Re: [RFC 7/11] virtio_pci: new, capability-aware driver.

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

On Wed, Jan 11, 2012 at 05:21:53PM +0000, Stefan Hajnoczi wrote:
> On Wed, Jan 11, 2012 at 3:39 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Wed, Jan 11, 2012 at 02:28:48PM +0000, Stefan Hajnoczi wrote:
> >> On Wed, Jan 11, 2012 at 9:10 AM, Benjamin Herrenschmidt
> >> <benh@kernel.crashing.org> wrote:
> >> > On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote:
> >> >>
> >> >> This is also an opportunity to stop using CPU physical addresses in
> >> >> the ring and instead perform DMA like a normal PCI device (use bus
> >> >> addresses).
> >> >
> >> > Euh why ?
> >>
> >> Because it's a paravirt hack that ends up hitting corner cases.  It's
> >> not possible to do virtio-pci passthrough under nested virtualization
> >> unless we use an IOMMU.  Imagine passing virtio-net from L0 into the
> >> L2 guest (i.e. PCI-passthrough).  If virtio-pci is really "PCI" this
> >> should be possible but it's not when we use physical addresses instead
> >> of bus addresses.
> >>
> >> Stefan
> >
> > It won't be hard to show siginificant performance regression if
> > we do this. Hard to justify for something as niche as nested virt.
> 
> For x86 this should be mostly a nop.

Maybe it should, but AFAIK it isn't.

> For ppc and SPARC architectures maybe you're right.  I still think
> it's a design flaw because if virtio v2 doesn't use bus addresses then
> it will simply not be possible to do passthrough for nested virt and
> other cases we haven't hit yet.
> 
> Stefan

virtio-pci does not implement things like SRIOV or FLR so it
won't work anyway.

If we ever fix this, and if we really want to pass through a virtio
device (why?) using an emulated iommu seems silly - we probably want a
PV IOMMU as well.

-- 
MST