Like others have said typically the microcode update is applied by both the BIOS and kernel. Typically the kernel has the latest microcode. However, there coul

Like others have said typically the microcode update is applied by both the BIOS and kernel. Typically the kernel has the latest microcode. However, there could be an issue where updating the microcode at kernel time can cause an issue due to a microcode version (or lack of) from the BIOS. Only other thing I could think of is the microcode update that disables TSX, I believe, could only disable it if TSX wasn't used yet. So I suppose it's also possible that the microcode has to be updated by the BIOS because some boot process prevents the kernel from being effective with its microcode update step.