前面提到今天犯了一个低级错误，是安装gentoo时编译内核不正确造成无法启动。如图：

后来google了一番，在这篇文章中找到了答案。解决办法是在编译内核时把下面的选项选中（我前面没有选中前两项）：

Device Drivers --->
Generic Driver Options --->
(/sbin/hotplug) path to uevent helper
[*] Maintain a devtmpfs filesystem to mount at /dev
[*] Automount devtmpfs at /dev, after the kernel mounted the rootfs
[*] Select only drivers that don't need compile-time external firmware
[*] Prevent firmware from being built