From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 41hMn06R9yzF2D1 for ; Fri, 3 Aug 2018 06:52:20 +1000 (AEST) 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 In-Reply-To: List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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