x86/time: improve TSC / CPU freq calibration accuracy

While the problem report was for extreme errors, even smaller ones would
better be avoided: The calculated period to run calibration loops over
can (and usually will) be shorter than the actual time elapsed between
first and last platform timer and TSC reads. Adjust values returned from
the init functions accordingly.

On a Skylake system I've tested this on accuracy (using HPET) went from
detecting in some cases more than 220kHz too high a value to about
±2kHz. On other systems (or on this system, but with PMTMR) the original
error range was much smaller, with less (in some cases only very little)
improvement.

Reported-by: James Dingwall <james-xen@dingwall.me.uk>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/time.c

@@ -287,6 +287,23 @@ static char *freq_string(u64 freq)
     return s;
 }
 
+static uint64_t adjust_elapsed(uint64_t elapsed, uint32_t actual,
+                               uint32_t target)
+{
+    if ( likely(actual > target) )
+    {
+        /*
+         * A (perhaps significant) delay before the last timer read (e.g. due
+         * to a SMI or NMI) can lead to (perhaps severe) inaccuracy if not
+         * accounting for the time elapsed beyond the originally calculated
+         * duration of the calibration interval.
+         */
+        elapsed = muldiv64(elapsed, target, actual);
+    }
+
+    return elapsed * CALIBRATE_FRAC;
+}
+
 /************************************************************
  * PLATFORM TIMER 1: PROGRAMMABLE INTERVAL TIMER (LEGACY PIT)
  */
@@ -455,7 +472,7 @@ static int64_t __init init_hpet(struct platform_timesource *pts)
     while ( (elapsed = hpet_read32(HPET_COUNTER) - count) < target )
         continue;
 
-    return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
+    return adjust_elapsed(rdtsc_ordered() - start, elapsed, target);
 }
 
 static void resume_hpet(struct platform_timesource *pts)
@@ -505,7 +522,7 @@ static s64 __init init_pmtimer(struct platform_timesource *pts)
     while ( (elapsed = (inl(pmtmr_ioport) - count) & mask) < target )
         continue;
 
-    return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
+    return adjust_elapsed(rdtsc_ordered() - start, elapsed, target);
 }
 
 static struct platform_timesource __initdata plt_pmtimer =