osdep: Document differences in rounding macros

Make it obvious which macros are safe in which situations.

Useful since QEMU_ALIGN_UP and ROUND_UP both purport to do
the same thing, but differ on whether the alignment must be
a power of 2.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <1469129688-22848-4-git-send-email-eblake@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

include/qemu/osdep.h

@@ -158,7 +158,8 @@ extern int daemon(int, int);
 /* 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 */
@@ -175,6 +176,9 @@ extern int daemon(int, int);
 /* Check if pointer p is n-bytes aligned */
 #define QEMU_PTR_IS_ALIGNED(p, n) QEMU_IS_ALIGNED((uintptr_t)(p), (n))
 
+/* 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