[PATCH 3/3] block/nvme: Align NVMeRegs structure to 4KiB and mark it packed

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

In commit e5ff22ba9fc we changed the doorbells register
declaration but forgot to mark the structure packed (as
MMIO registers), allowing the compiler to optimize it.
Fix by marking it packed, and align it to avoid:

  block/nvme.c: In function ‘nvme_create_queue_pair’:
  block/nvme.c:252:22: error: taking address of packed member of ‘struct <anonymous>’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
      252 |     q->sq.doorbell = &s->regs->doorbells[idx * s->doorbell_scale].sq_tail;
          |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: e5ff22ba9fc ("block/nvme: Pair doorbell registers")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 block/nvme.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/block/nvme.c b/block/nvme.c
index be80ea1f410..2f9f560ccd5 100644
--- a/block/nvme.c
+++ b/block/nvme.c
@@ -12,6 +12,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qemu/units.h"
 #include <linux/vfio.h>
 #include "qapi/error.h"
 #include "qapi/qmp/qdict.h"
@@ -82,13 +83,15 @@ typedef struct {
 } NVMeQueuePair;
 
 /* Memory mapped registers */
-typedef struct {
+typedef struct QEMU_PACKED {
     NvmeBar ctrl;
     struct {
         uint32_t sq_tail;
         uint32_t cq_head;
     } doorbells[];
-} NVMeRegs;
+} QEMU_ALIGNED(4 * KiB) NVMeRegs;
+
+QEMU_BUILD_BUG_ON(offsetof(NVMeRegs, doorbells[1]) != 4096 + 8);
 
 #define INDEX_ADMIN     0
 #define INDEX_IO(n)     (1 + n)
-- 
2.26.2