[PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities

[Jacob Pan]

Having a single UAPI version to govern the user-kernel data structures
makes compatibility check straightforward. On the contrary, supporting
combinations of multiple versions of the data can be a nightmare to
maintain.

This patch defines a unified UAPI version to be used for compatibility
checks between user and kernel.

---
v2: Rewrite extension rules to disallow adding new members. Use padding
    and union extensions only.
---

Signed-off-by: Liu Yi L <yi.l.liu@xxxxxxxxx>
Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Link: https://lkml.org/lkml/2020/2/3/1126
---
 include/uapi/linux/iommu.h | 53 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
index d7bcbc5f79b0..25dba9198d7f 100644
--- a/include/uapi/linux/iommu.h
+++ b/include/uapi/linux/iommu.h
@@ -8,6 +8,59 @@
 
 #include <linux/types.h>
 
+/**
+ * Current version of the IOMMU user API. This is intended for query
+ * between user and kernel to determine compatible data structures.
+ *
+ * UAPI version can be bumped up with the following rules:
+ * 1. All data structures passed between user and kernel space share
+ *    the same version number. i.e. any extension to any structure
+ *    results in version number increment.
+ *
+ * 2. Data structures are open to extension but closed to modification.
+ *    Extensions are allowed in two places:
+ *    - the padding bytes with a flag bit for each new member
+ *    - new union members at the end of each structure
+ *
+ *    No new members can be added after padding bytes are exhausted.
+ *    The reason is that the union size can change when new members are
+ *    added, having new member at the end would break backward
+ *    compatibility. Expansion of the union would move the new member
+ *    to different offset between versions.
+ *
+ *    Flag bits can be added without size change but existing ones
+ *    cannot be altered.
+ *
+ * 3. Versions are backward compatible.
+ *
+ * 4. Version to size lookup is supported by kernel internal API for each
+ *    API function type. @version is mandatory for new data structures
+ *    and must be at the beginning with type of __u32.
+ */
+#define IOMMU_UAPI_VERSION	1
+static inline int iommu_get_uapi_version(void)
+{
+	return IOMMU_UAPI_VERSION;
+}
+
+/*
+ * Supported UAPI features that can be reported to user space.
+ * These types represent the capability available in the kernel.
+ *
+ * REVISIT: UAPI version also implies the capabilities. Should we
+ * report them explicitly?
+ */
+enum IOMMU_UAPI_DATA_TYPES {
+	IOMMU_UAPI_BIND_GPASID,
+	IOMMU_UAPI_CACHE_INVAL,
+	IOMMU_UAPI_PAGE_RESP,
+	NR_IOMMU_UAPI_TYPE,
+};
+
+#define IOMMU_UAPI_CAP_MASK ((1 << IOMMU_UAPI_BIND_GPASID) |	\
+				(1 << IOMMU_UAPI_CACHE_INVAL) |	\
+				(1 << IOMMU_UAPI_PAGE_RESP))
+
 #define IOMMU_FAULT_PERM_READ	(1 << 0) /* read */
 #define IOMMU_FAULT_PERM_WRITE	(1 << 1) /* write */
 #define IOMMU_FAULT_PERM_EXEC	(1 << 2) /* exec */
-- 
2.7.4