--- arch/x86/kvm/x86.c

+++ 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

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

/* 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, &tsc_timestamp);

ktime_get_ts(&ts);

monotonic_to_bootbased(&ts);

+ kernel_ns = timespec_to_ns(&ts);

local_irq_restore(flags);

+ /*

+ * 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.tsc_timestamp = tsc_timestamp;

+ 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_v

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);