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This will cause kernel > to crash early in current_save_fsgs(). #UD is the expected behaviour of the FSGS instructions if they're not enabled. Are you saying that this specific CPU enumerates FSGSBASE in CPUID, and permits setting CR4.FSGSBASE (without #GP for a reserved bit), and the FSGS instructions still do not function? What happens if you read CR4 back after trying to set the bit? ~Andrew