[PATCH] xen/gnttab: Shrink struct gnttab_copy_buf

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Move the booleans into earlier padding.  No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
CC: Michal Orzel <michal.orzel@xxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Julien Grall <julien@xxxxxxx>
CC: Roger Pau Monné <roger@xxxxxxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>

On 64bit, this shrinks from 64 bytes to 56 bytes.

On arm32, it's 40 bytes before and after.  Given that we can't reuse the
tail-padding from struct gnttab_copy_ptr, I'm not sure there's an obvious
improvement to be had:

  xen.git/xen$ pahole xen-syms-arm32-after -E -C gnttab_copy_buf
  struct gnttab_copy_buf {
        struct gnttab_copy_ptr {
                union {
                        /* typedef grant_ref_t -> uint32_t */ unsigned int ref; 
         /*     0     4 */
                        /* typedef xen_pfn_t -> uint64_t */ long long unsigned 
int gmfn; /*     0     8 */
                } u;                                                            
         /*     0     8 */
                /* typedef domid_t -> uint16_t */ short unsigned int domid;     
         /*     8     2 */
                /* typedef uint16_t */ short unsigned int offset;               
         /*    10     2 */
        }ptr; /*     0    16 */

        /* XXX last struct has 4 bytes of padding */

        /* typedef uint16_t */ short unsigned int         len;                  
         /*    16     2 */
        _Bool                      read_only;                                   
         /*    18     1 */
        _Bool                      have_grant;                                  
         /*    19     1 */
        _Bool                      have_type;                                   
         /*    20     1 */

        /* XXX 3 bytes hole, try to pack */

        struct domain *            domain;                                      
         /*    24     4 */
        /* typedef mfn_t */ struct {
                long unsigned int  mfn;                                         
         /*    28     4 */
        } mfn; /*    28     4 */
        struct page_info *         page;                                        
         /*    32     4 */
        void *                     virt;                                        
         /*    36     4 */

        /* size: 40, cachelines: 1, members: 9 */
        /* sum members: 37, holes: 1, sum holes: 3 */
        /* paddings: 1, sum paddings: 4 */
        /* last cacheline: 40 bytes */
  };
---
 xen/common/grant_table.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/xen/common/grant_table.c b/xen/common/grant_table.c
index 5e79c012b695..d2c3c73be900 100644
--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2927,13 +2927,14 @@ struct gnttab_copy_buf {
     uint16_t len;
 
     /* Mapped etc. */
+    bool read_only;
+    bool have_grant;
+    bool have_type;
+
     struct domain *domain;
     mfn_t mfn;
     struct page_info *page;
     void *virt;
-    bool read_only;
-    bool have_grant;
-    bool have_type;
 };
 
 static int gnttab_copy_lock_domain(domid_t domid, struct gnttab_copy_buf *buf)

base-commit: 2043d22b7e926f508cc23391b4c75325fa7211e9
-- 
2.39.5