fwctl/cxl: Fix uuid_t usage in uapi

The uuid_t type is kernel internal, and Paul reports the following build
error when it is used in a uapi header:

    usr/include/cxl/features.h:59:9: error: unknown type name ‘uuid_t’

Create a uuid type (__uapi_uuid_t) compatible with the longstanding
definition uuid/uuid.h for userspace builds, and use uuid_t directly for
kernel builds.

Fixes: 9b8e73c ("cxl: Move cxl feature command structs to user header")
Link: https://patch.msgid.link/r/174430961702.617339.13963021112051029933.stgit@dwillia2-xfh.jf.intel.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reported-by: Paul E. McKenney <paulmck@kernel.org>
Closes: http://lore.kernel.org/f6489337-67c7-48c8-b48a-58603ec15328@paulmck-laptop
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202504050434.Eb4vugh5-lkp@intel.com/
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

include/uapi/cxl/features.h

@@ -8,10 +8,19 @@
 #define _UAPI_CXL_FEATURES_H_
 
 #include <linux/types.h>
-#ifndef __KERNEL__
-#include <uuid/uuid.h>
-#else
+
+typedef unsigned char __uapi_uuid_t[16];
+
+#ifdef __KERNEL__
 #include <linux/uuid.h>
+/*
+ * Note, __uapi_uuid_t is 1-byte aligned on modern compilers and 4-byte
+ * aligned on others. Ensure that __uapi_uuid_t in a struct is placed at
+ * a 4-byte aligned offset, or the structure is packed, to ensure
+ * consistent padding.
+ */
+static_assert(sizeof(__uapi_uuid_t) == sizeof(uuid_t));
+#define __uapi_uuid_t uuid_t
 #endif
 
 /*
@@ -60,7 +69,7 @@ struct cxl_mbox_get_sup_feats_in {
  * Get Supported Features Supported Feature Entry
  */
 struct cxl_feat_entry {
-	uuid_t uuid;
+	__uapi_uuid_t uuid;
 	__le16 id;
 	__le16 get_feat_size;
 	__le16 set_feat_size;
@@ -110,7 +119,7 @@ struct cxl_mbox_get_sup_feats_out {
  * CXL spec r3.2 section 8.2.9.6.2 Table 8-99
  */
 struct cxl_mbox_get_feat_in {
-	uuid_t uuid;
+	__uapi_uuid_t uuid;
 	__le16 offset;
 	__le16 count;
 	__u8 selection;
@@ -143,7 +152,7 @@ enum cxl_get_feat_selection {
  */
 struct cxl_mbox_set_feat_in {
 	__struct_group(cxl_mbox_set_feat_hdr, hdr, /* no attrs */,
-		uuid_t uuid;
+		__uapi_uuid_t uuid;
 		__le32 flags;
 		__le16 offset;
 		__u8 version;