From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9AD4EC11D05 for ; Thu, 20 Feb 2020 14:38:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 787CA20656 for ; Thu, 20 Feb 2020 14:38:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728042AbgBTOio (ORCPT ); Thu, 20 Feb 2020 09:38:44 -0500 Received: from foss.arm.com ([217.140.110.172]:44120 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727761AbgBTOio (ORCPT ); Thu, 20 Feb 2020 09:38:44 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AF19C31B; Thu, 20 Feb 2020 06:38:43 -0800 (PST) Received: from [10.1.196.37] (e121345-lin.cambridge.arm.com [10.1.196.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 788F03F6CF; Thu, 20 Feb 2020 06:38:41 -0800 (PST) Subject: Re: [RFC PATCH 0/5] Removing support for 32bit KVM/arm host To: Marc Zyngier Cc: Marek Szyprowski , Vladimir Murzin , Russell King , kvm@vger.kernel.org, Arnd Bergmann , Suzuki K Poulose , Quentin Perret , Christoffer Dall , Krzysztof Kozlowski , Bartlomiej Zolnierkiewicz , James Morse , Julien Thierry , Paolo Bonzini , Will Deacon , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org References: <20200210141324.21090-1-maz@kernel.org> <621a0a92-6432-6c3e-cb69-0b601764fa69@samsung.com> <43446bd5e884ae92f243799cbe748871@kernel.org> <3f7f3b6c8b758b6d2134364616c6bc1e@kernel.org> From: Robin Murphy Message-ID: <5a984189-78bb-2707-3714-13edcee9e8f5@arm.com> Date: Thu, 20 Feb 2020 14:38:40 +0000 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <3f7f3b6c8b758b6d2134364616c6bc1e@kernel.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 20/02/2020 2:01 pm, Marc Zyngier wrote: > On 2020-02-20 13:32, Robin Murphy wrote: >> On 20/02/2020 1:15 pm, Marc Zyngier wrote: >>> Hi Marek, >>> >>> On 2020-02-20 12:44, Marek Szyprowski wrote: >>>> Hi Marc, >>>> >>>> On 10.02.2020 15:13, Marc Zyngier wrote: >>>>> KVM/arm was merged just over 7 years ago, and has lived a very quiet >>>>> life so far. It mostly works if you're prepared to deal with its >>>>> limitations, it has been a good prototype for the arm64 version, >>>>> but it suffers a few problems: >>>>> >>>>> - It is incomplete (no debug support, no PMU) >>>>> - It hasn't followed any of the architectural evolutions >>>>> - It has zero users (I don't count myself here) >>>>> - It is more and more getting in the way of new arm64 developments >>>> >>>> That is a bit sad information. Mainline Exynos finally got everything >>>> that was needed to run it on the quite popular Samsung Exynos5422-based >>>> Odroid XU4/HC1/MC1 boards. According to the Odroid related forums it is >>>> being used. We also use it internally at Samsung. >>> >>> Something like "too little, too late" springs to mind, but let's be >>> constructive. Is anyone using it in a production environment, where >>> they rely on the latest mainline kernel having KVM support? >>> >>> The current proposal is to still have KVM support in 5.6, as well as >>> ongoing support for stable kernels. If that's not enough, can you please >>> explain your precise use case? >> >> Presumably there's no *technical* reason why the stable subset of v7 >> support couldn't be stripped down and brought back private to arch/arm >> if somebody really wants and is willing to step up and look after it? > > There is no technical reason at all, just a maintenance effort. > > The main killer is the whole MMU code, which I'm butchering with NV, > and that I suspect Will will also turn upside down with his stuff. > Not to mention the hypercall interface that will need a complete overhaul. > > If we wanted to decouple the two, we'd need to make the MMU code, the > hypercalls, arm.c and a number of other bits private to 32bit. Right, the prospective kvm-arm maintainer's gameplan would essentially be an equivalent "move virt/kvm/arm to arch/arm/kvm" patch, but then ripping out all the Armv8 and GICv3 gubbins instead. Yes, there would then be lots of *similar* code to start with, but it would only diverge further as v8 architecture development continues independently. Anyway, I just thought it seemed worth saying out loud, to reassure folks that a realistic middle-ground between "yay bye!" and "oh no the end of the world!" does exist, namely "someone else's problem" :) Robin.