From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936055Ab0BZNEQ (ORCPT ); Fri, 26 Feb 2010 08:04:16 -0500 Received: from tx2ehsobe003.messaging.microsoft.com ([65.55.88.13]:37823 "EHLO TX2EHSOBE005.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935891Ab0BZNEO (ORCPT ); Fri, 26 Feb 2010 08:04:14 -0500 X-SpamScore: -20 X-BigFish: VPS-20(zz1432R98dN936eMzz1202hzzz32i6bh87h61h) X-Spam-TCS-SCL: 0:0 X-FB-DOMAIN-IP-MATCH: fail X-WSS-ID: 0KYG9MQ-01-A2M-02 X-M-MSG: Date: Fri, 26 Feb 2010 14:04:01 +0100 From: Joerg Roedel To: Alexander Graf CC: Avi Kivity , Marcelo Tosatti , kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/5] KVM: SVM: Optimize nested svm msrpm merging Message-ID: <20100226130401.GD12689@amd.com> References: <1267118149-15737-1-git-send-email-joerg.roedel@amd.com> <1267118149-15737-3-git-send-email-joerg.roedel@amd.com> <4B87A248.1050300@redhat.com> <20100226122502.GC12689@amd.com> <4E7D93ED-E5FC-4A64-B9B0-E2F644CD2B68@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <4E7D93ED-E5FC-4A64-B9B0-E2F644CD2B68@suse.de> Organization: Advanced Micro Devices =?iso-8859-1?Q?GmbH?= =?iso-8859-1?Q?=2C_Karl-Hammerschmidt-Str=2E_34=2C_85609_Dornach_bei_M=FC?= =?iso-8859-1?Q?nchen=2C_Gesch=E4ftsf=FChrer=3A_Thomas_M=2E_McCoy=2C_Giuli?= =?iso-8859-1?Q?ano_Meroni=2C_Andrew_Bowd=2C_Sitz=3A_Dornach=2C_Gemeinde_A?= =?iso-8859-1?Q?schheim=2C_Landkreis_M=FCnchen=2C_Registergericht_M=FCnche?= =?iso-8859-1?Q?n=2C?= HRB Nr. 43632 User-Agent: Mutt/1.5.20 (2009-06-14) X-OriginalArrivalTime: 26 Feb 2010 13:04:02.0054 (UTC) FILETIME=[2ACFCA60:01CAB6E4] X-Reverse-DNS: ausb3extmailp02.amd.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 26, 2010 at 01:28:29PM +0100, Alexander Graf wrote: > > On 26.02.2010, at 13:25, Joerg Roedel wrote: > > > On Fri, Feb 26, 2010 at 12:28:24PM +0200, Avi Kivity wrote: > >>> +static void add_msr_offset(u32 offset) > >>> +{ > >>> + u32 old; > >>> + int i; > >>> + > >>> +again: > >>> + for (i = 0; i< MSRPM_OFFSETS; ++i) { > >>> + old = msrpm_offsets[i]; > >>> + > >>> + if (old == offset) > >>> + return; > >>> + > >>> + if (old != MSR_INVALID) > >>> + continue; > >>> + > >>> + if (cmpxchg(&msrpm_offsets[i], old, offset) != old) > >>> + goto again; > >>> + > >>> + return; > >>> + } > >>> + > >>> + /* > >>> + * If this BUG triggers the msrpm_offsets table has an overflow. Just > >>> + * increase MSRPM_OFFSETS in this case. > >>> + */ > >>> + BUG(); > >>> +} > >> > >> Why all this atomic cleverness? The possible offsets are all > >> determined statically. Even if you do them dynamically (makes sense > >> when considering pmu passthrough), it's per-vcpu and therefore > >> single threaded (just move msrpm_offsets into vcpu context). > > > > The msr_offset table is the same for all guests. It doesn't make sense > > to keep it per vcpu because it will currently look the same for all > > vcpus. For standard guests this array contains 3 entrys. It is marked > > with __read_mostly for the same reason. > > I'm still not convinced on this way of doing things. If it's static, > make it static. If it's dynamic, make it dynamic. Dynamically > generating a static list just sounds plain wrong to me. Stop. I had a static list in the first version of the patch. This list was fine except the fact that a developer needs to remember to update this list if the list of non-intercepted msrs is expanded. The whole reason for a dynamically built list is to take the task of maintaining the list away from the developer and remove a possible source of hard to find bugs. This is what the current approach does. Joerg