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

From: Michael S. Tsirkin
Date: Fri Nov 20 2015 - 01:57:13 EST

On Thu, Nov 19, 2015 at 01:59:05PM -0800, Andy Lutomirski wrote:
> On Nov 19, 2015 5:45 AM, "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.

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