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

[Jason Wang <jasowang@redhat.com>]

On 11/23/2015 05:36 PM, 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.
>
>> - 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?

Yes it is. This patch keeps this fallback (e.g using
address_space_memory) for the transport without an iommu implementation.

>
>> 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).
>

Good to know this. Thanks for the testing.