E.3060506@redhat.com> Date: Tue, 22 Jun 2010 09:58:54 +0200 From: Jes Sorensen User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100430 Fedora/3.0.4-2.fc12 Lightning/1.0b2pre Thunderbird/3.0.4 MIME-Version: 1.0 To: "Zhang, Yanmin" CC: Avi Kivity , LKML , kvm@vger.kernel.org, Ingo Molnar , Fr??d??ric Weisbecker , Arnaldo Carvalho de Melo , Cyrill Gorcunov , Lin Ming , Sheng Yang , Marcelo Tosatti , oerg Roedel , Gleb Natapov , Zachary Amsden , zhiteng.huang@intel.com, tim.c.chen@intel.com, Peter Zijlstra Subject: Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest os statistics collection in guest os References: <1277112680.2096.509.camel@ymzhang.sh.intel.com> <4C1F50D0.70205@redhat.com> <1277171344.2096.567.camel@ymzhang.sh.intel.com> <4C2062D8.20609@redhat.com> <1277192873.2096.690.camel@ymzhang.sh.intel.com> In-Reply-To: <1277192873.2096.690.camel@ymzhang.sh.intel.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2610 Lines: 50 On 06/22/10 09:47, Zhang, Yanmin wrote: > On Tue, 2010-06-22 at 09:14 +0200, Jes Sorensen wrote: >> On 06/22/10 03:49, Zhang, Yanmin wrote: >>> On Mon, 2010-06-21 at 14:45 +0300, Avi Kivity wrote: >>> So I think above discussion is around how to expose PMU hardware to guest os. I will >>> also check this method after the para virt interface is done. >> >> You should be able to expose the counters as read-only to the guest. KVM >> allows you to specify whether or not a guest has read, write or >> read/write access. If you allowed read access of the counters that would >> safe a fair bit of hyper calls. > Thanks. KVM is good in register access permission configuration. But things are not so > simple like that if we consider real running environment. Host kernel might schedule > guest os vcpu thread to other cpus, or other non-kvm processes might preempt the vcpu > thread on this cpu. > > To support such capability you said, we have to implement the direct exposition of PMU > hardware to guest os eventually. If the guest is rescheduled to another CPU, or you get a preemption, you have a VMEXIT. The vcpu thread will not migrate while it is running, so you can handle it while the the VMEXIT is being serviced. 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. > Besides the para virt perf interface, I'm also considering the direct exposition > of PMU hardware to guest os. But that will be another very different implementation. We > should not combine it with pv interface. Perhaps our target is to implement both, so > unmodified guest os could get support on perf statistics. That was what I was looking at initially, but it got stalled. I think it will make sense to build it on top of the infrastructure you have already posted, so once that settles it will definitely be easier to do. Cheers, Jes -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/