Re: [RFC PATCH 1/6] xen/domctl: Refine grant_opts into grant_version

[Jan Beulich <jbeulich@xxxxxxxx>]

On 29.10.2024 19:16, Alejandro Vallejo wrote:
> grant_opts is overoptimizing for space packing in a hypercall that
> doesn't warrant the effort. Tweak the ABI without breaking it in order
> to remove the bitfield by extending it to 8 bits.
> 
> Xen only supports little-endian systems, so the transformation from
> uint32_t to uint8_t followed by 3 octets worth of padding is not an ABI
> breakage.
> 
> No functional change
> 
> Signed-off-by: Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>
> ---
>  xen/include/public/domctl.h | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)

This isn't a complete patch, is it? I expect it'll break the build without
users of the field also adjusted.

> --- a/xen/include/public/domctl.h
> +++ b/xen/include/public/domctl.h
> @@ -90,11 +90,18 @@ struct xen_domctl_createdomain {
>      int32_t max_grant_frames;
>      int32_t max_maptrack_frames;
>  
> -/* Grant version, use low 4 bits. */
> -#define XEN_DOMCTL_GRANT_version_mask    0xf
> -#define XEN_DOMCTL_GRANT_version(v)      ((v) & 
> XEN_DOMCTL_GRANT_version_mask)
> +    /*
> +     * Maximum grant table version the domain can be configured with.
> +     *
> +     * Domains always start with v1 (if CONFIG_GRANT_TABLE) and can be bumped
> +     * to use up to `max_grant_version` via GNTTABOP_set_version.
> +     *
> +     * Must be zero iff !CONFIG_GRANT_TABLE.
> +     */
> +    uint8_t max_grant_version;
>  
> -    uint32_t grant_opts;
> +    /* Unused */
> +    uint8_t rsvd0[3];
>  
>  /*
>   * Enable altp2m mixed mode.

Just to mention it: I think while binary compatible, this is still on the edge
of needing an interface version bump. We may get away without as users of the
removed identifiers will still notice by way of observing build failures.

Jan