diff --git a/arch/ia64/include/asm/xen/interface.h b/arch/ia64/include/asm/xen/interface.h
index fbb519828aa1..09d5f7fd9db1 100644
--- a/arch/ia64/include/asm/xen/interface.h
+++ b/arch/ia64/include/asm/xen/interface.h
@@ -77,6 +77,7 @@ DEFINE_GUEST_HANDLE(int);
 DEFINE_GUEST_HANDLE(long);
 DEFINE_GUEST_HANDLE(void);
 DEFINE_GUEST_HANDLE(uint64_t);
+DEFINE_GUEST_HANDLE(uint32_t);
 
 typedef unsigned long xen_pfn_t;
 DEFINE_GUEST_HANDLE(xen_pfn_t);
diff --git a/arch/x86/include/asm/xen/interface.h b/arch/x86/include/asm/xen/interface.h
index a1f2db5f1170..cbf0c9d50b92 100644
--- a/arch/x86/include/asm/xen/interface.h
+++ b/arch/x86/include/asm/xen/interface.h
@@ -56,6 +56,7 @@ DEFINE_GUEST_HANDLE(int);
 DEFINE_GUEST_HANDLE(long);
 DEFINE_GUEST_HANDLE(void);
 DEFINE_GUEST_HANDLE(uint64_t);
+DEFINE_GUEST_HANDLE(uint32_t);
 #endif
 
 #ifndef HYPERVISOR_VIRT_START
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 312c9e3cb635..fe06bf4ef0e3 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -62,6 +62,15 @@
 #include <asm/reboot.h>
 #include <asm/stackprotector.h>
 #include <asm/hypervisor.h>
+#include <asm/mwait.h>
+
+#ifdef CONFIG_ACPI
+#include <linux/acpi.h>
+#include <asm/acpi.h>
+#include <acpi/pdc_intel.h>
+#include <acpi/processor.h>
+#include <xen/interface/platform.h>
+#endif
 
 #include "xen-ops.h"
 #include "mmu.h"
@@ -200,13 +209,17 @@ static void __init xen_banner(void)
 static __read_mostly unsigned int cpuid_leaf1_edx_mask = ~0;
 static __read_mostly unsigned int cpuid_leaf1_ecx_mask = ~0;
 
+static __read_mostly unsigned int cpuid_leaf1_ecx_set_mask;
+static __read_mostly unsigned int cpuid_leaf5_ecx_val;
+static __read_mostly unsigned int cpuid_leaf5_edx_val;
+
 static void xen_cpuid(unsigned int *ax, unsigned int *bx,
 		      unsigned int *cx, unsigned int *dx)
 {
 	unsigned maskebx = ~0;
 	unsigned maskecx = ~0;
 	unsigned maskedx = ~0;
-
+	unsigned setecx = 0;
 	/*
 	 * Mask out inconvenient features, to try and disable as many
 	 * unsupported kernel subsystems as possible.
@@ -214,9 +227,18 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
 	switch (*ax) {
 	case 1:
 		maskecx = cpuid_leaf1_ecx_mask;
+		setecx = cpuid_leaf1_ecx_set_mask;
 		maskedx = cpuid_leaf1_edx_mask;
 		break;
 
+	case CPUID_MWAIT_LEAF:
+		/* Synthesize the values.. */
+		*ax = 0;
+		*bx = 0;
+		*cx = cpuid_leaf5_ecx_val;
+		*dx = cpuid_leaf5_edx_val;
+		return;
+
 	case 0xb:
 		/* Suppress extended topology stuff */
 		maskebx = 0;
@@ -232,9 +254,75 @@ static void xen_cpuid(unsigned int *ax, unsigned int *bx,
 
 	*bx &= maskebx;
 	*cx &= maskecx;
+	*cx |= setecx;
 	*dx &= maskedx;
+
 }
 
+static bool __init xen_check_mwait(void)
+{
+#ifdef CONFIG_ACPI
+	struct xen_platform_op op = {
+		.cmd			= XENPF_set_processor_pminfo,
+		.u.set_pminfo.id	= -1,
+		.u.set_pminfo.type	= XEN_PM_PDC,
+	};
+	uint32_t buf[3];
+	unsigned int ax, bx, cx, dx;
+	unsigned int mwait_mask;
+
+	/* We need to determine whether it is OK to expose the MWAIT
+	 * capability to the kernel to harvest deeper than C3 states from ACPI
+	 * _CST using the processor_harvest_xen.c module. For this to work, we
+	 * need to gather the MWAIT_LEAF values (which the cstate.c code
+	 * checks against). The hypervisor won't expose the MWAIT flag because
+	 * it would break backwards compatibility; so we will find out directly
+	 * from the hardware and hypercall.
+	 */
+	if (!xen_initial_domain())
+		return false;
+
+	ax = 1;
+	cx = 0;
+
+	native_cpuid(&ax, &bx, &cx, &dx);
+
+	mwait_mask = (1 << (X86_FEATURE_EST % 32)) |
+		     (1 << (X86_FEATURE_MWAIT % 32));
+
+	if ((cx & mwait_mask) != mwait_mask)
+		return false;
+
+	/* We need to emulate the MWAIT_LEAF and for that we need both
+	 * ecx and edx. The hypercall provides only partial information.
+	 */
+
+	ax = CPUID_MWAIT_LEAF;
+	bx = 0;
+	cx = 0;
+	dx = 0;
+
+	native_cpuid(&ax, &bx, &cx, &dx);
+
+	/* Ask the Hypervisor whether to clear ACPI_PDC_C_C2C3_FFH. If so,
+	 * don't expose MWAIT_LEAF and let ACPI pick the IOPORT version of C3.
+	 */
+	buf[0] = ACPI_PDC_REVISION_ID;
+	buf[1] = 1;
+	buf[2] = (ACPI_PDC_C_CAPABILITY_SMP | ACPI_PDC_EST_CAPABILITY_SWSMP);
+
+	set_xen_guest_handle(op.u.set_pminfo.pdc, buf);
+
+	if ((HYPERVISOR_dom0_op(&op) == 0) &&
+	    (buf[2] & (ACPI_PDC_C_C1_FFH | ACPI_PDC_C_C2C3_FFH))) {
+		cpuid_leaf5_ecx_val = cx;
+		cpuid_leaf5_edx_val = dx;
+	}
+	return true;
+#else
+	return false;
+#endif
+}
 static void __init xen_init_cpuid_mask(void)
 {
 	unsigned int ax, bx, cx, dx;
@@ -261,6 +349,9 @@ static void __init xen_init_cpuid_mask(void)
 	/* Xen will set CR4.OSXSAVE if supported and not disabled by force */
 	if ((cx & xsave_mask) != xsave_mask)
 		cpuid_leaf1_ecx_mask &= ~xsave_mask; /* disable XSAVE & OSXSAVE */
+
+	if (xen_check_mwait())
+		cpuid_leaf1_ecx_set_mask = (1 << (X86_FEATURE_MWAIT % 32));
 }
 
 static void xen_set_debugreg(int reg, unsigned long val)
diff --git a/include/xen/interface/platform.h b/include/xen/interface/platform.h
index c1684680431b..861b359b44aa 100644
--- a/include/xen/interface/platform.h
+++ b/include/xen/interface/platform.h
@@ -200,7 +200,7 @@ DEFINE_GUEST_HANDLE_STRUCT(xenpf_getidletime_t);
 #define XEN_PM_CX   0
 #define XEN_PM_PX   1
 #define XEN_PM_TX   2
-
+#define XEN_PM_PDC  3
 /* Px sub info type */
 #define XEN_PX_PCT   1
 #define XEN_PX_PSS   2
@@ -293,6 +293,7 @@ struct xenpf_set_processor_pminfo {
 	union {
 		struct xen_processor_power          power;/* Cx: _CST/_CSD */
 		struct xen_processor_performance    perf; /* Px: _PPC/_PCT/_PSS/_PSD */
+		GUEST_HANDLE(uint32_t)              pdc;
 	};
 };
 DEFINE_GUEST_HANDLE_STRUCT(xenpf_set_processor_pminfo);