首先复制配置文件：
#cd /usr/src/sys/amd64/conf
#mkdir /root/kernel
#cp GENERIC /root/kernel/MY_KERNEL_CONF
#ln -s /root/kernel/MY_KERNEL_CONF
这样作的好处是更新源代码之后如果像要重新编译内核，就不会需要重新弄一个配制文件了。
第二步是准备硬件清单
#dmesg > hardware
#vi hardware
就可以查看机器的硬件配制了
接下来开始修改配置文件
vi MY_KERNEL_CONF
其中我的改动有：
ident GENERIC => ident MY_KERNEL
这个是内核的名字，以此来与之前的内核相区分
接下来的大部分是操作系统核心相关的东东，如文件系统，进程调度等，大部分我都是直接保留，注释掉了ipv6 相关的选项以及与FreeBSD4兼容的选项，因为我在FreebSD.org上看到的amd64 架构是从FreeBSD5.3 开始的
到后面就是根据硬件配制清单定制驱动，根据需求删减
配制完毕后开始编译
#cd /usr/src
#make buildkernel KERNCONF=MY_KERNEL_CONF
很不幸在链接时报错退出：
linking kernel.debug
umass.o(.text+0xcf): In function `umass_detach':
/usr/src/sys/dev/usb/storage/umass.c:2825: undefined reference to `xpt_bus_deregister'
umass.o(.text+0xf3):/usr/src/sys/dev/usb/storage/umass.c:2828: undefined reference to `cam_sim_free'
umass.o(.text+0xd6c): In function `umass_std_transform':
/usr/src/sys/dev/usb/storage/umass.c:3613: undefined reference to `xpt_done'
u9):/usr/src/sys/dev/usb/storage/umass.c:2689: undefined reference to `cam_sim_alloc'
umass.o(.text+0x2b54):/usr/src/sys/dev/usb/storage/umass.c:2711: undefined reference to `xpt_bus_register'
umass.o(.text+0x2db0):/usr/src/sys/dev/usb/storage/umass.c:2770: undefined reference to `xpt_periph'
umass.o(.text+0x2db5):/usr/src/sys/dev/usb/storage/umass.c:2770: undefined reference to `xpt_create_path'
umass.o(.text+0x2ec9):/usr/src/sys/dev/usb/storage/umass.c:2702: undefined reference to `cam_simq_free'
umass.o(.text+0x2f2d):/usr/src/sys/dev/usb/storage/umass.c:2779: undefined reference `xpt_free_path'
umass.o(.text+0x308d): In function `umass_cam_action':
/usr/src/sys/dev/usb/storage/umass.c:2849: undefined reference to `xpt_done'
umass.o(.text+0x317b):/usr/src/sys/dev/usb/storage/umass.c:3131: undefined reference to `xpt_done'
umass.o(.text+0x31ca):/usr/src/sys/dev/usb/storage/umass.c:2900: undefined reference to `xpt_done'
umass.o(.text+0x329b):/usr/src/sys/dev/usb/storage/umass.c:3121: undefined reference to `xpt_done'
umass.o(.text+0x3324):/usr/src/sys/dev/usb/storage/umass.c:2936: undefined reference to `xpt_done'
umass.o(.text+0x33b2):/usr/src/sys/dev/usb/storage/umass.c:3095: more undefined references to `xpt_done' fo.o(.text+0x35be):/usr/src/sys/dev/usb/storage/umass.c:3073: undefined reference to `xpt_done'
umass.o(.text+0x3819):/usr/src/sys/dev/usb/storage/umass.c:3004: undefined reference to `xpt_done'
umass.o(.text+0x39bf):/usr/src/sys/dev/usb/storage/umass.c:2994: undefined reference to `xpt_done'
umass.o(.text+0x3a07):/usr/src/sys/dev/usb/storage/umass.c:2982: undefined reference to `xpt_done'
umass.o(.text+0xdfa):/usr/src/sys/dev/usb/storage/umass.c:3325: more undefined references to `xpt_done' follow
*** Error code 1
Stop in /usr/
GOOGLE找到答案，原来是我在配置文件里含入了
device umass
却没有按照后面的注释含入
device scbus
device da
解决方法是直接将umass 也给它注释掉
重新build，成功!
# make installkernel KERNCONF=MY_KERNEL_CONF
搞定，重启
察看内核版本：
uname -i
$ uname -i
MY_KERNEL
然后上网、登录SSH、VNC、读光盘等等都没有问题
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