x86, doc: Documentation for early microcode loading

Documenation for early loading microcode methodology.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Link: http://lkml.kernel.org/r/1356075872-3054-2-git-send-email-fenghua.yu@intel.com


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

Documentation/x86/early-microcode.txt

+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:
+kernel/x86/microcode/GenuineIntel.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 kernel
+mkdir kernel/x86
+mkdir kernel/x86/microcode
+cp ../microcode.bin kernel/x86/microcode/GenuineIntel.bin
+find .|cpio -oc >../ucode.cpio
+cd ..
+cat ucode.cpio /boot/initrd-3.5.0.img >/boot/initrd-3.5.0.ucode.img