Re: [Qemu-devel] [PATCH RFC] fixup! virtio: convert to use DMA api

["Michael S. Tsirkin" <mst@redhat.com>]

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