From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932805AbeCFKwY (ORCPT ); Tue, 6 Mar 2018 05:52:24 -0500 Received: from foss.arm.com ([217.140.101.70]:36500 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932589AbeCFKwX (ORCPT ); Tue, 6 Mar 2018 05:52:23 -0500 Date: Tue, 06 Mar 2018 10:52:15 +0000 Message-ID: <86woyp4gtc.wl-marc.zyngier@arm.com> From: Marc Zyngier To: Peter Maydell Cc: Auger Eric , lkml - Kernel Mailing List , arm-mail-list , kvmarm@lists.cs.columbia.edu Subject: Re: [REPOST PATCH] arm/arm64: KVM: Add PSCI version selection API In-Reply-To: References: <20180215175803.6870-1-marc.zyngier@arm.com> <86o9k63f7a.wl-marc.zyngier@arm.com> <40c74835-da24-485f-14bb-a0c357c7e79b@redhat.com> <81f8b489-883b-58c8-851b-c624ca72c28d@arm.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM/1.14.9 (=?ISO-8859-4?Q?Goj=F2?=) APEL/10.8 EasyPG/1.0.0 Emacs/25.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Organization: ARM Ltd MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 06 Mar 2018 10:12:42 +0000, peter maydell wrote: > > On 6 March 2018 at 09:50, Marc Zyngier wrote: > > On 05/03/18 20:37, Auger Eric wrote: > >> On 05/03/18 17:31, Peter Maydell wrote: > >>> That also means that we will fail migration from a new kernel where > >>> we've specifically asked for PSCI 0.2 to an old PSCI-0.2-only kernel > >>> (because the KVM_REG_ARM_PSCI_VERSION reg will appear in the migration > >>> stream even if its value is the one value that matches the old kernel > >>> behaviour). I don't know if we care about that. > >> > >> Do you know when are we likely to force PSCI 0.2 on a new kernel? At > >> which layer is the decision supposed to be made and on which criteria? > > > > No decent SW should need this. But if you've written a guest that cannot > > work if it doesn't get "2" as response to PSCI_VERSION, you can override it. > > ...but if you want to be able to migrate back from a new kernel to > an old one, then you need to ask the new kernel for 0.2 so it > behaves the same way as the old one. As it stands this code wouldn't > let you do that migration even if you did specifically ask for 0.2. > (As I said, I don't know if we care about that.) Absolutely. The moment we introduce a new sysreg, we create a migration barrier. I'm not sure how the kernel can help in this respect. M. -- Jazz is not dead, it just smell funny.