diff -r 640f8b15b9dd arch/i386/kernel/cpu/cpufreq/powernow-k8.c
--- a/arch/i386/kernel/cpu/cpufreq/powernow-k8.c	Fri Apr  7 01:32:54 2006 +0100
+++ b/arch/i386/kernel/cpu/cpufreq/powernow-k8.c	Mon Apr 10 19:33:49 2006 -0400
@@ -48,6 +48,37 @@
 #define VERSION "version 1.60.0"
 #include "powernow-k8.h"
 
+/* Xen support */
+
+#ifdef CONFIG_XEN_PRIVILEGED_GUEST
+int xen_access_msr(u32 msr, int write, u32* out1, u32* out2, u32 in1, u32 in2) {
+	dom0_op_t op;
+	op.cmd = DOM0_MSR;
+  op.u.msr.write = write;
+  op.u.msr.cpu_mask = 1; /* only first CPU: not clear how to read multiple CPUs */
+  op.u.msr.msr = msr;
+  op.u.msr.in1 = in1;
+  op.u.msr.in2 = in2;
+	BUG_ON(HYPERVISOR_dom0_op(&op));
+
+  if (!write) {
+    *out1 = op.u.msr.out1; /* low 32 bits */
+    *out2 = op.u.msr.out2; /* high 32 bits */
+  }
+
+  return 0;
+}
+
+#define cpu_rdmsr(msr, val1, val2) xen_access_msr((msr), 0, &(val1), &(val2), 0, 0)
+#define cpu_wrmsr(msr, val1, val2) xen_access_msr((msr), 1, NULL, NULL, (val1), (val2))
+
+#else
+
+#define cpu_rdmsr(msr, val1, val2) rdmsr(msr, val1, val2)
+#define cpu_wrmsr(msr, val1, val2) wrmsr(msr, val1, val2)
+
+#endif
+
 /* serialize freq changes  */
 static DECLARE_MUTEX(fidvid_sem);
 
@@ -98,7 +129,7 @@ static int pending_bit_stuck(void)
 {
 	u32 lo, hi;
 
-	rdmsr(MSR_FIDVID_STATUS, lo, hi);
+	cpu_rdmsr(MSR_FIDVID_STATUS, lo, hi);
 	return lo & MSR_S_LO_CHANGE_PENDING ? 1 : 0;
 }
 
@@ -116,7 +147,7 @@ static int query_current_values_with_pen
 			dprintk("detected change pending stuck\n");
 			return 1;
 		}
-		rdmsr(MSR_FIDVID_STATUS, lo, hi);
+		cpu_rdmsr(MSR_FIDVID_STATUS, lo, hi);
 	} while (lo & MSR_S_LO_CHANGE_PENDING);
 
 	data->currvid = hi & MSR_S_HI_CURRENT_VID;
@@ -145,13 +176,13 @@ static void fidvid_msr_init(void)
 	u32 lo, hi;
 	u8 fid, vid;
 
-	rdmsr(MSR_FIDVID_STATUS, lo, hi);
+	cpu_rdmsr(MSR_FIDVID_STATUS, lo, hi);
 	vid = hi & MSR_S_HI_CURRENT_VID;
 	fid = lo & MSR_S_LO_CURRENT_FID;
 	lo = fid | (vid << MSR_C_LO_VID_SHIFT);
 	hi = MSR_C_HI_STP_GNT_BENIGN;
 	dprintk("cpu%d, init lo 0x%x, hi 0x%x\n", smp_processor_id(), lo, hi);
-	wrmsr(MSR_FIDVID_CTL, lo, hi);
+	cpu_wrmsr(MSR_FIDVID_CTL, lo, hi);
 }
 
 
@@ -173,7 +204,7 @@ static int write_new_fid(struct powernow
 		fid, lo, data->plllock * PLL_LOCK_CONVERSION);
 
 	do {
-		wrmsr(MSR_FIDVID_CTL, lo, data->plllock * PLL_LOCK_CONVERSION);
+		cpu_wrmsr(MSR_FIDVID_CTL, lo, data->plllock * PLL_LOCK_CONVERSION);
 		if (i++ > 100) {
 			printk(KERN_ERR PFX "internal error - pending bit very stuck - no further pstate changes possible\n");
 			return 1;
@@ -215,7 +246,7 @@ static int write_new_vid(struct powernow
 		vid, lo, STOP_GRANT_5NS);
 
 	do {
-		wrmsr(MSR_FIDVID_CTL, lo, STOP_GRANT_5NS);
+		cpu_wrmsr(MSR_FIDVID_CTL, lo, STOP_GRANT_5NS);
 		if (i++ > 100) {
 			printk(KERN_ERR PFX "internal error - pending bit very stuck - no further pstate changes possible\n");
 			return 1;
@@ -294,7 +325,7 @@ static int core_voltage_pre_transition(s
 		smp_processor_id(),
 		data->currfid, data->currvid, reqvid, data->rvo);
 
-	rdmsr(MSR_FIDVID_STATUS, lo, maxvid);
+	cpu_rdmsr(MSR_FIDVID_STATUS, lo, maxvid);
 	maxvid = 0x1f & (maxvid >> 16);
 	dprintk("ph1 maxvid=0x%x\n", maxvid);
 	if (reqvid < maxvid) /* lower numbers are higher voltages */
@@ -892,6 +923,19 @@ static int transition_frequency(struct p
 
 	res = transition_fid_vid(data, fid, vid);
 
+#ifdef CONFIG_XEN_PRIVILEGED_GUEST
+  {
+    dom0_op_t op;
+    int rc;
+    // printk("powernow-k8: notifying Xen of transition to %d khz on cpu %d\n", freqs.new, freqs.cpu);
+    op.cmd = DOM0_SETCPUFREQ;
+    op.u.setcpufreq.cpumap = (1 << freqs.cpu);
+    op.u.setcpufreq.khz = freqs.new;
+    rc = HYPERVISOR_dom0_op(&op);
+    // printk("powernow-k8: notified Xen of transition to %d khz on cpu %d (rc %d)\n", freqs.new, freqs.cpu, rc);
+  }
+#endif
+
 	freqs.new = find_khz_freq_from_fid(data->currfid);
 	for_each_cpu_mask(i, cpu_core_map[data->cpu]) {
 		freqs.cpu = i;
diff -r 640f8b15b9dd include/xen/interface/dom0_ops.h
--- a/include/xen/interface/dom0_ops.h	Fri Apr  7 01:32:54 2006 +0100
+++ b/include/xen/interface/dom0_ops.h	Mon Apr 10 19:33:49 2006 -0400
@@ -283,6 +283,28 @@ typedef struct dom0_getpageframeinfo2 {
     GUEST_HANDLE(ulong) array;
 } dom0_getpageframeinfo2_t;
 DEFINE_GUEST_HANDLE(dom0_getpageframeinfo2_t);
+
+/*
+ * Notify hypervisor of a CPU core frequency change completed
+ * by cpufreq driver in dom0, triggering an internal timer
+ * recalibration.
+ *
+ * This should be called after the new frequency has stabilized.
+ *
+ * The CPUs specified may include any CPU or core (if cores have
+ * independent PLLs). In an SMP or multi-core system, it may
+ * take a while for the recalibration function to be scheduled
+ * on the intended target CPUs; there is no guarantee this will
+ * happen by the time this call returns.
+ *
+ */
+#define DOM0_SETCPUFREQ       30
+typedef struct dom0_setcpufreq {
+    /* IN variables */
+    cpumap_t cpumap;
+    unsigned long khz;
+} dom0_setcpufreq_t;
+DEFINE_GUEST_HANDLE(dom0_setcpufreq_t);
 
 /*
  * Request memory range (@mfn, @mfn+@nr_mfns-1) to have type @type.
@@ -496,6 +518,7 @@ typedef struct dom0_op {
         struct dom0_shadow_control    shadow_control;
         struct dom0_setdomainmaxmem   setdomainmaxmem;
         struct dom0_getpageframeinfo2 getpageframeinfo2;
+        struct dom0_setcpufreq        setcpufreq;
         struct dom0_add_memtype       add_memtype;
         struct dom0_del_memtype       del_memtype;
         struct dom0_read_memtype      read_memtype;