Re: [PATCH v7 6/6] coco: guest: arm64: Replace dummy CCA device with sysfs ABI

["Dan Williams (nvidia)" <djbw@kernel.org>]

Aneesh Kumar K.V (Arm) wrote:
> The SMCCC firmware driver now creates the arm-smccc platform device and
> instantiates the CCA RSI auxiliary devices once the RSI ABI is discovered.
> The arm64-specific arm-cca-dev platform device stub is therefore no longer
> needed.
> 
> However, userspace has used the arm-cca-dev platform device to detect Arm
> CCA Realm guests [1]. Removing it without a replacement would break that
> detection and would also leave userspace depending on kernel device-model
> details.
> 
> Add /sys/firmware/cca/realm_guest as a stable, architecture-provided ABI
> for detecting whether the kernel is running as an Arm CCA Realm guest. The
> file returns 1 in Realm world and 0 otherwise, similar to the existing s390
> /sys/firmware/uv/prot_virt_guest interface for protected virtualization
> guests.
> 
> Remove the dummy arm-cca-dev registration now that userspace has a
> dedicated CCA Realm guest indicator, and document the new ABI in
> Documentation/ABI/testing/sysfs-firmware-cca.

I would have expected an attribute in /sys/class/tsm/tsmX to be the
common protected guest indicator. Then, if you need to distinguish the
architecture that registered that tsm it would be in the name of the
parent device for the tsm class device. 

That also gives you the property that a uevent has signalled the arrival
of tsm guest services. Otherwise, userspace still needs some custom
device-model details to know when it can start issuing tsm requests.

Is auxilliary device arrival too late in the flow for what systemd
needs?