Re: [RFC 10/20] iommu/iommufd: Add IOMMU_DEVICE_GET_INFO

[Jason Gunthorpe]

On Thu, Sep 23, 2021 at 11:15:24AM +0100, Jean-Philippe Brucker wrote:

> So we can only tell userspace "No_snoop is not supported" (provided we
> even want to allow them to enable No_snoop). Users in control of stage-1
> tables can create non-cacheable mappings through MAIR attributes.

My point is that ARM is using IOMMU_CACHE to control the overall
cachability of the DMA 

ie not specifying IOMMU_CACHE requires using the arch specific DMA
cache flushers. 

Intel never uses arch specifc DMA cache flushers, and instead is
abusing IOMMU_CACHE to mean IOMMU_BLOCK_NO_SNOOP on DMA that is always
cachable.

These are different things and need different bits. Since the ARM path
has a lot more code supporting it, I'd suggest Intel should change
their code to use IOMMU_BLOCK_NO_SNOOP and abandon IOMMU_CACHE.

Which clarifies what to do here as uAPI - these things need to have
different bits and Intel's should still have NO SNOOP in the
name. What the no-snoop bit is called on other busses can be clarified
in comments if that case ever arises.

Jason