On Fri, Jan 16, 2026 at 12:12:53PM +0000, Ben Horgan wrote:

On 1/16/26 12:02, Ben Horgan wrote:

quoted

On 1/16/26 11:57, Catalin Marinas wrote:

quoted

On Mon, Jan 12, 2026 at 04:58:29PM +0000, Ben Horgan wrote:

quoted

In the test__props_mismatch() kunit test we rely on the struct mpam_props
being packed to ensure memcmp doesn't consider packing. Making it packed
reduces the alignment of the features bitmap and so breaks a requirement
for the use of atomics. As we don't rely on the set/clear of these bits
being atomic, just make them non-atomic.

Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
---
Changes since v2:
Add comment (Jonathan)
---
 drivers/resctrl/mpam_internal.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index 17cdc3080d58..e8971842b124 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h

@@ -200,8 +200,12 @@ struct mpam_props {
 } PACKED_FOR_KUNIT;
 
 #define mpam_has_feature(_feat, x)	test_bit(_feat, (x)->features)
-#define mpam_set_feature(_feat, x)	set_bit(_feat, (x)->features)
-#define mpam_clear_feature(_feat, x)	clear_bit(_feat, (x)->features)
+/*
+ * The non-atomic get/set operations are used because if struct mpam_props is
+ * packed, the alignment requirements for atomics aren't met.
+ */
+#define mpam_set_feature(_feat, x)	__set_bit(_feat, (x)->features)
+#define mpam_clear_feature(_feat, x)	__clear_bit(_feat, (x)->features)

After discussing privately, I can see how test__props_mismatch() can
end up with unaligned atomics on the mmap_props::features array. Happy to
pick it up for 6.19 (probably the first patch as well, though that's
harmless).

Yes please.

quoted

Is there a Fixes tag here for future reference?

Yes, the mpam_set/clear macros were introduced in

The mpam_set_clear() actually comes after in:
c10ca83a7783 arm_mpam: Merge supported features during mpam_enable() into mpam_class
but I think the fixes below is still the correct one as it is where we could 
first start seeing the problem.

quoted

Fixes: 8c90dc68a5de ("arm_mpam: Probe the hardware features resctrl supports")

Yes, I left the original as that's the one first introducing the atomic
bitops on this structure.

-- 
Catalin