Early load microcode ==================== By Fenghua Yu <fenghua.yu@intel.com> Kernel can update microcode in early phase of boot time. Loading microcode early can fix CPU issues before they are observed during kernel boot time. Microcode is stored in an initrd file. The microcode is read from the initrd file and loaded to CPUs during boot time. The format of the combined initrd image is microcode in cpio format followed by the initrd image (maybe compressed). Kernel parses the combined initrd image during boot time. The microcode file in cpio name space is: on Intel: kernel/x86/microcode/GenuineIntel.bin on AMD : kernel/x86/microcode/AuthenticAMD.bin During BSP boot (before SMP starts), if the kernel finds the microcode file in the initrd file, it parses the microcode and saves matching microcode in memory. If matching microcode is found, it will be uploaded in BSP and later on in all APs. The cached microcode patch is applied when CPUs resume from a sleep state. There are two legacy user space interfaces to load microcode, either through /dev/cpu/microcode or through /sys/devices/system/cpu/microcode/reload file in sysfs. In addition to these two legacy methods, the early loading method described here is the third method with which microcode can be uploaded to a system's CPUs. The following example script shows how to generate a new combined initrd file in /boot/initrd-3.5.0.ucode.img with original microcode microcode.bin and original initrd image /boot/initrd-3.5.0.img. mkdir initrd cd initrd mkdir -p kernel/x86/microcode cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin (or AuthenticAMD.bin) find . | cpio -o -H newc >../ucode.cpio cd .. cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img