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[209.51.188.17]) by mx.google.com with ESMTPS id b66-20020ae9eb45000000b0071017f8baaesi12829506qkg.9.2023.03.02.05.26.47 for (version=TLS1_2 cipher=ECDHE-ECDSA-CHACHA20-POLY1305 bits=256/256); Thu, 02 Mar 2023 05:26:47 -0800 (PST) Received-SPF: pass (google.com: domain of qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; Authentication-Results: mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=YF1xeOy+; spf=pass (google.com: domain of qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom="qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org"; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pXix4-0003bq-Fh; Thu, 02 Mar 2023 08:26:30 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pXiws-0003RG-Kv for qemu-arm@nongnu.org; Thu, 02 Mar 2023 08:26:18 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pXiwr-0004Wl-49 for qemu-arm@nongnu.org; Thu, 02 Mar 2023 08:26:18 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677763570; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=a2nxODxGNCf2GEDTesp0abZC6ZU/bJOaxWOZgcF9cDk=; b=YF1xeOy+yZbr1BtYb79mouQyE2p22I7B+jImUa9c6zfaqJJ+mZzTjOXSUowuwwByvkqFDP khqhZBx/PFKI3Cs/E/ZJcC/jiClknDafWeAU1cmDh9Xa0gu81qVhCHDkA2+bwuDx1M5kWo HVBn2F1iyHhzfBUpYaLFDv8lVvipBN8= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-314-f5WNOqXyO6-aa48-cPrfJg-1; Thu, 02 Mar 2023 08:26:09 -0500 X-MC-Unique: f5WNOqXyO6-aa48-cPrfJg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 1B06F800B23; Thu, 2 Mar 2023 13:26:09 +0000 (UTC) Received: from localhost (unknown [10.39.192.174]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BEDC740C83B6; Thu, 2 Mar 2023 13:26:08 +0000 (UTC) From: Cornelia Huck To: Andrea Bolognani Cc: Peter Maydell , Thomas Huth , Laurent Vivier , qemu-arm@nongnu.org, qemu-devel@nongnu.org, kvm@vger.kernel.org, Eric Auger , "Dr. David Alan Gilbert" , Juan Quintela , Gavin Shan , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Richard Henderson Subject: Re: [PATCH v6 1/2] arm/kvm: add support for MTE In-Reply-To: Organization: Red Hat GmbH References: <20230228150216.77912-1-cohuck@redhat.com> <20230228150216.77912-2-cohuck@redhat.com> <874jr4dbcr.fsf@redhat.com> <87sfeoblsa.fsf@redhat.com> User-Agent: Notmuch/0.37 (https://notmuchmail.org) Date: Thu, 02 Mar 2023 14:26:06 +0100 Message-ID: <87fsanmgi9.fsf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.133.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-arm@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org Sender: qemu-arm-bounces+alex.bennee=linaro.org@nongnu.org X-TUID: y/eKF5J9a21X On Wed, Mar 01 2023, Andrea Bolognani wrote: > On Wed, Mar 01, 2023 at 03:15:17PM +0100, Cornelia Huck wrote: >> On Wed, Mar 01 2023, Andrea Bolognani wrote: >> > I'm actually a bit confused. The documentation for the mte property >> > of the virt machine type says >> > >> > mte >> > Set on/off to enable/disable emulating a guest CPU which implements >> > the Arm Memory Tagging Extensions. The default is off. >> > >> > So why is there a need to have a CPU property in addition to the >> > existing machine type property? >> >> I think the state prior to my patches is actually a bit confusing: the >> user needs to set a machine type property (causing tag memory to be >> allocated), which in turn enables a cpu feature. Supporting the machine >> type property for KVM does not make much sense IMHO: we don't allocate >> tag memory for KVM (in fact, that would not work). We have to keep the >> previous behaviour, and explicitly instructing QEMU to create cpus with >> a certain feature via a cpu property makes the most sense to me. > > I agree that a CPU feature makes more sense. > >> We might want to tweak the documentation for the machine property to >> indicate that it creates tag memory and only implicitly enables mte but >> is a pre-req for it -- thoughts? > > I wonder if it would be possible to flip things around, so that the > machine property is retained with its existing behavior for backwards > compatibility, but both for KVM and for TCG the CPU property can be > used on its own? > > Basically, keeping the default of machine.mte to off when cpu.mte is > not specified, but switching it to on when it is. This way, you'd be > able to simply use > > -machine virt -cpu xxx,mte=on > > to enable MTE, regardless of whether you're using KVM or TCG, instead > of requiring the above for KVM and > > -machine virt,mte=on -cpu xxx > > for TCG. The machine prop is a bool; that means that we cannot distinguish between "user did not set mte at all" and "user explicitly set mte=off" -- I think we want -machine virt,mte=off -cpu xxx,mte=on to generate an error, but that would still imply that we'd need to error out for -machine virt -cpu xxx,mte=on as well. We could make the machine prop OnOffAuto, but that looks like overkill to me. > > Note that, from libvirt's point of view, there's no advantage to > doing things that way instead of what you already have. Handling the > additional machine property is a complete non-issue. But it would > make things nicer for people running QEMU directly, I think. I'm tempted to simply consider this to be another wart of the QEMU command line :)