Re: [PATCH RFC 00/19] IOMMUFD Dirty Tracking

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, May 05, 2022 at 07:40:37AM +0000, Tian, Kevin wrote:
 
> In concept this is an iommu property instead of a domain property.

Not really, domains shouldn't be changing behaviors once they are
created. If a domain supports dirty tracking and I attach a new device
then it still must support dirty tracking.

I suppose we may need something here because we need to control when
domains are re-used if they don't have the right properties in case
the system iommu's are discontiguous somehow.

ie iommufd should be able to assert that dirty tracking is desired and
an existing non-dirty tracking capable domain will not be
automatically re-used.

We don't really have the right infrastructure to do this currently.

> From this angle IMHO it's more reasonable to report this IOMMU
> property to userspace via a device capability. If all devices attached
> to a hwpt claim IOMMU dirty tracking capability, the user can call
> set_dirty_tracking() on the hwpt object. 

Inherent domain properties need to be immutable or, at least one-way,
like enforced coherent, or it just all stops making any kind of sense.

> Once dirty tracking is enabled on a hwpt, further attaching a device
> which doesn't claim this capability is simply rejected.

It would be OK to do as enforced coherent does as flip a domain
permanently into dirty-tracking enabled, or specify a flag at domain
creation time.

Jason