Re: [Qemu-devel] [RFC] virtio: convert to use DMA api

[Cornelia Huck <cornelia.huck@de.ibm.com>]

On Mon, 23 Nov 2015 15:41:11 +0800
Jason Wang <jasowang@redhat.com> wrote:

> Currently, all virtio devices bypass IOMMU completely. This is because
> address_space_memory is assumed and used during DMA emulation. This
> patch converts the virtio core API to use DMA API. This idea is
> 
> - introducing a new transport specific helper to query the dma address
>   space. (only pci version is implemented).
> - query and use this address space during virtio device guest memory
>   accessing
> 
> With this virtiodevices will not bypass IOMMU anymore. Little tested with
> intel_iommu=on with virtio guest DMA series posted in
> https://lkml.org/lkml/2015/10/28/64.
> 
> TODO:
> - Feature bit for this

I'm still not convinced about that feature bit stuff. It just feels
wrong to use a mechanism that conveys negotiable device features to
configure what is basically a platform/hypervisor feature. I'd rather
see this out of the virtio layer and into the pci layer.

> - Implement this for all transports

Is it OK to just keep a fallback to today's implementation for
transports for which the iommu concept doesn't make sense and that will
always have an identity mapping?

> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  hw/block/virtio-blk.c             |  2 +-
>  hw/char/virtio-serial-bus.c       |  2 +-
>  hw/scsi/virtio-scsi.c             |  2 +-
>  hw/virtio/virtio-pci.c            |  9 +++++++++
>  hw/virtio/virtio.c                | 36 +++++++++++++++++++--------------
>  include/hw/virtio/virtio-access.h | 42 +++++++++++++++++++++++++++++----------
>  include/hw/virtio/virtio-bus.h    |  1 +
>  include/hw/virtio/virtio.h        |  2 +-
>  8 files changed, 67 insertions(+), 29 deletions(-)

FWIW, this doesn't seem to break for the s390-ccw-virtio machine (only
snifftested).