From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752320AbcGAPNX (ORCPT ); Fri, 1 Jul 2016 11:13:23 -0400 Received: from mail-wm0-f66.google.com ([74.125.82.66]:34480 "EHLO mail-wm0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751922AbcGAPNV (ORCPT ); Fri, 1 Jul 2016 11:13:21 -0400 Subject: Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= References: <20160630205429.16480-1-rkrcmar@redhat.com> <20160630205429.16480-4-rkrcmar@redhat.com> <20160701124421.GA2301@potion> <20160701143827.GE27840@potion> Cc: Andrew Honig , linux-kernel@vger.kernel.org, kvm , "Lan, Tianyu" , Igor Mammedov , Jan Kiszka , Peter Xu From: Paolo Bonzini Message-ID: <46f7b2e0-3225-8a9b-6155-3588b5f60f78@redhat.com> Date: Fri, 1 Jul 2016 17:12:15 +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: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/07/2016 17:06, Paolo Bonzini wrote: >>> >> > Should it? >> Yes, x2APIC ID cannot be changed in hardware and is initialized to the >> intitial APIC ID. >> Letting LAPIC_SET change x2APIC ID would allow scenarios where userspace >> reuses old VMs instead of building new ones after reconfiguration. >> I don't think it's a sensible use case and it it is currently broken, >> because we don't exit to userspace when changing APIC mode, so KVM would >> just set APIC ID to VCPU ID on any transition and userspace couldn't >> amend it. Forgot to reply about this: letting SET_LAPIC change x2APIC IDs is nonsense. In x2APIC mode + new capability disabled SET_LAPIC should ignore the id register altogether for backwards compatibility. In x2APIC mode + new capability enabled it should either ignore it, or fail if the x2APIC ID doesn't match the VCPU id. I suspect the latter is better because it would help catching the case where userspace is erroneously shifting the id left to bits 31-24. Paolo