prebuilt: failed extract-vmlinux attempt

README.adoc

@@ -582,12 +582,15 @@ but QEMU builds are pretty quick, and this further increases the probability of
 For gem5, do:
 
 ....
+git submodule update --init --depth 1 "$(./getvar linux_src_dir)"
 sudo apt-get install qemu-utils
 ./build-gem5
 ./run --gem5 --prebuilt
 ....
 
 `qemu-utils` is required because we currently distribute `.qcow2` files which <<gem5-qcow2,gem5 can't handle>>, so we need `qemu-img` to extract them first.
+
+The Linux kernel is required for `extract-vmlinux` to convert the compressed kernel image which QEMU understands into the raw vmlinux that gem5 understands: https://superuser.com/questions/298826/how-do-i-uncompress-vmlinuz-to-vmlinux
 ////
 
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