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, URIBL_BLOCKED,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 BE80EC4361B for ; Tue, 8 Dec 2020 17:22:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B9CE23B09 for ; Tue, 8 Dec 2020 17:22:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730544AbgLHRW3 (ORCPT ); Tue, 8 Dec 2020 12:22:29 -0500 Received: from mail.kernel.org ([198.145.29.99]:56858 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726703AbgLHRW3 (ORCPT ); Tue, 8 Dec 2020 12:22:29 -0500 Date: Tue, 8 Dec 2020 17:21:43 +0000 From: Catalin Marinas To: Marc Zyngier Cc: Steven Price , Peter Maydell , Haibo Xu , lkml - Kernel Mailing List , Juan Quintela , Richard Henderson , QEMU Developers , "Dr. David Alan Gilbert" , Thomas Gleixner , Will Deacon , kvmarm , arm-mail-list , Dave Martin Subject: Re: [PATCH v5 0/2] MTE support for KVM guest Message-ID: <20201208172143.GB13960@gaia> References: <20201119184248.4bycy6ouvaxqdiiy@kamzik.brq.redhat.com> <46fd98a2-ee39-0086-9159-b38c406935ab@arm.com> <0d0eb6da6a11f76d10e532c157181985@kernel.org> <20201207163405.GD1526@gaia> <874kkx5thq.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <874kkx5thq.wl-maz@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 07, 2020 at 07:03:13PM +0000, Marc Zyngier wrote: > On Mon, 07 Dec 2020 16:34:05 +0000, > Catalin Marinas wrote: > > On Mon, Dec 07, 2020 at 04:05:55PM +0000, Marc Zyngier wrote: > > > What I'd really like to see is a description of how shared memory > > > is, in general, supposed to work with MTE. My gut feeling is that > > > it doesn't, and that you need to turn MTE off when sharing memory > > > (either implicitly or explicitly). > > > > The allocation tag (in-memory tag) is a property assigned to a physical > > address range and it can be safely shared between different processes as > > long as they access it via pointers with the same allocation tag (bits > > 59:56). The kernel enables such tagged shared memory for user processes > > (anonymous, tmpfs, shmem). > > I think that's one case where the shared memory scheme breaks, as we > have two kernels in charge of their own tags, and they obviously can't > be synchronised Yes, if you can't trust the other entity to not change the tags, the only option is to do an untagged access. > > What we don't have in the architecture is a memory type which allows > > access to tags but no tag checking. To access the data when the tags > > aren't known, the tag checking would have to be disabled via either a > > prctl() or by setting the PSTATE.TCO bit. > > I guess that's point (3) in Steven's taxonomy. It still a bit ugly to > fit in an existing piece of userspace, specially if it wants to use > MTE for its own benefit. I agree it's ugly. For the device DMA emulation case, the only sane way is to mimic what a real device does - no tag checking. For a generic implementation, this means that such shared memory should not be mapped with PROT_MTE on the VMM side. I guess this leads to your point that sharing doesn't work for this scenario ;). > > The kernel accesses the user memory via the linear map using a match-all > > tag 0xf, so no TCO bit toggling. For user, however, we disabled such > > match-all tag and it cannot be enabled at run-time (at least not easily, > > it's cached in the TLB). However, we already have two modes to disable > > tag checking which Qemu could use when migrating data+tags. > > I wonder whether we will have to have something kernel side to > dump/reload tags in a way that matches the patterns used by live > migration. We have something related - ptrace dumps/resores the tags. Can the same concept be expanded to a KVM ioctl? -- Catalin