Re: Plan for /dev/ioasid RFC v2

[Alex Williamson]

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.  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.

> There is a certain niceness to what you are saying but it is not so
> practical without doing something bigger..
> 
> > Ideally vfio would also at least be able to register a type1 IOMMU
> > backend through the existing uapi, backed by this iommu code, ie. we'd
> > create a new "iommufd" (but without the user visible fd),   
> 
> It would be amazing to be able to achieve this, at least for me there
> are too many details be able to tell what that would look like
> exactly. I suggested once that putting the container ioctl interface
> in the drivers/iommu code may allow for this without too much trouble..

If we can't achieve this, then type1 legacy code is going to need to
live through an extended deprecation period.  I'm hoping that between
type1 and a native interface we'll have two paths into iommufd to vet
the design.  Thanks,

Alex