From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754976AbZFPKvR (ORCPT ); Tue, 16 Jun 2009 06:51:17 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753470AbZFPKvF (ORCPT ); Tue, 16 Jun 2009 06:51:05 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:43932 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752020AbZFPKvD (ORCPT ); Tue, 16 Jun 2009 06:51:03 -0400 Date: Tue, 16 Jun 2009 12:50:25 +0200 From: Ingo Molnar To: Avi Kivity Cc: "H. Peter Anvin" , Mathieu Desnoyers , Linus Torvalds , mingo@redhat.com, paulus@samba.org, acme@redhat.com, linux-kernel@vger.kernel.org, a.p.zijlstra@chello.nl, penberg@cs.helsinki.fi, vegard.nossum@gmail.com, efault@gmx.de, jeremy@goop.org, npiggin@suse.de, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods Message-ID: <20090616105025.GA2297@elte.hu> References: <20090615171845.GA7664@elte.hu> <20090615180527.GB4201@Krystal> <20090615182348.GC11248@elte.hu> <4A369573.1060504@zytor.com> <20090615184536.GH11248@elte.hu> <4A369914.4010903@zytor.com> <4A369ADF.2050305@redhat.com> <20090616083635.GE16229@elte.hu> <4A375D6B.3000105@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A375D6B.3000105@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Avi Kivity wrote: > On 06/16/2009 11:36 AM, Ingo Molnar wrote: >> >>>> I can try to find out internally what Intel's position on writing >>>> %cr2 is, but it'll take a while; however, KVM should be able to >>>> tell you if any random OS uses %cr2 writes (as should a static >>>> disassembly of their kernel.) >>>> >>> Linux is one such OS. When acting as a hypervisor it writes cr2 >>> to present its guests with their expected environment (any >>> hypervisor that uses virtualization extensions will of course need >>> to do this). >>> >> >> Ah, it does save/restore it in svm_vcpu_run. VMX can do this via its >> context structure (without explicit CR manipulations in host space), >> right? >> > > It's the other way around. svm switches the guest cr2 in hardware > (through svm->vmcb->save.cr2). The code you're referring to saves > and restores the host cr2, which is completely unnecessary. I'm > currently in the middle of dropping it :) Heh :) > vmx has no hardware support for switching cr2, so vmx_vcpu_run() > switches it using mov cr2. Given that it's pretty expensive, I've > switched it to write-if-changed, which dropped 70 cycles from the > vmexit latency. Yep, see my numbers elsewhere in this thread - the cost of a cr2 write is ~84 cycles on Nehalem. Ingo