Re: [PATCH v4 1/9] iommu: Introduce domain attachment handle

[Jason Gunthorpe]

On Wed, Apr 03, 2024 at 09:15:11AM +0800, Lu Baolu wrote:
> Currently, when attaching a domain to a device or its PASID, domain is
> stored within the iommu group. It could be retrieved for use during the
> window between attachment and detachment.
> 
> With new features introduced, there's a need to store more information
> than just a domain pointer. This information essentially represents the
> association between a domain and a device. For example, the SVA code
> already has a custom struct iommu_sva which represents a bond between
> sva domain and a PASID of a device. Looking forward, the IOMMUFD needs
> a place to store the iommufd_device pointer in the core, so that the
> device object ID could be quickly retrieved in the critical fault handling
> path.
> 
> Introduce domain attachment handle that explicitly represents the
> attachment relationship between a domain and a device or its PASID.
> A caller-specific data field can be used by the caller to store additional
> information beyond a domain pointer, depending on its specific use case.
> 
> Co-developed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> ---
>  drivers/iommu/iommu-priv.h |   9 +++
>  drivers/iommu/iommu.c      | 158 +++++++++++++++++++++++++++++++++----
>  2 files changed, 153 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iommu/iommu-priv.h b/drivers/iommu/iommu-priv.h
> index 5f731d994803..08c0667cef54 100644
> --- a/drivers/iommu/iommu-priv.h
> +++ b/drivers/iommu/iommu-priv.h
> @@ -28,4 +28,13 @@ void iommu_device_unregister_bus(struct iommu_device *iommu,
>  				 const struct bus_type *bus,
>  				 struct notifier_block *nb);
>  
> +struct iommu_attach_handle {
> +	struct iommu_domain		*domain;
> +	refcount_t			users;

I don't understand how the refcounting can be generally useful. There
is no way to free this:

> +	void				*priv;

When the refcount goes to zero.

Jason