Re: [PATCH v3 0/3] virtio DMA API core stuff
From: Michael S. Tsirkin
Date: Fri Nov 20 2015 - 02:47:40 EST
On Fri, Nov 20, 2015 at 08:56:46AM +0200, Michael S. Tsirkin wrote:
> 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
Or Xen code within QEMU can tweak this global internally
so users don't need to care.
> > 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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