On: I'm unsure of the security of simultaneous multithreading on modern x86 CPUs

[dragontamer]

Architecture: ARMv8 or ARMv7.

Microarchitecture: Neoverse N2, NeoverseE1, Apple Firestorm (all the same ARMv8 architecture, but all different microarchitectures)

Of these: Neoverse E1 has SMT. Apple Firestorm does not.

[floatboth → dragontamer]

Cavium ThunderX2 (RIP) had 4-way SMT