1 files changed, 19 insertions, 10 deletions
diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
index d7ce322fb93a..34810f6ae2b5 100644
--- a/include/uapi/linux/iommufd.h
+++ b/include/uapi/linux/iommufd.h
@@ -558,16 +558,25 @@ struct iommu_hw_info_vtd {
  * For the details of @idr, @iidr and @aidr, please refer to the chapters
  * from 6.3.1 to 6.3.6 in the SMMUv3 Spec.
  *
- * User space should read the underlying ARM SMMUv3 hardware information for
- * the list of supported features.
- *
- * Note that these values reflect the raw HW capability, without any insight if
- * any required kernel driver support is present. Bits may be set indicating the
- * HW has functionality that is lacking kernel software support, such as BTM. If
- * a VMM is using this information to construct emulated copies of these
- * registers it should only forward bits that it knows it can support.
- *
- * In future, presence of required kernel support will be indicated in flags.
+ * This reports the raw HW capability, and not all bits are meaningful to be
+ * read by userspace. Only the following fields should be used:
+ *
+ * idr[0]: ST_LEVEL, TERM_MODEL, STALL_MODEL, TTENDIAN , CD2L, ASID16, TTF
+ * idr[1]: SIDSIZE, SSIDSIZE
+ * idr[3]: BBML, RIL
+ * idr[5]: VAX, GRAN64K, GRAN16K, GRAN4K
+ *
+ * - S1P should be assumed to be true if a NESTED HWPT can be created
+ * - VFIO/iommufd only support platforms with COHACC, it should be assumed to be
+ *   true.
+ * - ATS is a per-device property. If the VMM describes any devices as ATS
+ *   capable in ACPI/DT it should set the corresponding idr.
+ *
+ * This list may expand in future (eg E0PD, AIE, PBHA, D128, DS etc). It is
+ * important that VMMs do not read bits outside the list to allow for
+ * compatibility with future kernels. Several features in the SMMUv3
+ * architecture are not currently supported by the kernel for nesting: HTTU,
+ * BTM, MPAM and others.
  */
 struct iommu_hw_info_arm_smmuv3 {
 	__u32 flags;