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=-7.5 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,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 4DA54C433ED for ; Tue, 27 Apr 2021 19:02:41 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (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 823E2613DC for ; Tue, 27 Apr 2021 19:02:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 823E2613DC Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:45688 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1lbSyl-0005nT-6h for qemu-devel@archiver.kernel.org; Tue, 27 Apr 2021 15:02:39 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:39890) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lbSx3-0004Qw-7H for qemu-devel@nongnu.org; Tue, 27 Apr 2021 15:00:53 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:51950) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1lbSx0-0000p4-JL for qemu-devel@nongnu.org; Tue, 27 Apr 2021 15:00:52 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1619550049; 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=pV0KUM/ofRHBSaAqTWs+59GgtKE/FeBXY4mIoOsF7CI=; b=SNXvmTNYB0gdLLptR9Z/Bae/A9mF55x8IDx09jPUCxLLInCeyA/u39NhKTNi9ZiNCyaRSn Szh+vRjlcJomqfugW/NkWRkNr7dIopwtizZmMt+X+KWaoVUGpSkT8xqaWy8RYDOsOFYb7s d+d1+L+xTo+WdC5KlUr0KONyQZj7Ujo= 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-520-3g5eBVl0PU6YHRNaZKHCrg-1; Tue, 27 Apr 2021 15:00:47 -0400 X-MC-Unique: 3g5eBVl0PU6YHRNaZKHCrg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7B96781840E; Tue, 27 Apr 2021 19:00:46 +0000 (UTC) Received: from work-vm (ovpn-114-253.ams2.redhat.com [10.36.114.253]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7DF9719714; Tue, 27 Apr 2021 19:00:45 +0000 (UTC) Date: Tue, 27 Apr 2021 20:00:42 +0100 From: "Dr. David Alan Gilbert" To: Alex Williamson Subject: Re: [PATCH] make vfio and DAX cache work together Message-ID: References: <20210426152203.379dab00@redhat.com> <20210427121850.68d2a8dd@redhat.com> MIME-Version: 1.0 In-Reply-To: <20210427121850.68d2a8dd@redhat.com> User-Agent: Mutt/2.0.6 (2021-03-06) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.218, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Dev Audsin , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Alex Williamson (alex.williamson@redhat.com) wrote: > On Tue, 27 Apr 2021 17:29:37 +0100 > Dev Audsin wrote: > > > Hi Alex > > > > Based on your comments and thinking a bit, wonder if it makes sense to > > allow DMA map for the DAX cache but make unexpected mappings to be not > > fatal. Please let me know your thoughts. > > I think you're still working on the assumption that simply making the > VM boot is an improvement, it's not. If there's a risk that a possible > DMA target for the device cannot be mapped, it's better that the VM > fail to boot than to expose that risk. Performance cannot compromise > correctness. > > We do allow DMA mappings to other device memory regions to fail > non-fatally with the logic that peer-to-peer DMA is often not trusted > to work by drivers and therefore support would be probed before > assuming that it works. I don't think that same logic applies here. > > Is there something about the definition of this particular region that > precludes it from being a DMA target for an assigned devices? It's never really the ram that's used. This area is really a chunk of VMA that's mmap'd over by (chunks of) normal files in the underlying exported filesystem. The actual RAM block itself is just a placeholder for the VMA, and is normally mapped PROT_NONE until an actual file is mapped on top of it. That cache bar is a mapping containing multiple separate file chunk mappings. So I guess the problems for VFIO are: a) At the start it's unmapped, unaccessible, unallocated ram. b) Later it's arbitrary chunks of ondisk files. [on a bad day, and it's bad even without vfio, someone truncates the file mapping] Dave > Otherwise if it's initially unpopulated, maybe something like the > RamDiscardManager could be used to insert DMA mappings as the region > becomes populated. > > Simply disabling mapping to boot with both features together, without > analyzing how that missing mapping affects their interaction is not > acceptable. Thanks, > > Alex > > > On Mon, Apr 26, 2021 at 10:22 PM Alex Williamson > > wrote: > > > > > > On Mon, 26 Apr 2021 21:50:38 +0100 > > > Dev Audsin wrote: > > > > > > > Hi Alex and David > > > > > > > > @Alex: > > > > > > > > Justification on why this region cannot be a DMA target for the device, > > > > > > > > virtio-fs with DAX is currently not compatible with NIC Pass through. > > > > When a SR-IOV VF attaches to a qemu process, vfio will try to pin the > > > > entire DAX Window but it is empty when the guest boots and will fail. > > > > A method to make VFIO and DAX to work together is to make vfio skip > > > > DAX cache. > > > > > > > > Currently DAX cache need to be set to 0, for the SR-IOV VF to be > > > > attached to Kata containers. Enabling both SR-IOV VF and DAX work > > > > together will potentially improve performance for workloads which are > > > > I/O and network intensive. > > > > > > Sorry, there's no actual justification described here. You're enabling > > > a VM with both features, virtio-fs DAX and VFIO, but there's no > > > evidence that they "work together" or that your use case is simply > > > avoiding a scenario where the device might attempt to DMA into the area > > > with this designation. With this change, if the device were to attempt > > > to DMA into this region, it would be blocked by the IOMMU, which might > > > result in a data loss within the VM. Justification of this change > > > needs to prove that this region can never be a DMA target for the > > > device, not simply that both features can be enabled and we hope that > > > they don't interact. Thanks, > > > > > > Alex > > > > > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK