Simplify logic a bit. Ensure data buffer is properly aligned,

especially for platforms where unaligned access is not allowed. Make
it possible to override the small buffer size.

A simple continuous read string test using libusb showed a reduction
in CPU usage from roughly 10% to less than 1% using a dual-core GHz
CPU, when the malloc() operation was skipped for small buffers.

MFC after:	2 weeks

diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
index 32ae342447f804ba56f25d9dc46a250878c3987e..1cd1287d257611f64c282fbf39982aa9dff96749 100644 (file)
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -646,10 +646,13 @@ 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_long com;
        int arg, error;
        u_int size;
-       u_char smalldata[128];
        caddr_t data;
 
        if (uap->com > 0xffffffff) {
@@ -682,10 +685,10 @@ sys_ioctl(struct thread *td, struct ioctl_args *uap)
                        data = (void *)&arg;
                        size = 0;
                } else {
-                       if (size <= sizeof(smalldata))
-                               data = smalldata;
-                       else
+                       if (size > SYS_IOCTL_SMALL_SIZE)
                                data = malloc((u_long)size, M_IOCTLOPS, M_WAITOK);
+                       else
+                               data = smalldata;
                }
        } else
                data = (void *)&uap->data;
@@ -707,7 +710,7 @@ sys_ioctl(struct thread *td, struct ioctl_args *uap)
                error = copyout(data, uap->data, (u_int)size);
 
 out:
-       if (size > 0 && data != (caddr_t)&smalldata)
+       if (size > SYS_IOCTL_SMALL_SIZE)
                free(data, M_IOCTLOPS);
        return (error);
 }