Re: [PATCH v11 7/7] coco: guest: arm64: Replace dummy CCA device with sysfs ABI

On Mon, 14 Sep 2026 11:35:11 +0530
"Aneesh Kumar K.V (Arm)" [off-list ref] 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.

My gut feeling would be to split this in to the new stuff in first patch
and the removal in second. That would make a revert simpler should it
turn out that someone has userspace that will actually notice and can't
be easily updated.  I'm not that bothered though as it will be your problem
not mine (hopefully!) ;)

[1] https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com (local)

Could make that a link tag.
 Link: https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com (local) #[1]

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>

One trivial thing inline.
Reviewed-by: Jonathan Cameron <redacted>

+static const struct attribute *cca_realm_attrs[] = {
+	&cca_realm_guest.attr,
+	NULL,

Given we don't want anything added after this, no comma ideally.

 };