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