Which processors support MONITOR/MWAIT at CPL > 0? Is that information documented anywhere? More specifically, on Linux is there something special that needs to be done to enable MONITOR/MWAIT at user privilege level? An application on Linux reports CPUID(1).ecx[3] == 1 on Nehalem (Xeon X5570 Dell r610) yet MONITOR/MWAIT result in # UD exception. Documentation on this is ambiguous.
http://www.intel.com/Assets/PDF/manual/253668.pdf http://www.intel.com/Assets/PDF/manual/253668.pdf
Intel® 64 and IA-32 Architectures
Software Developer's Manual
Volume 3A: System Programming Guide, Part 1
Order Number: 253668-033US
December 2009
14.4 MWAIT EXTENSIONS FOR ADVANCED POWER
MANAGEMENT
Page 14-10 Vol. 3
Says
At CPL=0, system software can specify desired C-state and sub C-state by using the
MWAIT hints register (EAX). Processors will not go to C-state and sub C-state deeper
than what is specified by the hint register. If CPL>0 and if MONITOR/MWAIT is
supported at CPL>0, the processor will only enter C1-state (regardless of the
C-state request in the hints register).
Executing MWAIT generates an exception on processors operating at a privilege level
where MONITOR/MWAIT are not supported.
Thanks.
