From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Date: Thu, 02 Aug 2018 11:01:26 -0500 Message-ID: <82ccef6ec3d95ee43f3990a4a2d0aea87eb45e89.camel@kernel.crashing.org> References: <20180727095804.GA25592@arm.com> <20180730093414.GD26245@infradead.org> <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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180802182959-mutt-send-email-mst@kernel.org> 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: "Michael S. Tsirkin" 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, 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.