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

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

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(-)

diff --git a/xen/include/public/domctl.h b/xen/include/public/domctl.h
index 353f831e402e..b3c8271e66ba 100644
--- 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.
-- 
2.47.0