From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752374AbcBLSNk (ORCPT ); Fri, 12 Feb 2016 13:13:40 -0500 Received: from mail-wm0-f41.google.com ([74.125.82.41]:36257 "EHLO mail-wm0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751273AbcBLSNi (ORCPT ); Fri, 12 Feb 2016 13:13:38 -0500 Subject: Re: [PART1 RFC 0/9] KVM: x86: Introduce SVM AVIC support To: Suravee Suthikulpanit , joro@8bytes.org, alex.williamson@redhat.com, gleb@kernel.org References: <1455285574-27892-1-git-send-email-suravee.suthikulpanit@amd.com> Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, wei@redhat.com, sherry.hurwitz@amd.com From: Paolo Bonzini X-Enigmail-Draft-Status: N1110 Message-ID: <56BE20CF.8030708@redhat.com> Date: Fri, 12 Feb 2016 19:13:35 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <1455285574-27892-1-git-send-email-suravee.suthikulpanit@amd.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I have a few questions about AVIC. On 12/02/2016 14:59, Suravee Suthikulpanit wrote: > CURRENT UNSUPPORT USE-CASES > =========================== > - Nested VM > - VM Migration I'm interested in what you mean with "VM migration". 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? (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