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

Tian, Kevin
2021-04-28T07:48:03Z

> From: Jason Gunthorpe <jgg@nvidia.com>
> 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. 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?

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?

Thanks
Kevin