Created attachment 186851 [details]
dmesg output after starting the VM

With kernel 4.2, starting one of my VMs instantly freezes the host system and creates Machine Check Exceptions on CPUs dedicated to that particula VM:

[12316.171917] mce: [Hardware Error]: CPU 3: Machine Check Exception: 5 Bank 17: be2000000003110a
[12316.171917] mce: [Hardware Error]: RIP !INEXACT! 10:<ffffffff813217fd> {intel_idle+0xbd/0x120}
[12316.171917] mce: [Hardware Error]: TSC 76fd7352bf6 ADDR fa137140 MISC 30f0083884509086 
[12316.171917] mce: [Hardware Error]: PROCESSOR 0:306f2 TIME 1441130705 SOCKET 0 APIC 6 microcode 2d
[12316.171917] mce: [Hardware Error]: Run the above through 'mcelog --ascii'
...

A bisection revealed that commit fd717f11015f673487ffc826e59b2bad69d20fe5 introduced the problem:

KVM: x86: apply guest MTRR virtualization on host reserved pages

Currently guest MTRR is avoided if kvm_is_reserved_pfn returns true.
However, the guest could prefer a different page type than UC for
such pages. A good example is that pass-throughed VGA frame buffer is
not always UC as host expected.

This patch enables full use of virtual guest MTRRs.

One could argue that the following warning is an obvious hint
[12311.584431] pmd_set_huge: Cannot satisfy [mem 0x383fe0000000-0x383fe0200000] with a huge-page mapping due to MTRR override.

but I'm able to run another VM without problems despite that warning.

Please let me know I you need additional information.