From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755098AbZFPIxm (ORCPT ); Tue, 16 Jun 2009 04:53:42 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752352AbZFPIxe (ORCPT ); Tue, 16 Jun 2009 04:53:34 -0400 Received: from mx2.redhat.com ([66.187.237.31]:53290 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751617AbZFPIxd (ORCPT ); Tue, 16 Jun 2009 04:53:33 -0400 Message-ID: <4A375D6B.3000105@redhat.com> Date: Tue, 16 Jun 2009 11:52:59 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1b3pre) Gecko/20090513 Fedora/3.0-2.3.beta2.fc11 Lightning/1.0pre Thunderbird/3.0b2 MIME-Version: 1.0 To: Ingo Molnar 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 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> In-Reply-To: <20090616083635.GE16229@elte.hu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 :) 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. -- error compiling committee.c: too many arguments to function