reading kernel tunables from userland ?

sorry for the possibly naive question, but is there
a way other than sysctl to read the value of kernel 'tunable'
variables ?

E.g. in if_nfe.c i see

TUNABLE_INT("hw.nfe.msi_disable", &msi_disable);
TUNABLE_INT("hw.nfe.msix_disable", &msix_disable);
TUNABLE_INT("hw.nfe.jumbo_disable", &jumbo_disable);

but sysctl is reporting none of them.

I seem to remember that there was a separate command to access
kernel tunables,
but cannot remember which one, and neither 'man -k tunable'
nor a google search return any useful answer...

thanks
luigi