From 0d91ea86a895b911fd7d999acb3f600706d9c8cd Mon Sep 17 00:00:00 2001 From: Fenghua Yu Date: Thu, 20 Dec 2012 23:44:21 -0800 Subject: x86, doc: Documentation for early microcode loading Documenation for early loading microcode methodology. Signed-off-by: Fenghua Yu Link: http://lkml.kernel.org/r/1356075872-3054-2-git-send-email-fenghua.yu@intel.com Signed-off-by: H. Peter Anvin --- Documentation/x86/early-microcode.txt | 43 +++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) create mode 100644 Documentation/x86/early-microcode.txt (limited to 'Documentation') diff --git a/Documentation/x86/early-microcode.txt b/Documentation/x86/early-microcode.txt new file mode 100644 index 000000000000..4aaf0dfb0cb8 --- /dev/null +++ b/Documentation/x86/early-microcode.txt @@ -0,0 +1,43 @@ +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: +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 -- cgit v1.2.3-59-g8ed1b