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=-2.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 71442C433E0 for ; Wed, 24 Jun 2020 11:20:46 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 3BD0620836 for ; Wed, 24 Jun 2020 11:20:46 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="gzmq3pX/" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3BD0620836 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Type: Content-Transfer-Encoding:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=XhN3unepTe9ZY5DmOObZ1Vmvrios55w53EhwH7rA8d0=; b=gzmq3pX/D6eFmXPHDTU5GTU19 XXA4Qtd/8eb2G1Feb3bUfQeGm8rF+h8Wdvu6gVEII43E0mVeWZ3xzPplbcMhneLAy+wwqYAunqBg8 xMNgdrvwIJuzsKNo5xz24tpMR9ddiQXnxWxPJzvoQ6n4fqH6sgcM53CvU4/AzU3qv2E2NQRCj1/pQ 29g4+LQj6EKw20PJDzzw+qrORw+fN4GTdBWLxoUKFrnTwJuk8zsuSClhKNU3u49LtrGzZTLDYWOsR BgW5GgsdI/Xk1H3/nXPKBjR3aETDx8uOIs9Uq4YHr3phDLVXiEycrEbOIuXW+aKctmfpYX0aREwiS rD2y823WA==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jo3Qf-0007dd-QC; Wed, 24 Jun 2020 11:18:57 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jo3QY-0007bN-Kh for linux-arm-kernel@lists.infradead.org; Wed, 24 Jun 2020 11:18:51 +0000 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 44A0B1F1; Wed, 24 Jun 2020 04:18:50 -0700 (PDT) Received: from [192.168.1.84] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 588F53F6CF; Wed, 24 Jun 2020 04:18:48 -0700 (PDT) Subject: Re: [RFC PATCH 0/2] MTE support for KVM guest To: Catalin Marinas References: <20200617123844.29960-1-steven.price@arm.com> <20200624093846.GA11863@gaia> <20200624103412.GD25945@arm.com> <20200624110904.GB11863@gaia> From: Steven Price Message-ID: <904edac0-3de7-35a6-a9bc-b983ccd3490c@arm.com> Date: Wed, 24 Jun 2020 12:18:46 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <20200624110904.GB11863@gaia> Content-Language: en-GB X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , Marc Zyngier , lkml - Kernel Mailing List , Dave P Martin , Thomas Gleixner , Will Deacon , "kvmarm@lists.cs.columbia.edu" , arm-mail-list Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 24/06/2020 12:09, Catalin Marinas wrote: > On Wed, Jun 24, 2020 at 12:03:35PM +0100, Steven Price wrote: >> On 24/06/2020 11:34, Dave Martin wrote: >>> On Wed, Jun 24, 2020 at 10:38:48AM +0100, Catalin Marinas wrote: >>>> On Tue, Jun 23, 2020 at 07:05:07PM +0100, Peter Maydell wrote: >>>>> On Wed, 17 Jun 2020 at 13:39, Steven Price wrote: >>>>>> These patches add support to KVM to enable MTE within a guest. It is >>>>>> based on Catalin's v4 MTE user space series[1]. >>>>>> >>>>>> [1] http://lkml.kernel.org/r/20200515171612.1020-1-catalin.marinas%40arm.com >>>>>> >>>>>> Posting as an RFC as I'd like feedback on the approach taken. >>>>> >>>>> What's your plan for handling tags across VM migration? >>>>> Will the kernel expose the tag ram to userspace so we >>>>> can copy it from the source machine to the destination >>>>> at the same time as we copy the actual ram contents ? >>>> >>>> Qemu can map the guest memory with PROT_MTE and access the tags directly >>>> with LDG/STG instructions. Steven was actually asking in the cover >>>> letter whether we should require that the VMM maps the guest memory with >>>> PROT_MTE as a guarantee that it can access the guest tags. >>>> >>>> There is no architecturally visible tag ram (tag storage), that's a >>>> microarchitecture detail. >>> >>> If userspace maps the guest memory with PROT_MTE for dump purposes, >>> isn't it going to get tag check faults when accessing the memory >>> (i.e., when dumping the regular memory content, not the tags >>> specifically). >>> >>> Does it need to map two aliases, one with PROT_MTE and one without, >>> and is that architecturally valid? >> >> Userspace would either need to have two mappings (I don't believe there are >> any architectural issues with that - but this could be awkward to arrange in >> some situations) or be careful to avoid faults. Basically your choices with >> one mapping are: >> >> 1. Disable tag checking (using prctl) when touching the memory. This works >> but means you lose tag checking for the VMM's own accesses during this code >> sequence. >> >> 2. Read the tag values and ensure you use the correct tag. This suffers >> from race conditions if the VM is still running. >> >> 3. Use one of the exceptions in the architecture that generates a Tag >> Unchecked access. Sadly the only remotely useful thing I can see in the v8 >> ARM is "A base register plus immediate offset addressing form, with the SP >> as the base register." - but making sure SP is in range of where you want to >> access would be a pain. > > Or: > > 4. Set PSTATE.TCO when accessing tagged memory in an unsafe way. > Ah yes, similar to (1) but much lower overhead ;) That's probably the best option - it can be hidden in a memcpy_ignoring_tags() function. However it still means that the VMM can't directly touch the guest's memory which might cause issues for the VMM. Steve _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel