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

[Jeremy Linton]

Hi,

On 10/10/25 7:38 AM, Greg KH wrote:
> On Fri, Oct 10, 2025 at 07:10:58AM -0500, Jeremy Linton wrote:
> > Hi,
> >
> >
> > On 7/30/25 8:07 AM, Greg KH wrote:
> > > On Wed, Jul 30, 2025 at 01:23:33PM +0100, Jonathan Cameron wrote:
> > > > On Wed, 30 Jul 2025 11:38:27 +0100
> > > > Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote:
> > > >
> > > > > On Wed, 30 Jul 2025 14:12:26 +0530
> > > > > "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxx> wrote:
> > > > >
> > > > > > Jason Gunthorpe <jgg@xxxxxxxx> writes:
> > > > > > > On Tue, Jul 29, 2025 at 06:10:45PM +0100, Jonathan Cameron wrote:
> > > > > > > > > +static struct platform_device cca_host_dev = {
> > > > > > > > Hmm. Greg is getting increasingly (and correctly in my view) grumpy with
> > > > > > > > platform devices being registered with no underlying resources etc as glue
> > > > > > > > layers.  Maybe some of that will come later.
> > > > > > >
> > > > > > > Is faux_device a better choice? I admit to not knowing entirely what
> > > > > > > it is for..
> > > > >
> > > > > I'll go with a cautious yes to faux_device. This case of a glue device
> > > > > with no resources and no reason to be on a particular bus was definitely
> > > > > the intent but I'm not 100% sure without trying it that we don't run
> > > > > into any problems.
> > > > >
> > > > > Not that many examples yet, but cpuidle-psci.c looks like a vaguely similar
> > > > > case to this one.
> > > > >
> > > > > All it really does is move the location of the device and
> > > > > smash together the device registration with probe/remove.
> > > > > That means the device disappears if probe() fails, which is cleaner
> > > > > in many ways than leaving a pointless stub behind.
> > > > >
> > > > > Maybe it isn't appropriate it if is actually useful to rmmod/modprobe the
> > > > > driver.
> > > > >
> > > > > +CC Greg on basis I may have wrong end of the stick ;)
> > > > This time with at least one less typo in Greg's email address.
> > >
> > > 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.
>
> Why?

Originally it was because without the tsm drivers loaded there wasn't 
any way for a CCA guest to know it is running in a confidential compute 
environment. That is a bit of a problem for generic distro kernels which 
don't want to load a bunch of functionality on devices that don't 
support it. So, this triggers the tsm module load, which in turn 
provides enough metadata for userspace to start attestation/whatever it 
needs. (Ex: systemd-detect-virt --cvm).

I think Jason clarifies whats going on too.




> > AFAIK, the faux device is currently missing a faux_device_id in
> > mod_devicetable, alias matching logic in file2alias, and probably a few
> > other things which keeps it from performing this function.
>
> How would a faux device ever expect to get auto-loaded?  That's not what
> is supposed to be happening here at all.
>
> If you have real hardware backing something, then use the real driver
> type.  that is NOT a faux driver, which is, as the name says, for "fake"
> devices that you wish to add to the device/driver tree.
>
> thanks,
>
> greg k-h