Okay, I found it. A SMT knob was added alongside in the L1TF fixes.
/sys/devices/system/cpu/smt
/sys/devices/system/cpu/smt/active
/sys/devices/system/cpu/smt/control
active: Tells whether SMT is active (enabled and siblings online)
control: Read/write interface to control SMT. Possible
values:
"on" SMT is enabled
"off" SMT is disabled
"forceoff" SMT is force disabled. Cannot be changed.
"notsupported" SMT is not supported by the CPU
If control status is "forceoff" or "notsupported" writes are rejected.
