iommu/arm-smmu-v3: Improve uAPI comment for IOMMU_HW_INFO_TYPE_ARM_SMMUV3

Be specific about what fields should be accessed in the idr result and
give other guidance to the VMM on how it should generate the
vIDR. Discussion on the list, and review of the qemu implementation
understood this needs to be clearer and more detailed.

Link: https://patch.msgid.link/r/0-v1-191e5e24cec3+3b0-iommufd_smmuv3_hwinf_jgg@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: jgunthorpe

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;