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=-5.2 required=3.0 tests=BAYES_00, 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 78C39C433B4 for ; Thu, 8 Apr 2021 18:21:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 3A15061108 for ; Thu, 8 Apr 2021 18:21:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232818AbhDHSVk (ORCPT ); Thu, 8 Apr 2021 14:21:40 -0400 Received: from mail.kernel.org ([198.145.29.99]:39382 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232666AbhDHSVa (ORCPT ); Thu, 8 Apr 2021 14:21:30 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 5B5F561130; Thu, 8 Apr 2021 18:21:15 +0000 (UTC) Date: Thu, 8 Apr 2021 19:21:12 +0100 From: Catalin Marinas To: David Hildenbrand Cc: Steven Price , Mark Rutland , Peter Maydell , "Dr. David Alan Gilbert" , Andrew Jones , Haibo Xu , Suzuki K Poulose , qemu-devel@nongnu.org, Marc Zyngier , Juan Quintela , Richard Henderson , linux-kernel@vger.kernel.org, Dave Martin , James Morse , linux-arm-kernel@lists.infradead.org, Thomas Gleixner , Will Deacon , kvmarm@lists.cs.columbia.edu, Julien Thierry Subject: Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature Message-ID: <20210408182112.GC7676@arm.com> References: <8977120b-841d-4882-2472-6e403bc9c797@redhat.com> <20210331092109.GA21921@arm.com> <86a968c8-7a0e-44a4-28c3-bac62c2b7d65@arm.com> <20210331184311.GA10737@arm.com> <20210407151458.GC21451@arm.com> <5e5bf772-1e4d-ca59-a9d8-058a72dfad4f@arm.com> <20210408141853.GA7676@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 08, 2021 at 08:16:17PM +0200, David Hildenbrand wrote: > On 08.04.21 16:18, Catalin Marinas wrote: > > On Wed, Apr 07, 2021 at 04:52:54PM +0100, Steven Price wrote: > > > On 07/04/2021 16:14, Catalin Marinas wrote: > > > > On Wed, Apr 07, 2021 at 11:20:18AM +0100, Steven Price wrote: > > > > > On 31/03/2021 19:43, Catalin Marinas wrote: > > > > > > When a slot is added by the VMM, if it asked for MTE in guest (I guess > > > > > > that's an opt-in by the VMM, haven't checked the other patches), can we > > > > > > reject it if it's is going to be mapped as Normal Cacheable but it is a > > > > > > ZONE_DEVICE (i.e. !kvm_is_device_pfn() + one of David's suggestions to > > > > > > check for ZONE_DEVICE)? This way we don't need to do more expensive > > > > > > checks in set_pte_at(). > > > > > > > > > > The problem is that KVM allows the VMM to change the memory backing a slot > > > > > while the guest is running. This is obviously useful for the likes of > > > > > migration, but ultimately means that even if you were to do checks at the > > > > > time of slot creation, you would need to repeat the checks at set_pte_at() > > > > > time to ensure a mischievous VMM didn't swap the page for a problematic one. > > > > > > > > Does changing the slot require some KVM API call? Can we intercept it > > > > and do the checks there? > > > > > > As David has already replied - KVM uses MMU notifiers, so there's not really > > > a good place to intercept this before the fault. > > > > > > > Maybe a better alternative for the time being is to add a new > > > > kvm_is_zone_device_pfn() and force KVM_PGTABLE_PROT_DEVICE if it returns > > > > true _and_ the VMM asked for MTE in guest. We can then only set > > > > PG_mte_tagged if !device. > > > > > > KVM already has a kvm_is_device_pfn(), and yes I agree restricting the MTE > > > checks to only !kvm_is_device_pfn() makes sense (I have the fix in my branch > > > locally). > > > > Indeed, you can skip it if kvm_is_device_pfn(). In addition, with MTE, > > I'd also mark a pfn as 'device' in user_mem_abort() if > > pfn_to_online_page() is NULL as we don't want to map it as Cacheable in > > Stage 2. It's unlikely that we'll trip over this path but just in case. > > > > (can we have a ZONE_DEVICE _online_ pfn or by definition they are > > considered offline?) > > By definition (and implementation) offline. When you get a page = > pfn_to_online_page() with page != NULL, that one should never be ZONE_DEVICE > (otherwise it would be a BUG). > > As I said, things are different when exposing dax memory via dax/kmem to the > buddy. But then, we are no longer talking about ZONE_DEVICE. Thanks David, it's clear now. -- Catalin