Commit 0d91ea86 authored by Fenghua Yu's avatar Fenghua Yu Committed by H. Peter Anvin

x86, doc: Documentation for early microcode loading

Documenation for early loading microcode methodology.
Signed-off-by: default avatarFenghua Yu <>
Link: default avatarH. Peter Anvin <>
parent ac2cbab2
Early load microcode
By Fenghua Yu <>
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:
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
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
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
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