Re: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Blue Swirl wrote:
>>>  I don't understand.  It's not a device that needs bouncing, it's a
>>> particular transfer.  This could be either due to the transfer 
>>> targeting
>>> mmio, or due to the transfer requiring a transformation.
>>>     
>>
>> Should the bouncing be something more much complex, for example
>> negotiated between the devices? Or maybe the devices set up a
>> transforming and non-transforming channel (which the other end should
>> be able to transform some more) and use them as needed?
>>   
>
> Yes.  We already have two cases:
>
> - may do partial request: useful for block storage where requests can 
> be huge
> - need full request: networking, where you can't send or receive half 
> a packet; on the other hand, packets are small (even with tso)
>
> You're adding a third case, always bounce, when the data undergoes a 
> transformation which can't happen in-place.

IMHO, IOMMU translation is distinct from mapping/data transformation.  I 
would think the IOMMU translation API would be different in that it took 
a physical address and returned a physical address.

The IOMMU DMA API (which could transform data potentially) should return 
a virtual address and data a physical address.  In the normal case 
(non-transforming IOMMU), it should just fall-through to CPU DMA API 
(which is just map/unmap).

The PCI DMA API would normally fall through to the IOMMU DMA API.

So we would have:

PCI DMA map(addr):
  if IOMMU:
     return IOMMU DMA map(addr)
  return CPU DMA map(addr)

IOMMU DMA map(addr):
  new_address = IOMMU translate(addr)
  if transform:
    return IOMMU byte-swap map(addr)
  return CPU DMA map(addr)

Regards,

Anthony Liguori