Re: Plan for /dev/ioasid RFC v2

[Jason Gunthorpe]

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@xxxxxxxxxx> 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.

In any event a module dependency from vfio to iommu is not bothersome,
while the other way certainly is.

> Likewise iommufd shouldn't have any on vfio.  As much as you
> dislike the symbol_get hack of the kvm-vfio device, it would be
> reasonable for iommufd to reach for a vfio symbol when an
> IOMMU_BIND_VFIO_DEVICE_FD ioctl is called.

We'd have to add a special ioctl to iommu for every new subsystem, it
doesn't scale. iommu is a core subsystem, vfio is a driver subsystem.
The direction of dependency is clear, I think.

Jason