Re: [PATCH v12 05/17] iommu: Add attach/detach_dev_pasid iommu interface

[Jason Gunthorpe]

On Fri, Aug 26, 2022 at 08:11:29PM +0800, Lu Baolu wrote:

> + * iommu_get_domain_for_dev_pasid() - Retrieve domain for @pasid of @dev
> + * @dev: the queried device
> + * @pasid: the pasid of the device
> + * @type: matched domain type, 0 for any match
> + *
> + * This is a variant of iommu_get_domain_for_dev(). It returns the existing
> + * domain attached to pasid of a device. It's only for internal use of the
> + * IOMMU subsystem. 

If it is only for external use then why is it exported?

I would add something like:

 Callers must hold a lock around this function, and both
 iommu_attach/detach_dev_pasid() whenever a domain of type is being
 manipulated. This API does not internally resolve races with
 attach/detach.

> + * detaching from the device PASID.
> + *
> + * Return: attached domain on success, NULL otherwise.
> + */
> +struct iommu_domain *iommu_get_domain_for_dev_pasid(struct device *dev,
> +						    ioasid_t pasid,
> +						    unsigned int type)
> +{
> +	struct iommu_domain *domain;
> +	struct iommu_group *group;
> +
> +	group = iommu_group_get(dev);
> +	if (!group)
> +		return NULL;
> +	/*
> +	 * The xarray protects its internal state with RCU. Hence the domain
> +	 * obtained is either NULL or fully formed.
> +	 */

This has nothing to do with RCU

xa_lock() is used to ensure that the domain pointer remains valid
while we check the type since it blocks concurrent xa_erase().

Jason