Outdated docs on detailed steps of using 9PFS

[thass0]

This part of the docs in the README seems to be outdated:

For 9pfs, also make sure that you configured vfscore to automatically mount a host shared filesystem: Under Library Configuration -> vfscore: Configuration select Automatically mount a root filesystem, set Default root filesystem to 9PFS, and ensure that Default root device is to fs0. This last option simplifies the use of the -e parameter of qemu-guest.

It seems to me that the quote refers to configuration options for vfscore that were replaced by this commit: a3b1eee

With the current version of Unikraft, the following steps allow booting the elfloader with the example/helloworld app again:

• git clone https://github.com/unikraft/app-elfloader elfloader && cd elfloader && ./scripts/setup.sh
• cd example/helloworld/ && make helloworld
• Set up the rootfs/ matching the ldd output for the helloworld binary (as described in the README, except I also add an /etc directory)
• UK_DEFCONFIG=$(pwd)/defconfigs/qemu-x86_64-9pfs make defconfig
• Configure Library Configuration -> vfscore -> Compiled-in filesystem table -> Configuration and pick 9PFS
• make -j$(nproc)
• ./workdir/unikraft/support/scripts/qemu-guest -k workdir/build/elfloader_qemu-x86_64 -e rootfs/ -a "/helloworld"

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