[PATCH 3/5] iommu: Introduce the domain op enforce_cache_coherency()

[Tian, Kevin]

> From: Jason Gunthorpe <jgg at nvidia.com>
> Sent: Wednesday, April 6, 2022 12:16 AM
> 
> This new mechanism will replace using IOMMU_CAP_CACHE_COHERENCY
> and
> IOMMU_CACHE to control the no-snoop blocking behavior of the IOMMU.
> 
> Currently only Intel and AMD IOMMUs are known to support this
> feature. They both implement it as an IOPTE bit, that when set, will cause
> PCIe TLPs to that IOVA with the no-snoop bit set to be treated as though
> the no-snoop bit was clear.
> 
> The new API is triggered by calling enforce_cache_coherency() before
> mapping any IOVA to the domain which globally switches on no-snoop
> blocking. This allows other implementations that might block no-snoop
> globally and outside the IOPTE - AMD also documents such an HW capability.
> 
> Leave AMD out of sync with Intel and have it block no-snoop even for
> in-kernel users. This can be trivially resolved in a follow up patch.
> 
> Only VFIO will call this new API.

Is it too restrictive? In theory vdpa may also implement a contract with
KVM and then wants to call this new API too?