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=-4.1 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 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 35CD7C433DF for ; Wed, 12 Aug 2020 02:36:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F083E206B2 for ; Wed, 12 Aug 2020 02:36:33 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="exl5FDCW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726355AbgHLCgd (ORCPT ); Tue, 11 Aug 2020 22:36:33 -0400 Received: from us-smtp-1.mimecast.com ([207.211.31.81]:33784 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726173AbgHLCgd (ORCPT ); Tue, 11 Aug 2020 22:36:33 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1597199791; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=w+xbO45pxEQ4RVFD3gdok49sH5sjhVfab6kjd9PTGOo=; b=exl5FDCWsy8eWTiZV28fdN0BtUREVKmheQm3MO+gCWIBFvhBgjkvEsh9Em4E6gQNsnqfid bUJQli5PNqXoorQcZvE3tO/2zwEbZoBs4HIXri8KoCfM+JfZPX0/Jgg9nfbmbtZCU/0nnX GbE9eOiBbFjp7eEgxMcMAYh/gwdk3zg= 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-261-RWjOC2UOO6mb6G7yCv5Feg-1; Tue, 11 Aug 2020 22:36:28 -0400 X-MC-Unique: RWjOC2UOO6mb6G7yCv5Feg-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9D544802B43; Wed, 12 Aug 2020 02:36:24 +0000 (UTC) Received: from x1.home (ovpn-112-71.phx2.redhat.com [10.3.112.71]) by smtp.corp.redhat.com (Postfix) with ESMTP id DE0BB5C1BD; Wed, 12 Aug 2020 02:36:18 +0000 (UTC) Date: Tue, 11 Aug 2020 20:36:18 -0600 From: Alex Williamson To: "Tian, Kevin" Cc: Jason Gunthorpe , "Jiang, Dave" , "vkoul@kernel.org" , "Dey, Megha" , "maz@kernel.org" , "bhelgaas@google.com" , "rafael@kernel.org" , "gregkh@linuxfoundation.org" , "tglx@linutronix.de" , "hpa@zytor.com" , "Pan, Jacob jun" , "Raj, Ashok" , "Liu, Yi L" , "Lu, Baolu" , "Kumar, Sanjay K" , "Luck, Tony" , "Lin, Jing" , "Williams, Dan J" , "kwankhede@nvidia.com" , "eric.auger@redhat.com" , "parav@mellanox.com" , "Hansen, Dave" , "netanelg@mellanox.com" , "shahafs@mellanox.com" , "yan.y.zhao@linux.intel.com" , "pbonzini@redhat.com" , "Ortiz, Samuel" , "Hossain, Mona" , "dmaengine@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" , "linux-pci@vger.kernel.org" , "kvm@vger.kernel.org" Subject: Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver Message-ID: <20200811203618.41aa6031@x1.home> In-Reply-To: References: <159534667974.28840.2045034360240786644.stgit@djiang5-desk3.ch.intel.com> <20200721164527.GD2021248@mellanox.com> <20200724001930.GS2021248@mellanox.com> <20200805192258.5ee7a05b@x1.home> <20200807121955.GS16789@nvidia.com> <20200811110036.7d337837@x1.home> Organization: Red Hat MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: dmaengine-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: dmaengine@vger.kernel.org On Wed, 12 Aug 2020 01:58:00 +0000 "Tian, Kevin" wrote: > > From: Alex Williamson > > Sent: Wednesday, August 12, 2020 1:01 AM > > > > On Mon, 10 Aug 2020 07:32:24 +0000 > > "Tian, Kevin" wrote: > > > > > > From: Jason Gunthorpe > > > > Sent: Friday, August 7, 2020 8:20 PM > > > > > > > > On Wed, Aug 05, 2020 at 07:22:58PM -0600, Alex Williamson wrote: > > > > > > > > > If you see this as an abuse of the framework, then let's identify those > > > > > specific issues and come up with a better approach. As we've discussed > > > > > before, things like basic PCI config space emulation are acceptable > > > > > overhead and low risk (imo) and some degree of register emulation is > > > > > well within the territory of an mdev driver. > > > > > > > > What troubles me is that idxd already has a direct userspace interface > > > > to its HW, and does userspace DMA. The purpose of this mdev is to > > > > provide a second direct userspace interface that is a little different > > > > and trivially plugs into the virtualization stack. > > > > > > No. Userspace DMA and subdevice passthrough (what mdev provides) > > > are two distinct usages IMO (at least in idxd context). and this might > > > be the main divergence between us, thus let me put more words here. > > > If we could reach consensus in this matter, which direction to go > > > would be clearer. > > > > > > First, a passthrough interface requires some unique requirements > > > which are not commonly observed in an userspace DMA interface, e.g.: > > > > > > - Tracking DMA dirty pages for live migration; > > > - A set of interfaces for using SVA inside guest; > > > * PASID allocation/free (on some platforms); > > > * bind/unbind guest mm/page table (nested translation); > > > * invalidate IOMMU cache/iotlb for guest page table changes; > > > * report page request from device to guest; > > > * forward page response from guest to device; > > > - Configuring irqbypass for posted interrupt; > > > - ... > > > > > > Second, a passthrough interface requires delegating raw controllability > > > of subdevice to guest driver, while the same delegation might not be > > > required for implementing an userspace DMA interface (especially for > > > modern devices which support SVA). For example, idxd allows following > > > setting per wq (guest driver may configure them in any combination): > > > - put in dedicated or shared mode; > > > - enable/disable SVA; > > > - Associate guest-provided PASID to MSI/IMS entry; > > > - set threshold; > > > - allow/deny privileged access; > > > - allocate/free interrupt handle (enlightened for guest); > > > - collect error status; > > > - ... > > > > > > We plan to support idxd userspace DMA with SVA. The driver just needs > > > to prepare a wq with a predefined configuration (e.g. shared, SVA, > > > etc.), bind the process mm to IOMMU (non-nested) and then map > > > the portal to userspace. The goal that userspace can do DMA to > > > associated wq doesn't change the fact that the wq is still *owned* > > > and *controlled* by kernel driver. However as far as passthrough > > > is concerned, the wq is considered 'owned' by the guest driver thus > > > we need an interface which can support low-level *controllability* > > > from guest driver. It is sort of a mess in uAPI when mixing the > > > two together. > > > > > > Based on above two reasons, we see distinct requirements between > > > userspace DMA and passthrough interfaces, at least in idxd context > > > (though other devices may have less distinction in-between). Therefore, > > > we didn't see the value/necessity of reinventing the wheel that mdev > > > already handles well to evolve an simple application-oriented usespace > > > DMA interface to a complex guest-driver-oriented passthrough interface. > > > The complexity of doing so would incur far more kernel-side changes > > > than the portion of emulation code that you've been concerned about... > > > > > > > > > > > I don't think VFIO should be the only entry point to > > > > virtualization. If we say the universe of devices doing user space DMA > > > > must also implement a VFIO mdev to plug into virtualization then it > > > > will be alot of mdevs. > > > > > > Certainly VFIO will not be the only entry point. and This has to be a > > > case-by-case decision. If an userspace DMA interface can be easily > > > adapted to be a passthrough one, it might be the choice. But for idxd, > > > we see mdev a much better fit here, given the big difference between > > > what userspace DMA requires and what guest driver requires in this hw. > > > > > > > > > > > I would prefer to see that the existing userspace interface have the > > > > extra needed bits for virtualization (eg by having appropriate > > > > internal kernel APIs to make this easy) and all the emulation to build > > > > the synthetic PCI device be done in userspace. > > > > > > In the end what decides the direction is the amount of changes that > > > we have to put in kernel, not whether we call it 'emulation'. For idxd, > > > adding special passthrough requirements (guest SVA, dirty tracking, > > > etc.) and raw controllability to the simple userspace DMA interface > > > is for sure making kernel more complex than reusing the mdev > > > framework (plus some degree of emulation mockup behind). Not to > > > mention the merit of uAPI compatibility with mdev... > > > > I agree with a lot of this argument, exposing a device through a > > userspace interface versus allowing user access to a device through a > > userspace interface are different levels of abstraction and control. > > In an ideal world, perhaps we could compose one from the other, but I > > don't think the existence of one is proof that the other is redundant. > > That's not to say that mdev/vfio isn't ripe for abuse in this space, > > but I'm afraid the test for that abuse is probably much more subtle. > > > > I'll also remind folks that LPC is coming up in just a couple short > > weeks and this might be something we should discuss (virtually) > > in-person. uconf CfPs are currently open. Thanks, > > > > Yes, LPC is a good place to reach consensus. btw I saw there is > already one VFIO topic called "device assignment/sub-assignment". > Do you think whether this can be covered under that topic, or > makes more sense to be a new one? All the things listed in the CFP are only potential topics to get ideas flowing, there is currently no proposal to talk about sub-assignment. I'd suggest submitting separate topics for each and if we run into time constraints we can ask that they might be combined together. Thanks, Alex