RE: [PATCH v2 1/4] iommu: Introduce the domain op enforce_cache_coherency()

Tian, Kevin
2022-04-08T08:05:52Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, April 7, 2022 11:24 PM
>
> 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 a 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.

I still didn't see the point of mandating a caller for a new API (and as
you pointed out iommufd will call it too).

[...]
> diff --git a/include/linux/intel-iommu.h b/include/linux/intel-iommu.h
> index 2f9891cb3d0014..1f930c0c225d94 100644
> --- a/include/linux/intel-iommu.h
> +++ b/include/linux/intel-iommu.h
> @@ -540,6 +540,7 @@ struct dmar_domain {
> u8 has_iotlb_device: 1;
> u8 iommu_coherency: 1; /* indicate coherency of
> iommu access */
> u8 iommu_snooping: 1; /* indicate snooping control
> feature */
> + u8 enforce_no_snoop : 1; /* Create IOPTEs with snoop control */

it reads like no_snoop is the result of the enforcement... Probably
force_snooping better matches the intention here.

Thanks
Kevin