Rico Board--AM437x--NFS启动失败

大家好，我在使用NFS作为跟文件系统来启动Rico Board，uboot的参数设置如下：
setenv ipaddr 192.168.1.100
setenv serverip 192.168.1.89
save
set bootcmd tftp 0x80200000 zImage ; tftp 0x80F80000 myir_ricoboard.dtb ; bootz 0x80200000 - 0x80F80000
save
setenv bootargs console=ttyO0,115200n8 root=/dev/nfs rw nfsroot=192.168.1.89:/home/xxxx/work/ricoboard/rootfs ip=192.168.1.100
ip=192.168.1.100


运行到以下地方卡死，打印信息如下：
.... ....
libphy: 4a101000.mdio:00 - Link is Up - 1000/Full
IP-Config: Guessing netmask 255.255.255.0
IP-Config: Complete:
     device=eth0, hwaddr=c4:be:84:cd:b5:46, ipaddr=192.168.1.100, mask=255.255.255.0, gw=255.255.255.255
     host=192.168.1.100, domain=, nis-domain=(none)
     bootserver=255.255.255.255, rootserver=192.168.1.89, rootpath=
VFS: Unable to mount root fs via NFS, trying floppy.
VFS: Cannot open root device "nfs" or unknown-block(2,0): error -6
Please append a correct "root=" boot option; here are the available partitions:
1f00             512 mtdblock0  (driver?)
1f01             512 mtdblock1  (driver?)
1f02             128 mtdblock2  (driver?)
1f03             128 mtdblock3  (driver?)
1f04              64 mtdblock4  (driver?)
1f05            5120 mtdblock5  (driver?)
1f06            9920 mtdblock6  (driver?)
b300         3776512 mmcblk0  driver: mmcblk
  b301           72261 mmcblk0p1 00000000-01
  b302         1526175 mmcblk0p2 00000000-02
  b303         2168775 mmcblk0p3 00000000-03
b310            2048 mmcblk0boot1  (driver?)
b308            2048 mmcblk0boot0  (driver?)
b318        15558144 mmcblk1  driver: mmcblk
  b319           72261 mmcblk1p1 00000000-01
  b31a        15478627 mmcblk1p2 00000000-02
Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(2,0)


①出现以上错误时，虚拟机中可以ping通开发板的，说明内核已经运行了，网络通信应该没有问题；
PING 192.168.1.100 (192.168.1.100) 56(84) bytes of data.
64 bytes from 192.168.1.100: icmp_seq=1 ttl=64 time=1.42 ms
64 bytes from 192.168.1.100: icmp_seq=2 ttl=64 time=1.31 ms
64 bytes from 192.168.1.100: icmp_seq=3 ttl=64 time=1.17 ms
64 bytes from 192.168.1.100: icmp_seq=4 ttl=64 time=1.41 ms
64 bytes from 192.168.1.100: icmp_seq=5 ttl=64 time=1.36 ms
64 bytes from 192.168.1.100: icmp_seq=6 ttl=64 time=1.11 ms
64 bytes from 192.168.1.100: icmp_seq=7 ttl=64 time=1.26 ms
64 bytes from 192.168.1.100: icmp_seq=8 ttl=64 time=0.897 ms
64 bytes from 192.168.1.100: icmp_seq=9 ttl=64 time=0.700 ms

②我虚拟机里的NFS服务是OK的，用sudo mount -t nfs 127.0.0.1:/home/xxxx/work/ricoboard/rootfs /mnt 测试过，可以挂载；
③虚拟机中防火墙也已经关闭；
④内核编译时，已经添加了对网络文件系统的支持；

个人实在是想不出还有啥地方不对了，但就是死活挂载不了文件系统，请高手忙看看！
多谢多谢！