suse_vtsc_tolerance=<val>
Reference: bsc#1026236

To avoid emulation of vTSC after live migration or save/restore allow
different clock frequency up to the specified value. If the frequency
is within the allowed range TSC access by the domU will be performed
at native speed. Otherwise TSC access will be emulated. It is up to
the hostadmin to decide how much tolerance all running domUs can
actually handle. The default is zero tolerance.

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -47,6 +47,9 @@
 static char __initdata opt_clocksource[10];
 string_param("clocksource", opt_clocksource);
 
+static unsigned int __ro_after_init opt_suse_vtsc_tolerance;
+integer_param("suse_vtsc_tolerance", opt_suse_vtsc_tolerance);
+
 unsigned long __read_mostly cpu_khz;  /* CPU clock frequency in kHz. */
 DEFINE_SPINLOCK(rtc_lock);
 unsigned long pit0_ticks;
@@ -2581,6 +2584,8 @@ int tsc_set_info(struct domain *d,
 
     switch ( tsc_mode )
     {
+        bool disable_vtsc;
+
     case TSC_MODE_DEFAULT:
     case TSC_MODE_ALWAYS_EMULATE:
         d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
@@ -2594,8 +2599,25 @@ int tsc_set_info(struct domain *d,
          * When a guest is created, gtsc_khz is passed in as zero, making
          * d->arch.tsc_khz == cpu_khz. Thus no need to check incarnation.
          */
+        disable_vtsc = d->arch.tsc_khz == cpu_khz;
+
+        if ( tsc_mode == TSC_MODE_DEFAULT && !disable_vtsc &&
+             opt_suse_vtsc_tolerance && is_hvm_domain(d) )
+        {
+            long khz_diff = ABS((long)cpu_khz - gtsc_khz);
+
+            disable_vtsc = khz_diff <= opt_suse_vtsc_tolerance;
+
+            printk(XENLOG_G_INFO "%pd: host has %lu kHz,"
+                   " domU expects %u kHz,"
+                   " difference of %ld is %s tolerance of %u\n",
+                   d, cpu_khz, gtsc_khz, khz_diff,
+                   disable_vtsc ? "within" : "outside",
+                   opt_suse_vtsc_tolerance);
+        }
+
         if ( tsc_mode == TSC_MODE_DEFAULT && host_tsc_is_safe() &&
-             (d->arch.tsc_khz == cpu_khz ||
+             (disable_vtsc ||
               (is_hvm_domain(d) &&
                hvm_get_tsc_scaling_ratio(d->arch.tsc_khz))) )
         {