From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:45507) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1asc4k-000899-4Y for qemu-devel@nongnu.org; Tue, 19 Apr 2016 16:16:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1asc4j-0005bi-5n for qemu-devel@nongnu.org; Tue, 19 Apr 2016 16:16:46 -0400 Date: Tue, 19 Apr 2016 23:16:32 +0300 From: "Michael S. Tsirkin" Message-ID: <20160419231437-mutt-send-email-mst@redhat.com> References: <1461004173.3765.73.camel@infradead.org> <20160419130732-mutt-send-email-mst@redhat.com> <20160419190520-mutt-send-email-mst@redhat.com> <20160419191914-mutt-send-email-mst@redhat.com> <1461083204.20056.8.camel@infradead.org> <20160419204907-mutt-send-email-mst@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Andy Lutomirski Cc: David Woodhouse , "qemu-devel@nongnu.org Developers" , "linux-kernel@vger.kernel.org" , Paolo Bonzini , peterx@redhat.com, Cornelia Huck , Stefan Hajnoczi , Kevin Wolf , Amit Shah , qemu-block@nongnu.org, Jason Wang , Alex Williamson , Andy Lutomirski , Christian Borntraeger , Wei Liu , Linux Virtualization , kvm list On Tue, Apr 19, 2016 at 11:01:38AM -0700, Andy Lutomirski wrote: > On Tue, Apr 19, 2016 at 10:49 AM, Michael S. Tsirkin wrote: > > On Tue, Apr 19, 2016 at 12:26:44PM -0400, David Woodhouse wrote: > >> On Tue, 2016-04-19 at 19:20 +0300, Michael S. Tsirkin wrote: > >> > > >> > > I thought that PLATFORM served that purpose. Woudn't the host > >> > > advertise PLATFORM support and, if the guest doesn't ack it, the host > >> > > device would skip translation? Or is that problematic for vfio? > >> > > >> > Exactly that's problematic for security. > >> > You can't allow guest driver to decide whether device skips security. > >> > >> Right. Because fundamentally, this *isn't* a property of the endpoint > >> device, and doesn't live in virtio itself. > >> > >> It's a property of the platform IOMMU, and lives there. > > > > It's a property of the hypervisor virtio implementation, and lives there. > > It is now, but QEMU could, in principle, change the way it thinks > about it so that virtio devices would use the QEMU DMA API but ask > QEMU to pass everything through 1:1. This would be entirely invisible > to guests but would make it be a property of the IOMMU implementation. > At that point, maybe QEMU could find a (platform dependent) way to > tell the guest what's going on. > > FWIW, as far as I can tell, PPC and SPARC really could, in principle, > set up 1:1 mappings in the guest so that the virtio devices would work > regardless of whether QEMU is ignoring the IOMMU or not -- I think the > only obstacle is that the PPC and SPARC 1:1 mappings are currectly set > up with an offset. I don't know too much about those platforms, but > presumably the layout could be changed so that 1:1 really was 1:1. > > --Andy Sure. Do you see any reason why the decision to do this can't be keyed off the virtio feature bit? -- MST