RE: [RFC] /dev/ioasid uAPI proposal

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Thursday, June 3, 2021 12:17 AM
>
[...] 
> > > If there are no hypervisor traps (does this exist?) then there is no
> > > way to involve the hypervisor here and the child IOASID should simply
> > > be a pointer to the guest's data structure that describes binding. In
> > > this case that IOASID should claim all PASIDs when bound to a
> > > RID.
> >
> > And in that case I think we should call that object something other
> > than an IOASID, since it represents multiple address spaces.
> 
> Maybe.. It is certainly a special case.
> 
> We can still consider it a single "address space" from the IOMMU
> perspective. What has happened is that the address table is not just a
> 64 bit IOVA, but an extended ~80 bit IOVA formed by "PASID, IOVA".

More accurately 64+20=84 bit IOVA 😊

> 
> If we are already going in the direction of having the IOASID specify
> the page table format and other details, specifying that the page

I'm leaning toward this direction now, after a discussion with Baolu.
He reminded me that a default domain is already created for each
device when it's probed by the iommu driver. So it looks workable
to expose a per-device capability query uAPI to user once a device
is bound to the ioasid fd. Once it's available, the user should be able
to judge what format/mode should be set when creating an IOASID.

> tabnle format is the 80 bit "PASID, IOVA" format is a fairly small
> step.

In concept this view is true. But when designing the uAPI possibly
we will not call it a 84bit format as the PASID table itself just
serves 20bit PASID space. 

Will think more how to mark it in the next version.

> 
> I wouldn't twist things into knots to create a difference, but if it
> is easy to do it wouldn't hurt either.
> 

Thanks
Kevin