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

[Anshuman Khandual]

On 04/05/2018 04:44 PM, Balbir Singh wrote:
> 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?

No they wont. This is just a RFC asking for suggestion/feedback on a
particular direction, will clean up the code later on once we agree
on this.