Style changes:

- Move two IOCTL related defines to the top of the C-file
- Add more comments describing the recently added IOCTL small size and
small align macros

1 files changed, 15 insertions, 4 deletions

diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
index 01996155e4cc..8dddc9b556f6 100644
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -75,6 +75,20 @@ __FBSDID("$FreeBSD$");
 
 #include <security/audit/audit.h>
 
+/*
+ * The following macro defines how many bytes will be allocated from
+ * the stack instead of memory allocated when passing the IOCTL data
+ * structures from userspace and to the kernel. Some IOCTLs having
+ * small data structures are used very frequently and this small
+ * buffer on the stack gives a significant speedup improvement for
+ * those requests. The value of this define should be greater or equal
+ * to 64 bytes and should also be power of two. The data structure is
+ * currently hard-aligned to a 8-byte boundary on the stack. This
+ * should currently be sufficient for all supported platforms.
+ */
+#define	SYS_IOCTL_SMALL_SIZE	128	/* bytes */
+#define	SYS_IOCTL_SMALL_ALIGN	8	/* bytes */
+
 int iosize_max_clamp = 0;
 SYSCTL_INT(_debug, OID_AUTO, iosize_max_clamp, CTLFLAG_RW,
     &iosize_max_clamp, 0, "Clamp max i/o size to INT_MAX");
@@ -646,10 +660,7 @@ struct ioctl_args {
 int
 sys_ioctl(struct thread *td, struct ioctl_args *uap)
 {
-#ifndef SYS_IOCTL_SMALL_SIZE
-#define	SYS_IOCTL_SMALL_SIZE 128
-#endif
-	u_char smalldata[SYS_IOCTL_SMALL_SIZE] __aligned(8);
+	u_char smalldata[SYS_IOCTL_SMALL_SIZE] __aligned(SYS_IOCTL_SMALL_ALIGN);
 	u_long com;
 	int arg, error;
 	u_int size;