From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030385AbcGKHo6 (ORCPT ); Mon, 11 Jul 2016 03:44:58 -0400 Received: from mail-lf0-f46.google.com ([209.85.215.46]:32780 "EHLO mail-lf0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030323AbcGKHoy (ORCPT ); Mon, 11 Jul 2016 03:44:54 -0400 Subject: Re: [PATCH v2 11/13] KVM: x86: add KVM_CAP_X2APIC_API To: Yang Zhang , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , linux-kernel@vger.kernel.org, kvm@vger.kernel.org References: <20160707171550.14675-1-rkrcmar@redhat.com> <20160707171550.14675-12-rkrcmar@redhat.com> <630f1159-53b4-aeca-ed36-c94e9bf75790@gmail.com> Cc: "Lan, Tianyu" , Igor Mammedov , Jan Kiszka , Peter Xu From: Paolo Bonzini Message-ID: <053e042b-c523-9c10-51e5-59a5ac164bc5@redhat.com> Date: Mon, 11 Jul 2016 09:44:49 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 MIME-Version: 1.0 In-Reply-To: <630f1159-53b4-aeca-ed36-c94e9bf75790@gmail.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 On 11/07/2016 08:06, Yang Zhang wrote: >> Changes to MSI addresses follow the format used by interrupt remapping >> unit. >> The upper address word, that used to be 0, contains upper 24 bits of >> the LAPIC >> address in its upper 24 bits. Lower 8 bits are reserved as 0. >> Using the upper address word is not backward-compatible either as we >> didn't >> check that userspace zeroed the word. Reserved bits are still not >> explicitly > > Does this means we cannot migrate the VM from KVM_CAP_X2APIC_API enabled > host to the disable host even VM doesn't have more than 255 VCPUs? Yes, but that's why KVM_CAP_X2APIC_API is enabled manually. The idea is that QEMU will not use KVM_CAP_X2APIC_API except on the newest machine type. If interrupt remapping is on, KVM_CAP_X2APIC_API is needed even with 8 VCPUs, I think. Otherwise KVM will believe that 0xff is "broadcast" rather than "cluster 0, CPUs 0-7". Paolo