Re: [for-4.20][PATCH 1/2] device-tree: bootfdt: Fix build issue when CONFIG_PHYS_ADDR_T_32=y

[Julien Grall <julien.grall.oss@xxxxxxxxx>]

On Mon, 27 Jan 2025 at 07:46, Michal Orzel <michal.orzel@xxxxxxx> wrote:

On Arm32, when CONFIG_PHYS_ADDR_T_32 is set, a build failure is observed:
common/device-tree/bootfdt.c: In function 'build_assertions':
./include/xen/macros.h:47:31: error: static assertion failed: "!(alignof(struct membanks) != 8)"
   47 | #define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
      |                               ^~~~~~~~~~~~~~
common/device-tree/bootfdt.c:31:5: note: in expansion of macro 'BUILD_BUG_ON'
   31 |     BUILD_BUG_ON(alignof(struct membanks) != 8);

When CONFIG_PHYS_ADDR_T_32 is set, paddr_t is defined as unsigned long,
therefore the struct membanks alignment is 4B. Fix it.

Usually, we add a BUILD_BUG_ON when other parts of the code rely on a specific property (in this case alignment). Can you explain in the commit message why the new check is still ok?

Cheers,

Fixes: 2209c1e35b47 ("xen/arm: Introduce a generic way to access memory bank structures")
Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
---
 xen/common/device-tree/bootfdt.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/common/device-tree/bootfdt.c b/xen/common/device-tree/bootfdt.c
index 47386d4fffea..511700ccc2ba 100644
--- a/xen/common/device-tree/bootfdt.c
+++ b/xen/common/device-tree/bootfdt.c
@@ -27,8 +27,8 @@ static void __init __maybe_unused build_assertions(void)
      */
     BUILD_BUG_ON((offsetof(struct membanks, bank) !=
                  offsetof(struct meminfo, bank)));
-    /* Ensure "struct membanks" is 8-byte aligned */
-    BUILD_BUG_ON(alignof(struct membanks) != 8);
+    /* Ensure "struct membanks" is paddr aligned */
+    BUILD_BUG_ON(alignof(struct membanks) != sizeof(paddr_t));
 }

 static bool __init device_tree_node_is_available(const void *fdt, int node)
--
2.25.1