It depends on the severity of the bug and the level of foresight of the hardware designer. In the worst case (e.g. the FDIV bug on the Pentium) you have to reca

It depends on the severity of the bug and the level of foresight of the hardware designer. In the worst case (e.g. the FDIV bug on the Pentium) you have to recall the CPU. Obviously, this is very bad. That's why modern CPUs have what are called "chicken bits" or "kill bits", which the BIOS or OS can set to disable specific features. The most recent use of kill bits I can recall is Intel disabling the TSX instructions on the Haswell line of CPUs. Finally, the least invasive option is to issue a microcode update, alters the way that x86 instructions get decoded to avoid the problematic behavior. Microcode updates are issued as software patches, and they're actually in the Linux kernel source tree. If you want to learn more about all three of these fix techniques, in addition to learning a bit about the steps that Intel and AMD take to prevent such bugs in the first place, I highly recommend this CCC talk: https://media.ccc.de/v/32c3-7171-when_hardware_must_just_work https://media.ccc.de/v/32c3-7171-when_hardware_must_just_wor....