Comment 1 for bug 1694561

It appears a kernel panic is happening immediately while attempting to boot 4.12-rc3, it doesn't happen on 4.12-rc2.
Adding earlyprintk=efi to the grub command line options, I get the following message (which explains why the bootloader stops at 'loading initial ramdisk...'):

This was meticulously copied from a photo of a GRUB screen, this change is undoubtedly the culprit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/diff/arch/x86/mm/pat.c?id=v4.12-rc3&id2=v4.12-rc2
(direct commit at)
https://github.com/torvalds/linux/commit/cbed27cdf0e3f7ea3b2259e86b9e34df02be3fe4

[ 0.000000] Kernel panic - not syncing: x86/PAT: PAT enabled, but not supported by secondary CPU
[ 0.000000] CPU: 0 PID: 0 Comm: swapper Not tainted 4.12.0-041200rc3-generic #201705282131
[ 0.000000] Hardware name: HP HP Spectre x360 Convertible /802D, BIOS F.45 04/21/2017
[ 0.000000] Call Trace:
[ 0.000000] dump_stack+0x63/0x8d
[ 0.000000] panic+0xe4/0x22d
[ 0.000000] pat_init+0x13d/0x140
[ 0.000000] generic_set_all+0x1ab/0x300
[ 0.000000] mtrr_bp_init+0x1e5/0x227
[ 0.000000] setup_arch+0x53a/0xca8
[ 0.000000] ? printk+0x52/0x6e
[ 0.000000] ? early_idt_handler_array+0x120/0x120
[ 0.000000] start_kernel+0xb7/0x46c
[ 0.000000] ? early_idt_handler_array+0x120/0x120
[ 0.000000] x86_64_start_reservations+0x29/0x2b
[ 0.000000] x86_64_start_kernel+0x154/0x177
[ 0.000000] secondary_startup_64+0x9f/0x9f
[ 0.000000] ---[ end Kernel panic - not syncing: x86/PAT: PAT enabled, but not supported by secondary CPU
[ 0.000000]

This change causes a kernel panic on my Intel 5500U processor, I literally cannot boot my computer. This commit results in a serious regression for multicore Intel CPUs, 4.12-rc3 and drm-tip result in an immediate kernel panic attempting to boot. This is unacceptable.