Hi, i'm getting this problem with linux 4.3.0-rcX branch.
Full log is attached. My hardware is based on intel skylake i7-6700k on
z170 chipset and it is working properly. It does boot correctly with
older kernels (3.12).
With mce=off on xen parameters it does boot unti during pci detection
where it hangs (see attached log).

I tried with both xen 4.4.2 and 4.6.0 with the same effect

[    2.957615] VFS: Disk quotas d(XEN) Fatal machine check: MCE: Fatal
error happened on CPUs ff
(XEN)
(XEN) ****************************************
(XEN)
(XEN)    The processor has reported a hardware error which cannot
(XEN)    be recovered from.  Xen will now reboot the machine.
(XEN) mce.c:1533: Begin dump mc_info
(XEN) CPU0: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU0: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU1: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU1: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU2: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU2: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU3: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) CPU3: Machine Check Exception:                5
(XEN) Bank 4: ba00000011000402[               0]
(XEN) mce.c:1536: End dump mc_info, 8 mcinfo dumped
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) HARDWARE ERROR
(XEN) ****************************************
(XEN)
(XEN) Reboot in five seconds...

Any help?

Thanks,
John.