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
