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

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Message-ID: <20251015115044.GE3938986@ziepe.ca>
Date: Wed, 15 Oct 2025 08:50:44 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	Jeremy Linton <jeremy.linton@....com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Dan Williams <dan.j.williams@...el.com>, linux-coco@...ts.linux.dev,
	kvmarm@...ts.linux.dev, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, aik@....com, lukas@...ner.de,
	Samuel Ortiz <sameo@...osinc.com>,
	Xu Yilun <yilun.xu@...ux.intel.com>,
	Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [RFC PATCH v1 11/38] KVM: arm64: CCA: register host tsm platform
 device

On Wed, Oct 15, 2025 at 11:58:25AM +0200, Greg KH wrote:
> On Wed, Oct 15, 2025 at 03:22:28PM +0530, Aneesh Kumar K.V wrote:
> > Jason Gunthorpe <jgg@...pe.ca> writes:
> > 
> > > 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.
> > >
> > 
> > IIUC, auxiliary_device needs a parent device, and the documentation
> > explains that it’s intended for cases where a large driver is split into
> > multiple dependent smaller ones.

Which is the case here, you have a SMC interface that you want to
fracture into multiple subsystems.

> > If we want to use auxiliary_device for this case, what would serve as
> > the parent device?

You probably need to make a platform device for the discovered PSCI
interface from the firmware. Looks like DT will already have one, ACPI
could invent one..
 
> The real device that has the resources you wish to share access to.  Are
> there physical resources here you are sharing?  If so, that device is
> the parent.  If there is no such thing, then just make a bunch of faux
> devices and be done with it :)

At the very bottom of the stack it looks like the PSCI interface is
discovered first through DT/ACPI. The PSCI interface has RPCs that are
then used to discover if SMC/etc/etc are present and along the way it
makes platform devices to plug in subsystems to it based on what it
can discover.

It is just not sharing "resources" in the traditional sense, PSCI has
no registers or interrupts, yet it is a service provided by the
platform firmare.

Again faux devices don't serve the need here to load modules and do
driver binding.

Jason