diff --git a/sys/x86/include/specialreg.h b/sys/x86/include/specialreg.h
--- a/sys/x86/include/specialreg.h
+++ b/sys/x86/include/specialreg.h
@@ -904,6 +904,9 @@
 /* MSR IA32_MCU_OPT_CTRL */
 #define	IA32_RNGDS_MITG_DIS	0x00000001
 
+/* MSR IA32_PM_ENABLE */
+#define	IA32_PM_ENABLE_HWP_ENABLE	(1ULL << 0)
+
 /* MSR IA32_HWP_CAPABILITIES */
 #define	IA32_HWP_CAPABILITIES_HIGHEST_PERFORMANCE(x)	(((x) >> 0) & 0xff)
 #define	IA32_HWP_CAPABILITIES_GUARANTEED_PERFORMANCE(x)	(((x) >> 8) & 0xff)