diff --git a/qemu/include/qemu/osdep.h b/qemu/include/qemu/osdep.h
index c3c61fe8..530d1f2e 100644
--- a/qemu/include/qemu/osdep.h
+++ b/qemu/include/qemu/osdep.h
@@ -141,7 +141,8 @@
 /* Round number down to multiple */
 #define QEMU_ALIGN_DOWN(n, m) ((n) / (m) * (m))
 
-/* Round number up to multiple */
+/* Round number up to multiple. Safe when m is not a power of 2 (see
+ * ROUND_UP for a faster version when a power of 2 is guaranteed) */
 #define QEMU_ALIGN_UP(n, m) QEMU_ALIGN_DOWN((n) + (m) - 1, (m))
 
 /* Check if n is a multiple of m */
@@ -172,6 +173,9 @@
 #define QEMU_PTR_IS_ALIGNED(p, n) QEMU_IS_ALIGNED((uintptr_t)(p), (n))
 #endif
 
+/* Round number up to multiple. Requires that d be a power of 2 (see
+ * QEMU_ALIGN_UP for a safer but slower version on arbitrary
+ * numbers) */
 #ifndef ROUND_UP
 #define ROUND_UP(n,d) (((n) + (d) - 1) & -(d))
 #endif