Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform device

Jason Gunthorpe
2025-10-10T13:59:25Z

On Fri, Oct 10, 2025 at 07:10:58AM -0500, Jeremy Linton wrote:
> > Yes, use faux_device if you need/want a struct device to represent
> > something in the tree and it does NOT have any real platform resources
> > behind it. That's explicitly what it was designed for.
>
> Right, but this code is intended to trigger the kmod/userspace module
> loader.

Faux devices are not intended to be bound, it says so right on the label:

* A "simple" faux bus that allows devices to be created and added
* automatically to it. This is to be used whenever you need to create a
* device that is not associated with any "real" system resources, and do
* not want to have to deal with a bus/driver binding logic. It is
^^^^^^^^^^^^^^^^^^^^^^^^^^
* intended to be very simple, with only a create and a destroy function
* available.

auxiliary_device is quite similar to faux except it is intended to be
bound to drivers, supports module autoloading and so on.

What you have here is the platform firmware provides the ARM SMC
(Secure Monitor Call Calling Convention) interface which is a generic
function call multiplexer between the OS and ARM firmware.

Then we have things like the TSM subsystem that want to load a driver
to use calls over SMC if the underlying platform firmware supports the
RSI group of SMC APIs. You'd have a TSM subsystem driver that uses the
RSI call group over SMC that autobinds when the RSI call group is
detected when the SMC is first discovered.

So you could use auxiliary_device, you'd consider SMC itself to be the
shared HW block and all the auxiliary drivers are per-subsystem
aspects of that shared SMC interface. It is not a terrible fit for
what it was intended for at least.

Jason