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=-7.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 4FCA6C4167B for ; Mon, 7 Dec 2020 16:46:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 11E5B23787 for ; Mon, 7 Dec 2020 16:46:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727773AbgLGQqF (ORCPT ); Mon, 7 Dec 2020 11:46:05 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:43561 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725804AbgLGQqE (ORCPT ); Mon, 7 Dec 2020 11:46:04 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1607359477; 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=UgdjE4fKcDjZBY/heUx1DQEDENSCD7DYWWLFE/GwBEg=; b=AiaaUCVjHrh0hsEJDm7XD1DLKeFeODroWOcMEKElNocjWdD6QJiazBUI6e+XlMMrZPnGGr F8O9ilw/PxpDF1ddAFb+1BEL4WknsTAl3iMl/P4/8yg5toi04zaRiuI7HZDq59d6oQgwBj 0JTOE9AJxHD1aKb4FGaFjtyH+5jzj9g= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-212-xjKu7mQ6NxSw7npi5lgtfA-1; Mon, 07 Dec 2020 11:44:36 -0500 X-MC-Unique: xjKu7mQ6NxSw7npi5lgtfA-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 1C09D1922020; Mon, 7 Dec 2020 16:44:34 +0000 (UTC) Received: from work-vm (ovpn-114-87.ams2.redhat.com [10.36.114.87]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 32D6A60C0F; Mon, 7 Dec 2020 16:44:30 +0000 (UTC) Date: Mon, 7 Dec 2020 16:44:28 +0000 From: "Dr. David Alan Gilbert" To: Steven Price , dgibson@redhat.com Cc: Peter Maydell , Haibo Xu , lkml - Kernel Mailing List , Juan Quintela , Marc Zyngier , Richard Henderson , QEMU Developers , Catalin Marinas , Thomas Gleixner , Will Deacon , kvmarm , arm-mail-list , Dave Martin Subject: Re: [PATCH v5 0/2] MTE support for KVM guest Message-ID: <20201207164428.GD3135@work-vm> References: <20201119153901.53705-1-steven.price@arm.com> <20201119184248.4bycy6ouvaxqdiiy@kamzik.brq.redhat.com> <46fd98a2-ee39-0086-9159-b38c406935ab@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.14.6 (2020-07-11) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Price (steven.price@arm.com) wrote: > On 07/12/2020 15:27, Peter Maydell wrote: > > On Mon, 7 Dec 2020 at 14:48, Steven Price wrote: > > > Sounds like you are making good progress - thanks for the update. Have > > > you thought about how the PROT_MTE mappings might work if QEMU itself > > > were to use MTE? My worry is that we end up with MTE in a guest > > > preventing QEMU from using MTE itself (because of the PROT_MTE > > > mappings). I'm hoping QEMU can wrap its use of guest memory in a > > > sequence which disables tag checking (something similar will be needed > > > for the "protected VM" use case anyway), but this isn't something I've > > > looked into. > > > > It's not entirely the same as the "protected VM" case. For that > > the patches currently on list basically special case "this is a > > debug access (eg from gdbstub/monitor)" which then either gets > > to go via "decrypt guest RAM for debug" or gets failed depending > > on whether the VM has a debug-is-ok flag enabled. For an MTE > > guest the common case will be guests doing standard DMA operations > > to or from guest memory. The ideal API for that from QEMU's > > point of view would be "accesses to guest RAM don't do tag > > checks, even if tag checks are enabled for accesses QEMU does to > > memory it has allocated itself as a normal userspace program". > > Sorry, I know I simplified it rather by saying it's similar to protected VM. > Basically as I see it there are three types of memory access: > > 1) Debug case - has to go via a special case for decryption or ignoring the > MTE tag value. Hopefully this can be abstracted in the same way. > > 2) Migration - for a protected VM there's likely to be a special method to > allow the VMM access to the encrypted memory (AFAIK memory is usually kept > inaccessible to the VMM). For MTE this again has to be special cased as we > actually want both the data and the tag values. > > 3) Device DMA - for a protected VM it's usual to unencrypt a small area of > memory (with the permission of the guest) and use that as a bounce buffer. > This is possible with MTE: have an area the VMM purposefully maps with > PROT_MTE. The issue is that this has a performance overhead and we can do > better with MTE because it's trivial for the VMM to disable the protection > for any memory. Those all sound very similar to the AMD SEV world; there's the special case for Debug that Peter mentioned; migration is ...complicated and needs special case that's still being figured out, and as I understand Device DMA also uses a bounce buffer (and swiotlb in the guest to make that happen). I'm not sure about the stories for the IBM hardware equivalents. Dave > The part I'm unsure on is how easy it is for QEMU to deal with (3) without > the overhead of bounce buffers. Ideally there'd already be a wrapper for > guest memory accesses and that could just be wrapped with setting TCO during > the access. I suspect the actual situation is more complex though, and I'm > hoping Haibo's investigations will help us understand this. > > Thanks, > > Steve > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK