Re: [PATCH v2 1/4] iommu: Add private_data_owner to iommu_domain

[Baolu Lu <baolu.lu@linux.intel.com>]

On 2/28/25 09:31, Nicolin Chen wrote:
> Steal two bits from the 32-bit "type" in struct iommu_domain to store a
> new tag for private data owned by either dma-iommu or iommufd.
> 
> Set the domain->private_data_owner in dma-iommu and iommufd. These will
> be used to replace the sw_msi function pointer.
> 
> Suggested-by: Jason Gunthorpe<jgg@nvidia.com>
> Signed-off-by: Nicolin Chen<nicolinc@nvidia.com>
> ---
>   include/linux/iommu.h                | 7 ++++++-
>   drivers/iommu/dma-iommu.c            | 2 ++
>   drivers/iommu/iommufd/hw_pagetable.c | 3 +++
>   3 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index e93d2e918599..4f2774c08262 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -209,8 +209,13 @@ struct iommu_domain_geometry {
>   #define IOMMU_DOMAIN_PLATFORM	(__IOMMU_DOMAIN_PLATFORM)
>   #define IOMMU_DOMAIN_NESTED	(__IOMMU_DOMAIN_NESTED)
>   
> +#define IOMMU_DOMAIN_DATA_OWNER_NONE (0U)
> +#define IOMMU_DOMAIN_DATA_OWNER_DMA (1U)
> +#define IOMMU_DOMAIN_DATA_OWNER_IOMMUFD (2U)
> +
>   struct iommu_domain {
> -	unsigned type;
> +	u32 type : 30;
> +	u32 private_data_owner : 2;

Is there any special consideration for reserving only 2 bits for the
private data owner? Is it possible to allocate more bits so that it
could be more extensible for the future?

For example, current iommu core allows a kernel device driver to
allocate a paging domain and replace the default domain for kernel DMA.
This suggests the private data owner bits may be needed beyond iommu-dma
and iommufd.

Thanks,
baolu