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    SubjectRe: [PATCH RFC] fixup! virtio: convert to use DMA api
    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 <mst@redhat.com> 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

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