How to mount rootfs on FastModel

Hi, I am learning building arm64 kernel on: https://wiki.linaro.org/HowTo/BuildArm64Kernel

I followed the page and started the kernel, because I didn't indicate a mmc deivce, the console outputs as below: VFS: Cannot open root device "vda2" or unknown-block(254,2): error -6 Please append a correct "root=" boot option; here are the available partitions: Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(254,2)

I had download the pre-built root filesystem, but I don't know how to use it. Could anyone tell me how to mount rootfs? Thank you!