diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 600807b..08bc2ff 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -357,6 +357,9 @@ struct kvm_vcpu_arch {
 	struct page *time_page;
 
 	bool singlestep; /* guest is single stepped by KVM */
+	u64 last_guest_tsc;
+	u64 last_kernel_ns;
+
 	bool nmi_pending;
 	bool nmi_injected;
 
diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index 53235fd..44b9ae7 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -11,4 +11,42 @@ void pvclock_read_wallclock(struct pvclock_wall_clock *wall,
 			    struct pvclock_vcpu_time_info *vcpu,
 			    struct timespec *ts);
 
+/*
+ * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
+ * yielding a 64-bit result.
+ */
+static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
+{
+	u64 product;
+#ifdef __i386__
+	u32 tmp1, tmp2;
+#endif
+
+	if (shift < 0)
+		delta >>= -shift;
+	else
+		delta <<= shift;
+
+#ifdef __i386__
+	__asm__ (
+		"mul  %5       ; "
+		"mov  %4,%%eax ; "
+		"mov  %%edx,%4 ; "
+		"mul  %5       ; "
+		"xor  %5,%5    ; "
+		"add  %4,%%eax ; "
+		"adc  %5,%%edx ; "
+		: "=A" (product), "=r" (tmp1), "=r" (tmp2)
+		: "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
+#elif defined(__x86_64__)
+	__asm__ (
+		"mul %%rdx ; shrd $32,%%rdx,%%rax"
+		: "=a" (product) : "0" (delta), "d" ((u64)mul_frac) );
+#else
+#error implement me!
+#endif
+
+	return product;
+}
+
 #endif /* _ASM_X86_PVCLOCK_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b2c02a2..a857d05 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -47,6 +47,7 @@
 #include <asm/desc.h>
 #include <asm/mtrr.h>
 #include <asm/mce.h>
+#include <asm/pvclock.h>
 
 #define MAX_IO_MSRS 256
 #define CR0_RESERVED_BITS						\
@@ -633,6 +634,8 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
 	struct kvm_vcpu_arch *vcpu = &v->arch;
 	void *shared_kaddr;
 	unsigned long this_tsc_khz;
+	s64 kernel_ns, max_kernel_ns;
+	u64 tsc_timestamp;
 
 	if ((!vcpu->time_page))
 		return;
@@ -646,15 +649,52 @@ static void kvm_write_guest_time(struct kvm_vcpu *v)
 
 	/* Keep irq disabled to prevent changes to the clock */
 	local_irq_save(flags);
-	kvm_get_msr(v, MSR_IA32_TSC, &vcpu->hv_clock.tsc_timestamp);
+	//kvm_get_msr(v, MSR_IA32_TSC, &vcpu->hv_clock.tsc_timestamp);
+	kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
 	ktime_get_ts(&ts);
 	monotonic_to_bootbased(&ts);
 	local_irq_restore(flags);
+	kernel_ns = timespec_to_ns(&ts);
+
+	/*
+	 * Time as measured by the TSC may go backwards when resetting the base
+	 * tsc_timestamp.  The reason for this is that the TSC resolution is
+	 * higher than the resolution of the other clock scales.  Thus, many
+	 * possible measurments of the TSC correspond to one measurement of any
+	 * other clock, and so a spread of values is possible.  This is not a
+	 * problem for the computation of the nanosecond clock; with TSC rates
+	 * around 1GHZ, there can only be a few cycles which correspond to one
+	 * nanosecond value, and any path through this code will inevitably
+	 * take longer than that.  However, with the kernel_ns value itself,
+	 * the precision may be much lower, down to HZ granularity.  If the
+	 * first sampling of TSC against kernel_ns ends in the low part of the
+	 * range, and the second in the high end of the range, we can get:
+	 *
+	 * (TSC - offset_low) * S + kns_old > (TSC - offset_high) * S + kns_new
+	 *
+	 * As the sampling errors potentially range in the thousands of cycles,
+	 * it is possible such a time value has already been observed by the
+	 * guest.  To protect against this, we must compute the system time as
+	 * observed by the guest and ensure the new system time is greater.
+	 */
+	max_kernel_ns = 0;
+	if (vcpu->hv_clock.tsc_timestamp && vcpu->last_guest_tsc) {
+		max_kernel_ns = vcpu->last_guest_tsc -
+			vcpu->hv_clock.tsc_timestamp;
+		max_kernel_ns = pvclock_scale_delta(max_kernel_ns,
+				vcpu->hv_clock.tsc_to_system_mul,
+				vcpu->hv_clock.tsc_shift);
+		max_kernel_ns += vcpu->last_kernel_ns;
+	}
+
+	if (max_kernel_ns > kernel_ns)
+		kernel_ns = max_kernel_ns;
 
 	/* With all the info we got, fill in the values */
 
-	vcpu->hv_clock.system_time = ts.tv_nsec +
-				     (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
+	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
+	vcpu->last_kernel_ns = kernel_ns;
+	vcpu->last_guest_tsc = tsc_timestamp;
 
 	/*
 	 * The interface expects us to write an even number signaling that the
@@ -3695,6 +3735,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	kvm_x86_ops->prepare_guest_switch(vcpu);
 	kvm_load_guest_fpu(vcpu);
 
+	kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
+
 	local_irq_disable();
 
 	clear_bit(KVM_REQ_KICK, &vcpu->requests);
