Re: s390-iommu.c default domain conversion

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 5/9/22 7:35 PM, Jason Gunthorpe wrote:
> Hi s390 folks/Matthew
> 
> Since everyone is looking at iommu support for the nested domains,
> could we also tackle the default domain conversion please? s390 is one
> of the last 4 drivers that need it.
> 
>  From what I can see it looks like when detach_dev() is called it
> expects the platform's dma_ops to work in arch/s390/pci/pci_dma.c ?

Yes

> 
> Has anyone thought about converting the dma_ops to use the normal DMA
> API iommu support and run it through the iommu driver instead of
> through the dma_ops?
> 
> Alternatively perhaps we can keep the dma_ops with some iommu
> side-change.

It has come up before.  So ultimately the goal is to be driving the dma 
through the default iommu domain (via dma-iommu) rather than directly in 
the dma_ops?  One of our main concerns is performance loss from s390-ism 
optimizations in the dma_ops like RPCIT avoidance / lazy map + global 
flush (maybe I am wrong or something can be generalized through the api 
- or something is already there that will work).  Of course there are 
some obvious benefits too, we have some duplication of work between 
s390-iommu.c and arch/s390/pci/pci_dma.c

I think the reality is that Niklas and I need to have a close look and 
do some testing on our end to see what it will take and if we can get 
acceptable performance from a conversion, then get back to you.

Thanks,
Matt