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 41hH3p5gtSzF21L for ; Fri, 3 Aug 2018 03:19:49 +1000 (AEST) Date: Thu, 2 Aug 2018 20:19:41 +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: <20180802200646-mutt-send-email-mst@kernel.org> References: <20180730125100-mutt-send-email-mst@kernel.org> <20180730111802.GA9830@infradead.org> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <82ccef6ec3d95ee43f3990a4a2d0aea87eb45e89.camel@kernel.crashing.org> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Aug 02, 2018 at 11:01:26AM -0500, Benjamin Herrenschmidt wrote: > On Thu, 2018-08-02 at 18:41 +0300, Michael S. Tsirkin wrote: > > > > > I don't completely agree: > > > > > > 1 - VIRTIO_F_IOMMU_PLATFORM is a property of the "other side", ie qemu > > > for example. It indicates that the peer bypasses the normal platform > > > iommu. The platform code in the guest has no real way to know that this > > > is the case, this is a specific "feature" of the qemu implementation. > > > > > > 2 - VIRTIO_F_PLATFORM_DMA (or whatever you want to call it), is a > > > property of the guest platform itself (not qemu), there's no way the > > > "peer" can advertize it via the virtio negociated flags. At least for > > > us. I don't know for sure whether that would be workable for the ARM > > > case. In our case, qemu has no idea at VM creation time that the VM > > > will turn itself into a secure VM and thus will require bounce > > > buffering for IOs (including virtio). > > > > > > So unless we have another hook for the arch code to set > > > VIRTIO_F_PLATFORM_DMA on selected (or all) virtio devices from the > > > guest itself, I don't see that as a way to deal with it. > > > > > > > The other issue is VIRTIO_F_IO_BARRIER > > > > which is very vaguely defined, and which needs a better definition. > > > > And last but not least we'll need some text explaining the challenges > > > > of hardware devices - I think VIRTIO_F_PLATFORM_DMA + VIRTIO_F_IO_BARRIER > > > > is what would basically cover them, but a good description including > > > > an explanation of why these matter. > > > > > > Ben. > > > > > > > So is it true that from qemu point of view there is nothing special > > going on? You pass in a PA, host writes there. > > Yes, qemu doesn't see a different. It's the guest that will bounce the > pages via a pool of "insecure" pages that qemu can access. Normal pages > in a secure VM come from PAs that qemu cannot physically access. > > Cheers, > Ben. > 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? -- MST