Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

[Jason Gunthorpe]

On Wed, Apr 28, 2021 at 07:47:56AM +0000, Tian, Kevin wrote:
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Wednesday, April 28, 2021 1:12 AM
> > 
> [...]
> > One option is VFIO can keep its group FD but nothing else will have
> > anthing like it. However I don't much like the idea that VFIO will
> > have a special and unique programming model to do that same things
> > other subsystem will do. That will make it harder for userspace to
> > implement.
> 
> Hi, Jason,
> 
> I have a question here. Based on discussions so far, it's clearly that the
> new ioasid uAPI will differ from existing VFIO uAPI a lot, e.g. ioasid-
> centric operations, no group fd, no incompatible domains, etc. Then 
> I wonder how we plan to support legacy VFIO applications in this 
> transition phase. 

I suspect the VFIO group fd will have to be registered with
/dev/ioasid in addition to each device if we are to retain the same
model.

> Earlier you ever mentioned the desire of directly replacing
> /dev/vfio/vfio with /dev/ioasid and having ioasid to present both
> VFIO and new uAPI. Doesn't it imply that we have to copy the VFIO
> container/group semantics into /dev/ioasid although it's a special
> programming model only for VFIO?

I gave that as a something to think about, if it doesn't work out then
it is just a bad idea to discard.

> Alternatively we could keep all the container/group legacy within VFIO
> and having /dev/ioasid support only the new uAPI semantics. In this case
> VFIO will include a shim iommu backend to connect its legacy uAPI into 
> drivers/ioasid backend functions for backward compatibility. Then VFIO
> will also support a new model which only uses its device uAPI to bind
> to new ioasid fd w/o using any legacy container/group/iommu uAPI.
> Does this sound a plan? 

It may be where we end up.. Though I fear it will make it overly
complex inside VFIO to access the new stuff. It would be very nice if
we could see a path where VFIO insides could only deal with the
in-kernel ioasid handles, whatever they are.

Jason