From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751504AbcBLTz1 (ORCPT ); Fri, 12 Feb 2016 14:55:27 -0500 Received: from mail-by2on0090.outbound.protection.outlook.com ([207.46.100.90]:12659 "EHLO na01-by2-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750852AbcBLTzZ (ORCPT ); Fri, 12 Feb 2016 14:55:25 -0500 Authentication-Results: amd.com; dkim=none (message not signed) header.d=none;amd.com; dmarc=none action=none header.from=amd.com; Subject: Re: [PART1 RFC 0/9] KVM: x86: Introduce SVM AVIC support To: Paolo Bonzini , , , References: <1455285574-27892-1-git-send-email-suravee.suthikulpanit@amd.com> <56BE20CF.8030708@redhat.com> CC: , , , From: Suravee Suthikulpanit Message-ID: <56BE3897.5060507@amd.com> Date: Sat, 13 Feb 2016 02:55:03 +0700 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 MIME-Version: 1.0 In-Reply-To: <56BE20CF.8030708@redhat.com> Content-Type: text/plain; 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I've noticed that, > because x2APIC is disabled by default, it's possible that a VM on a > non-AVIC host will fail when moved to an AVIC host. I would prefer if > you kept x2APIC working through VMEXITs. Getting the full performance > benefit would require disabling x2APIC of course (or waiting for > improved support in the processor), but at least there would be no surprise. > > Is it just this, or is there anything else? For VM Migration, I meant the process of taking a snapshot of the VM and later on loading it back to resume where it left off, and not necessary to a different type of system. This would require saving off the VAPIC backing page of each vCPU along with the rest of per-VM AVIC data structures. IIUC, this might require changes in the QEmu as well. Moving VM non-AVIC host to AVIC host, and vice versa, would be tricky. I also see you points about supporting x2APIC. Let me look more into it, and then get back to you on this. > > (I'm quite surprised that x2APIC is not supported. MSRs are faster than > memory accesses due to the cost of TLB misses, which can be hefty for VMs). > > I have a few other doubts about the architecture that hopefully you (or > Wei Huang too) can clarify before I can review this patch meaningfully. > I have replied to other patches with these questions. > > Paolo > I'll go through those and get back to you separately. Regards, Suravee