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

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

On Mon, Nov 23, 2015 at 10:36:45AM +0100, Cornelia Huck wrote:
> 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.

I agree it's not something that needs to be negotiated.

But given that we are changing device and driver behaviour in a drastic
way, it seems prudent to have a way for guest and host to discover that,
even if it's just in case.

Whether it's a feature bit or something else pci-specific,
depends on whether this makes sense for any other transport.

> > - 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?

Is there a reason why iommu does not make any sense for ccw or mmio?


> > 
> > 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).