RE: Plan for /dev/ioasid RFC v2

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe
> Sent: Tuesday, June 29, 2021 7:13 AM
> 
> On Mon, Jun 28, 2021 at 05:09:02PM -0600, Alex Williamson wrote:
> > On Mon, 28 Jun 2021 19:48:18 -0300
> > Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > > On Mon, Jun 28, 2021 at 04:31:45PM -0600, Alex Williamson wrote:
> > >
> > > > I'd expect that /dev/iommu will be used by multiple subsystems.  All
> > > > will want to bind devices to address spaces, so shouldn't binding a
> > > > device to an iommufd be an ioctl on the iommufd, ie.
> > > > IOMMU_BIND_VFIO_DEVICE_FD.  Maybe we don't even need "VFIO" in
> there and
> > > > the iommufd code can figure it out internally.
> > >
> > > It wants to be the other way around because iommu_fd is the lower
> > > level subsystem. We don't/can't teach iommu_fd how to convert a fd
> > > number to a vfio/vdpa/etc/etc, we teach all the things building on
> > > iommu_fd how to change a fd number to an iommu - they already
> > > necessarily have an inter-module linkage.
> >
> > These seem like peer subsystems, like vfio and kvm.  vfio shouldn't
> > have any hard dependencies on the iommufd module, especially so long as
> > we have the legacy type1 code.
> 
> It does, the vfio_device implementation has to tell the iommu subsystem
> what kind of device behavior it has and possibly interact with the
> iommu subsystem with it in cases like PASID. This was outlined in part
> of the RFC.

Right. PASID is managed by specific device driver in this RFC and provided
as routing information to iommu_fd when the device is attached to an 
IOASID. Another point is about PASID virtualization (vPASID->pPASID),
which is established by having the user to register its vPASID when doing
the attach call. vfio device driver needs to use this information in the
mediation path. In concept vPASID is not relevant to iommu_fd which only 
cares about pPASID. Having vPASID registered via iommu_fd uAPI and 
then indirectly communicated to vfio device driver looks not a clean
way in the first place.

Thanks
Kevin