[RFC] virtio: Use DMA MAP API for devices without an IOMMU

[Balbir Singh]

On Thu, Apr 5, 2018 at 8:56 PM, Anshuman Khandual
<khandual at linux.vnet.ibm.com> wrote:
> There are certian platforms which would like to use SWIOTLB based DMA API
> for bouncing purpose without actually requiring an IOMMU back end. But the
> virtio core does not allow such mechanism. Right now DMA MAP API is only
> selected for devices which have an IOMMU and then the QEMU/host back end
> will process all incoming SG buffer addresses as IOVA instead of simple
> GPA which is the case for simple bounce buffers after being processed with
> SWIOTLB API. To enable this usage, it introduces an architecture specific
> function which will just make virtio core front end select DMA operations
> structure.
>
> Signed-off-by: Anshuman Khandual <khandual at linux.vnet.ibm.com>
> ---
> This RFC is just to get some feedback. Please ignore the function call
> back into the architecture. It can be worked out properly later on. But
> the question is can we have virtio devices in the guest which would like
> to use SWIOTLB based (or any custom DMA API based) bounce buffering with
> out actually being an IOMMU devices emulated by QEMU/host as been with
> the current VIRTIO_F_IOMMU_PLATFORM virtio flag ?
>
>  arch/powerpc/platforms/pseries/iommu.c | 6 ++++++
>  drivers/virtio/virtio_ring.c           | 4 ++++
>  include/linux/virtio.h                 | 2 ++
>  3 files changed, 12 insertions(+)
>
> diff --git a/arch/powerpc/platforms/pseries/iommu.c b/arch/powerpc/platforms/pseries/iommu.c
> index 06f02960b439..dd15fbddbe89 100644
> --- a/arch/powerpc/platforms/pseries/iommu.c
> +++ b/arch/powerpc/platforms/pseries/iommu.c
> @@ -1396,3 +1396,9 @@ static int __init disable_multitce(char *str)
>  __setup("multitce=", disable_multitce);
>
>  machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init);
> +
> +bool is_virtio_dma_platform(void)
> +{
> +       return true;
> +}
> +EXPORT_SYMBOL(is_virtio_dma_platform);
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 71458f493cf8..9f205a79d378 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -144,6 +144,10 @@ struct vring_virtqueue {
>
>  static bool vring_use_dma_api(struct virtio_device *vdev)
>  {
> +       /* Use DMA API even for virtio devices without an IOMMU */
> +       if (is_virtio_dma_platform())
> +               return true;
> +
>         if (!virtio_has_iommu_quirk(vdev))
>                 return true;
>
> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> index 988c7355bc22..d8bb83d753ea 100644
> --- a/include/linux/virtio.h
> +++ b/include/linux/virtio.h
> @@ -200,6 +200,8 @@ static inline struct virtio_driver *drv_to_virtio(struct device_driver *drv)
>  int register_virtio_driver(struct virtio_driver *drv);
>  void unregister_virtio_driver(struct virtio_driver *drv);
>
> +extern bool is_virtio_dma_platform(void);
> +

Where is the default implementation for non-pseries platforms? Will they compile
after these changes?

Balbir