Re: [Qemu-devel] [PATCH 13/13] pseries: Implement IOMMU and DMA for PAPR PCI devices

[Benjamin Herrenschmidt]

On Fri, 2012-03-09 at 21:58 +1100, David Gibson wrote:
> > What about virtio?
> 
> virtio doesn't use virtualized PCI DMA, it uses direct hypervisor
> access to guest memory, by guest physical address.  It *shouldn't*,
> but it does - that's the way it's specced and that's the way the guest
> kernel expects it to be.  It could be fixed with a new feature bit,
> but that's a project for another day.

More precisely, the patch don't break virtio as virtio just bypasses
this all.

Also having virtio go through the iommu might not be such a great idea,
it should definitely remain optional. The ability of virtio to go
straight to guest memory has some significant performance advantages.

Cheers,
Ben.