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

[David Gibson]

On Fri, Mar 09, 2012 at 11:23:58AM +0100, Paolo Bonzini wrote:
> Il 09/03/2012 06:01, David Gibson ha scritto:
> > Currently the pseries machine emulation does not support DMA for emulated
> > PCI devices, because the PAPR spec always requires a (guest visible,
> > paravirtualized) IOMMU which was not implemented.  Now that we have
> > infrastructure for IOMMU emulation, we can correct this and allow PCI DMA
> > for pseries.
> > 
> > With the existing PAPR IOMMU code used for VIO devices, this is almost
> > trivial. We use a single DMAContext for each (virtual) PCI host bridge,
> > which is the usual configuration on real PAPR machines (which often have
> > _many_ PCI host bridges).
> 
> 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.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson