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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=ham 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 16A94C43142 for ; Thu, 2 Aug 2018 20:52:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id BFDCD2154D for ; Thu, 2 Aug 2018 20:52:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BFDCD2154D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731952AbeHBWpI (ORCPT ); Thu, 2 Aug 2018 18:45:08 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:48532 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730951AbeHBWpH (ORCPT ); Thu, 2 Aug 2018 18:45:07 -0400 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id EC7C140241F2; Thu, 2 Aug 2018 20:52:16 +0000 (UTC) Received: from redhat.com (ovpn-117-57.ams2.redhat.com [10.36.117.57]) by smtp.corp.redhat.com (Postfix) with SMTP id 7B722215670D; Thu, 2 Aug 2018 20:52:11 +0000 (UTC) Date: Thu, 2 Aug 2018 23:52:10 +0300 From: "Michael S. Tsirkin" To: Benjamin Herrenschmidt Cc: Christoph Hellwig , Will Deacon , Anshuman Khandual , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, aik@ozlabs.ru, robh@kernel.org, joe@perches.com, elfring@users.sourceforge.net, david@gibson.dropbear.id.au, jasowang@redhat.com, mpe@ellerman.id.au, linuxram@us.ibm.com, haren@linux.vnet.ibm.com, paulus@samba.org, srikar@linux.vnet.ibm.com, robin.murphy@arm.com, jean-philippe.brucker@arm.com, marc.zyngier@arm.com Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Message-ID: <20180802225738-mutt-send-email-mst@kernel.org> References: <20180730155633-mutt-send-email-mst@kernel.org> <20180731173052.GA17153@infradead.org> <3d6e81511571260de1c8047aaffa8ac4df093d2e.camel@kernel.crashing.org> <20180801081637.GA14438@arm.com> <20180801083639.GF26378@infradead.org> <26c1d3d50d8e081eed44fe9940fbefed34598cbd.camel@kernel.crashing.org> <20180802182959-mutt-send-email-mst@kernel.org> <82ccef6ec3d95ee43f3990a4a2d0aea87eb45e89.camel@kernel.crashing.org> <20180802200646-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.78 on 10.11.54.6 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Thu, 02 Aug 2018 20:52:17 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.7]); Thu, 02 Aug 2018 20:52:17 +0000 (UTC) for IP:'10.11.54.6' DOMAIN:'int-mx06.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'mst@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 02, 2018 at 12:53:41PM -0500, Benjamin Herrenschmidt wrote: > On Thu, 2018-08-02 at 20:19 +0300, Michael S. Tsirkin wrote: > > > > I see. So yes, given that device does not know or care, using > > virtio features is an awkward fit. > > > > So let's say as a quick fix for you maybe we could generalize the > > xen_domain hack, instead of just checking xen_domain check some static > > branch. Then teach xen and others to enable that.> > > > OK but problem then becomes this: if you do this and virtio device appears > > behind a vIOMMU and it does not advertize the IOMMU flag, the > > code will try to use the vIOMMU mappings and fail. > > > > It does look like even with trick above, you need a special version of > > DMA ops that does just swiotlb but not any of the other things DMA API > > might do. > > > > Thoughts? > > Yes, this is the purpose of Anshuman original patch (I haven't looked > at the details of the patch in a while but that's what I told him to > implement ;-) : > > - Make virtio always use DMA ops to simplify the code path (with a set > of "transparent" ops for legacy) > > and > > - Provide an arch hook allowing us to "override" those "transparent" > DMA ops with some custom ones that do the appropriate swiotlb gunk. > > Cheers, > Ben. > Right but as I tried to say doing that brings us to a bunch of issues with using DMA APIs in virtio. Put simply DMA APIs weren't designed for guest to hypervisor communication. When we do (as is the case with PLATFORM_IOMMU right now) this adds a bunch of overhead which we need to get rid of if we are to switch to PLATFORM_IOMMU by default. We need to fix that. -- MST From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Date: Thu, 2 Aug 2018 23:52:10 +0300 Message-ID: <20180802225738-mutt-send-email-mst@kernel.org> References: <20180730155633-mutt-send-email-mst@kernel.org> <20180731173052.GA17153@infradead.org> <3d6e81511571260de1c8047aaffa8ac4df093d2e.camel@kernel.crashing.org> <20180801081637.GA14438@arm.com> <20180801083639.GF26378@infradead.org> <26c1d3d50d8e081eed44fe9940fbefed34598cbd.camel@kernel.crashing.org> <20180802182959-mutt-send-email-mst@kernel.org> <82ccef6ec3d95ee43f3990a4a2d0aea87eb45e89.camel@kernel.crashing.org> <20180802200646-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Benjamin Herrenschmidt Cc: robh@kernel.org, srikar@linux.vnet.ibm.com, mpe@ellerman.id.au, Will Deacon , linux-kernel@vger.kernel.org, linuxram@us.ibm.com, virtualization@lists.linux-foundation.org, Christoph Hellwig , paulus@samba.org, marc.zyngier@arm.com, joe@perches.com, robin.murphy@arm.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com, Anshuman Khandual List-Id: virtualization@lists.linuxfoundation.org On Thu, Aug 02, 2018 at 12:53:41PM -0500, Benjamin Herrenschmidt wrote: > On Thu, 2018-08-02 at 20:19 +0300, Michael S. Tsirkin wrote: > > > > I see. So yes, given that device does not know or care, using > > virtio features is an awkward fit. > > > > So let's say as a quick fix for you maybe we could generalize the > > xen_domain hack, instead of just checking xen_domain check some static > > branch. Then teach xen and others to enable that.> > > > OK but problem then becomes this: if you do this and virtio device appears > > behind a vIOMMU and it does not advertize the IOMMU flag, the > > code will try to use the vIOMMU mappings and fail. > > > > It does look like even with trick above, you need a special version of > > DMA ops that does just swiotlb but not any of the other things DMA API > > might do. > > > > Thoughts? > > Yes, this is the purpose of Anshuman original patch (I haven't looked > at the details of the patch in a while but that's what I told him to > implement ;-) : > > - Make virtio always use DMA ops to simplify the code path (with a set > of "transparent" ops for legacy) > > and > > - Provide an arch hook allowing us to "override" those "transparent" > DMA ops with some custom ones that do the appropriate swiotlb gunk. > > Cheers, > Ben. > Right but as I tried to say doing that brings us to a bunch of issues with using DMA APIs in virtio. Put simply DMA APIs weren't designed for guest to hypervisor communication. When we do (as is the case with PLATFORM_IOMMU right now) this adds a bunch of overhead which we need to get rid of if we are to switch to PLATFORM_IOMMU by default. We need to fix that. -- MST