[PATCH v12 7/7] coco: guest: arm64: Replace dummy CCA device with sysfs ABI
From: "Aneesh Kumar K.V (Arm)" <aneesh.kumar@kernel.org>
Date: 2026-09-21 05:39:18
Also in:
linux-coco, lkml
Subsystem:
the rest · Maintainer:
Linus Torvalds
The SMCCC firmware driver now creates the arm_smccc bus and instantiates the CCA RSI device 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 ABI for detecting whether the kernel is running as an Arm CCA Realm guest. Create the file only in Realm world, where it reads 1. 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. Link: https://lore.kernel.org/all/4a7d84b2-2ec4-4773-a2d5-7b63d5c683cf@arm.com (local) Reviewed-by: Jonathan Cameron <redacted> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org> --- Documentation/ABI/testing/sysfs-firmware-cca | 7 ++++ drivers/firmware/arm_rmm/rsi.c | 43 +++++++++++++++----- 2 files changed, 39 insertions(+), 11 deletions(-) create mode 100644 Documentation/ABI/testing/sysfs-firmware-cca
diff --git a/Documentation/ABI/testing/sysfs-firmware-cca b/Documentation/ABI/testing/sysfs-firmware-cca
new file mode 100644
index 000000000000..9cc355724f94
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-firmware-cca@@ -0,0 +1,7 @@ +What: /sys/firmware/cca/realm_guest +Date: May 2026 +Contact: Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org> +Description: Read-only. Present only when the kernel is running as an + Arm Confidential Compute Architecture (CCA) Realm guest, in + which case it reads 1. If the file does not exist, the kernel + is not running as a Realm guest.
diff --git a/drivers/firmware/arm_rmm/rsi.c b/drivers/firmware/arm_rmm/rsi.c
index 2dd69febedbd..425b27c0cad4 100644
--- a/drivers/firmware/arm_rmm/rsi.c
+++ b/drivers/firmware/arm_rmm/rsi.c@@ -7,14 +7,16 @@ #include <linux/memblock.h> #include <linux/psci.h> #include <linux/swiotlb.h> -#include <linux/platform_device.h> #include <linux/arm-rsi-cmds.h> +#include <linux/kobject.h> +#include <linux/sysfs.h> #include <asm/io.h> #include <asm/mem_encrypt.h> #include <asm/pgtable.h> static struct realm_config config; +static struct kobject *cca_kobj; unsigned long prot_ns_shared; EXPORT_SYMBOL(prot_ns_shared);
@@ -182,17 +184,36 @@ void __init arm64_rsi_init(void) static_branch_enable(&rsi_present); } -static struct platform_device rsi_dev = { - .name = "arm-cca-dev", - .id = PLATFORM_DEVID_NONE +static ssize_t realm_guest_show(struct kobject *kobj, + struct kobj_attribute *attr, char *buf) +{ + return sysfs_emit(buf, "1\n"); +} + +static struct kobj_attribute cca_realm_guest = + __ATTR(realm_guest, 0444, realm_guest_show, NULL); + +static const struct attribute *cca_realm_attrs[] = { + &cca_realm_guest.attr, + NULL }; -static int __init arm64_create_dummy_rsi_dev(void) +static int __init realm_sysfs_init(void) { - if (is_realm_world() && - platform_device_register(&rsi_dev)) - pr_err("failed to register rsi platform device\n"); - return 0; -} + int ret; -arch_initcall(arm64_create_dummy_rsi_dev) + if (!is_realm_world()) + return 0; + + cca_kobj = kobject_create_and_add("cca", firmware_kobj); + if (!cca_kobj) + return -ENOMEM; + + ret = sysfs_create_files(cca_kobj, cca_realm_attrs); + if (!ret) + return 0; + + kobject_put(cca_kobj); + return ret; +} +device_initcall(realm_sysfs_init);
--
2.43.0