From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH V4 1/2] perf ignore LBR and extra_regs. Date: Thu, 10 Jul 2014 11:02:04 +0200 Message-ID: <20140710090204.GU3935@laptop> References: <1404838181-3911-1-git-send-email-kan.liang@intel.com> <20140709141631.GE9918@twins.programming.kicks-ass.net> <37D7C6CF3E00A74B8858931C1DB2F077014D2005@SHSMSX103.ccr.corp.intel.com> <20140709145809.GG9918@twins.programming.kicks-ass.net> <37D7C6CF3E00A74B8858931C1DB2F077014D2729@SHSMSX103.ccr.corp.intel.com> <20140709164146.GH9918@twins.programming.kicks-ass.net> <37D7C6CF3E00A74B8858931C1DB2F077014D361B@SHSMSX103.ccr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "andi@firstfloor.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" To: "Liang, Kan" Return-path: Content-Disposition: inline In-Reply-To: <37D7C6CF3E00A74B8858931C1DB2F077014D361B@SHSMSX103.ccr.corp.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org /me reminds you of 78 char text wrap. On Wed, Jul 09, 2014 at 07:32:09PM +0000, Liang, Kan wrote: > > Sure; but what I meant was, check_msr() is broken when ran on such a > > kernel. You need to fix check_msr() to return failure on these 'ignored' > > MSRs, after all they don't function as expected, they're effectively broken. > > The function is designed to check if the MSRs can be safely accessed > (no #GP). It cannot guarantee the correctness of the MSRs. If KVM > applied patch 2 and guest applied patch 1, from the guest's > perspective, the MSRs can be accessed (no #GP triggered). So return > true is expected. It should not be a broken. You're not understanding. I know you wrote that function to do that. I'm saying that's wrong. Look at check_hw_exists() it explicitly checks for fake MSRs and reports them broken. These fake MSRs _ARE_ broken, they do not behave as expected. Not crashing is not the right consideration here, we're interested in higher order correct behaviour. > The only unexpected > thing for guest is that the counting/sampling result for LBR/extra reg > is always 0. But the patch is a short term fix to stop things from > crashing. I think it should be acceptable. Patch 2 is fine, patch 1, in particular your check_msr() routine is not.