Re: [PATCH v3] fwctl/cxl: Fix uuid_t usage in uapi

[Paul E. McKenney]

On Thu, Apr 10, 2025 at 11:27:40AM -0700, Dan Williams wrote:
> 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.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Reported-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Closes: http://lore.kernel.org/f6489337-67c7-48c8-b48a-58603ec15328@paulmck-laptop
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Closes: https://lore.kernel.org/oe-kbuild-all/202504050434.Eb4vugh5-lkp@xxxxxxxxx/
> Fixes: 9b8e73cdb141 ("cxl: Move cxl feature command structs to user header")
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>

Nice, thank you!!!

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
> Changes since v2:
> * Drop the tinkering with __align_of__ and just document the safety
>   rules (Jason)
> 
>  include/uapi/cxl/features.h |   21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/include/uapi/cxl/features.h b/include/uapi/cxl/features.h
> index d6db8984889f..490606d7694b 100644
> --- a/include/uapi/cxl/features.h
> +++ b/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;
>