Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753236Ab0FWBN0 (ORCPT ); Tue, 22 Jun 2010 21:13:26 -0400 Received: from mga02.intel.com ([134.134.136.20]:63278 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751678Ab0FWBNY (ORCPT ); Tue, 22 Jun 2010 21:13:24 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.53,463,1272870000"; d="scan'208";a="632729495" Subject: Re: [PATCH V2 1/5] ara virt interface of perf to support kvm guest os statistics collection in guest os From: "Zhang, Yanmin" To: Jes Sorensen 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 In-Reply-To: <4C206D3E.3060506@redhat.com> 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> <4C206D3E.3060506@redhat.com> Content-Type: text/plain; charset="ISO-8859-1" Date: Wed, 23 Jun 2010 09:13:44 +0800 Message-Id: <1277255624.2096.728.camel@ymzhang.sh.intel.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.0 (2.28.0-2.fc12) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2259 Lines: 41 On Tue, 2010-06-22 at 09:58 +0200, Jes Sorensen wrote: > 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. That doesn't resolve the issue that guest os software has to poll register. -- 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/