Re: [PATCH v6 01/14] iommu: Introduce a replace API for device pasid

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jan 09, 2025 at 07:20:01AM +0000, Tian, Kevin wrote:
> > From: Tian, Kevin
> > Sent: Thursday, January 9, 2025 3:08 PM
> > 
> > > From: Liu, Yi L <yi.l.liu@intel.com>
> > > Sent: Thursday, December 19, 2024 9:28 PM
> > >
> > > +/**
> > > + * iommu_replace_device_pasid - Replace the domain that a pasid is
> > > attached to
> > > + * @domain: the new iommu domain
> > > + * @dev: the attached device.
> > > + * @pasid: the pasid of the device.
> > > + * @handle: the attach handle.
> > > + *
> > > + * This API allows the pasid to switch domains. Return 0 on success, or an
> > > + * error. The pasid will keep the old configuration if replacement failed.
> > > + * This is supposed to be used by iommufd, and iommufd can guarantee
> > > that
> > > + * both iommu_attach_device_pasid() and iommu_replace_device_pasid()
> > > would
> > > + * pass in a valid @handle.
> > > + */
> > 
> > Better explain why a valid handle is required here.
> 
> Okay, it's because __iommu_set_group_pasid() requires the old domain now
> and the only way to retrieve it at this point is via a handle. It's probably also
> ok to directly store a domain pointer to the xarray when the handle is missing
> but that sounds more confusing.

I had shared a xarray approach to do that at one point, it apparently
was scary enough nobody picked it up :)

https://lore.kernel.org/linux-iommu/20240322165927.GG66976@ziepe.ca/

Jason