Re: [PATCH v3 0/3] virtio DMA API core stuff

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

On Thu, Nov 19, 2015 at 01:59:05PM -0800, Andy Lutomirski wrote:
> On Nov 19, 2015 5:45 AM, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >
> > On Tue, Oct 27, 2015 at 11:38:57PM -0700, Andy Lutomirski wrote:
> > > This switches virtio to use the DMA API unconditionally.  I'm sure
> > > it breaks things, but it seems to work on x86 using virtio-pci, with
> > > and without Xen, and using both the modern 1.0 variant and the
> > > legacy variant.
> >
> > So thinking hard about it, I don't see any real drawbacks to making this
> > conditional on a new feature bit, that Xen can then set..
> 
> Can you elaborate?  If I run QEMU, hosting Xen, hosting Linux, and the
> virtio device is provided by QEMU, then how does Xen set the bit?

You would run QEMU with the appropriate flag. E.g.
-global virtio-pci,use_platform_dma=on

> Similarly, how would Xen set the bit for a real physical device?
> 
> 
> --Andy

There's no need to set bits for physical devices I think: from security
point of view, using them from a VM isn't very different from using them
from host.



-- 
MST