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[GUIDE] Patching the kernel for Haswell CPUs, XCPM early reboot

05 November 2014 - 02:24 PM


The first version of OS X to support Haswell CPUs was Mountain Lion 10.8.5. At the same time, for certain CPUs including Haswell, Apple moved CPU power management from AppleIntelCPUPowerManagement.kext to the kernel. This technology is known as XCPM (Xnu CPU Power Management). This technology is enabled for Haswell CPUs automatically and can optionally be use with Ivy Bridge CPUs with the -xcpm kernel flag.

The problem is Apple's computers do not have a locked MSR 0xE2, yet most PC laptops do. When the kernel tries to write to this locked register, it causes a panic. And this panic happens so early in the process that it results in a reboot. Similar to how we must patch AppleIntelCPUPowerManagement.kext for locked CPU MSR 0xE2, the kernel must also be patched for CPUs using XCPM to avoid this reboot.

Background information can be found here: http://www.insanelym...063cl-i7-4700mq

How to patch the kernel

After creating your USB installer, you should have the "vanilla" kernel. Here's how to patch it:

For Yosemite 10.10.x:

# in Terminal, assuming your USB is called Installer
cp /Volumes/Installer/System/Library/Kernels/kernel ~/Desktop/kernel_backup
cp /Volumes/Installer/System/Library/Kernels/kernel ~/Desktop/kernel
cd ~/Desktop
# now patch the kernel at ~/Desktop/kernel
perl -pi -e 's|\xe2\x00\x00\x00\x02\x00\x00\x00|\x00\x00\x00\x00\x00\x00\x00\x00|g' kernel
perl -pi -e 's|\xe2\x00\x00\x00\x4c\x00\x00\x00|\x00\x00\x00\x00\x00\x00\x00\x00|g' kernel
perl -pi -e 's|\xe2\x00\x00\x00\x90\x01\x00\x00|\x00\x00\x00\x00\x00\x00\x00\x00|g' kernel
# patched kernel is now at ~/Desktop/kernel
sudo cp kernel /Volumes/Installer/System/Library/Kernels/kernel
After you install OS X to your HDD, you will have vanilla kernel on the HDD, leading to the same instant reboot problem. So, you must re-enter the OS X installer, and go into Terminal (using Utilities -> Terminal) and replace the kernel with the patched version:
# in Terminal from the OS X installer, assuming your HDD volume is called "Macintosh HD"
cp /System/Library/Kernels/kernel "/Volumes/Macintosh HD/System/Library/Kernels/kernel"
After that, you can exit the installer, and boot into the installed partition to finish installation and reach the desktop.

MD5 sums for the kernel

Check your md5 sums with:
md5 /Volumes/Installer/System/Library/Kernels/kernel
10.10/10.10.1 kernel
vanilla: 9f1768fddec369e545d938f0f4f76350
xcpm patched: 1938ebb0bd6cdc00930543025d3df887

10.10.2 kernel
vanilla: 8a985412c42c153601580fd1b8bc360e
xcpm patched: cc46457172315aea68d8e1f957d2746c

A note about Clover

The Clover bootloader has this patch built-in with the config.plist/KernelAndKextPatches/KernelPm option.

Keep in mind if you're not using Clover (and KernelPm), you will have to re-patch the (new) kernel after any OS X update. With Clover, it is automatic provided the code in Clover is updated (if necessary) to patch the (new) kernel.

Update 2015-01-26

For details see the blog article: https://racerrehabma...r-early-reboot/
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