How to use m5 readfile and m5 execfile in gem5? - gem5

Question

I expect m5 readfile will read a file from the host, and m5 execfile will execute a file from host, but the documentation is scarce and I couldn't get them to work.
Maybe the path is determined by readfile= in m5out/config.ini, but I don't know how to modify that setting when using fs.py; when I relaunch gem5 it overwrites that file.
I was able to use the related m5 writefile however with:
Guest:
echo mycontent > myfileguest
m5 writefile myfileguest myfilehost
Host:
cat m5out/myfilehost

Answer 1

readfile
As of gem5 68af229490fc811aebddf68b3e2e09e63a5fa475 you can set the readfile path with:
fs.py --script=myfile
Unlike writefile, the path is relative to the current directory on the host, not m5out.
Then when you run m5 readfile in the guest, it outputs the contents of the file to stdout.
This does in the fs.py script:
if options.script is not None:
test_sys.readfile = options.script
which seems to inherit from class System, which ends up serializing configurations to config.ini only for debugging purposes.
execfile
execfile is just a very light wrapper over readfile, that instead of writing the input file to stdout, writes it to /tmp/execfile, and runs the exec syscall on it from C.
However, as of 68af229490fc811aebddf68b3e2e09e63a5fa475, m5 does not:
open the file with O_CREAT
set the appropriate executable permissions
so you must do on guest:
touch /tmp/execfile
chmod +x /tmp/execfile
m5 execfile
I'd just rather do this with m5 readfile and some scripting.
m5 execfile could be useful as an init program however, but I find argument passing to init programs too clumsy and unrealiable.