http://www.ingmarverheij.com/damn-you-c-states-unexpected-xenserver-reboot/

http://support.citrix.com/article/CTX127395

Disable all power management in the BIOS:

• AMD Power Now
• Cool'n'Quiet
• Intel SpeedStep
• Enhanced Intel SpeedStep (EIST)

A result above 2 indicates you need to disable power management in the BIOS:

xenpm get-cpuidle-states | grep total | uniq

Disable all C-states, including C1E.

If the server BIOS has power management options that leave power management to the BIOS rather than the operating system such as Dell Active Power Controller or HP Power Regulator, also disable this by setting the power management option to OS Control.

Dell:

• Processor Settings → Processor C-State Control
• Processor Settings → Processor C1E

HP:

• Power Management Options → Advanced Power Mgmt Options → Minimum Processor Idle Power State → No C-States
• Power Management Options → Advanced Power Mgmt Options → Minimum Processor Idle Power Package State → No Package State

Intel:

• Advanced → CPU Configuration → Enhanced Intel SpeedStep Technology
• Advanced → CPU Configuration → Turbo Mode (Turbo Boost)
• Advanced → CPU Configuration → CPU C1E
• Advanced → CPU Configuration → CPU C3 Report
• Advanced → CPU Configuration → CPU C6 Report