x86/time: further improve TSC / CPU freq calibration accuracy

Calibration logic assumes that the platform timer (HPET or ACPI PM
timer) and the TSC are read at about the same time. This assumption may
not hold when a long latency event (e.g. SMI or NMI) occurs between the
two reads. Reduce the risk of reading uncorrelated values by doing at
least four pairs of reads, using the tuple where the delta between the
enclosing TSC reads was smallest. From the fourth iteration onwards bail
if the new TSC delta isn't better (smaller) than the best earlier one.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/time.c

@@ -289,9 +289,47 @@ static char *freq_string(u64 freq)
     return s;
 }
 
-static uint64_t adjust_elapsed(uint64_t elapsed, uint32_t actual,
-                               uint32_t target)
+static uint32_t __init read_pt_and_tsc(uint64_t *tsc,
+                                       const struct platform_timesource *pts)
 {
+    uint64_t tsc_prev = *tsc = rdtsc_ordered(), tsc_min = ~0;
+    uint32_t best = best;
+    unsigned int i;
+
+    for ( i = 0; ; ++i )
+    {
+        uint32_t pt = pts->read_counter();
+        uint64_t tsc_cur = rdtsc_ordered();
+        uint64_t tsc_delta = tsc_cur - tsc_prev;
+
+        if ( tsc_delta < tsc_min )
+        {
+            tsc_min = tsc_delta;
+            *tsc = tsc_cur;
+            best = pt;
+        }
+        else if ( i > 2 )
+            break;
+
+        tsc_prev = tsc_cur;
+    }
+
+    return best;
+}
+
+static uint64_t __init calibrate_tsc(const struct platform_timesource *pts)
+{
+    uint64_t start, end, elapsed;
+    unsigned int count = read_pt_and_tsc(&start, pts);
+    unsigned int target = CALIBRATE_VALUE(pts->frequency), actual;
+    unsigned int mask = (uint32_t)~0 >> (32 - pts->counter_bits);
+
+    while ( ((pts->read_counter() - count) & mask) < target )
+        continue;
+
+    actual = (read_pt_and_tsc(&end, pts) - count) & mask;
+    elapsed = end - start;
+
     if ( likely(actual > target) )
     {
         /*
@@ -397,8 +435,7 @@ static u64 cf_check read_hpet_count(void)
 
 static int64_t __init cf_check init_hpet(struct platform_timesource *pts)
 {
-    uint64_t hpet_rate, start;
-    uint32_t count, target, elapsed;
+    uint64_t hpet_rate;
     /*
      * Allow HPET to be setup, but report a frequency of 0 so it's not selected
      * as a timer source. This is required so it can be used in legacy
@@ -469,13 +506,7 @@ static int64_t __init cf_check init_hpet(struct platform_timesource *pts)
 
     pts->frequency = hpet_rate;
 
-    count = hpet_read32(HPET_COUNTER);
-    start = rdtsc_ordered();
-    target = CALIBRATE_VALUE(hpet_rate);
-    while ( (elapsed = hpet_read32(HPET_COUNTER) - count) < target )
-        continue;
-
-    return adjust_elapsed(rdtsc_ordered() - start, elapsed, target);
+    return calibrate_tsc(pts);
 }
 
 static void cf_check resume_hpet(struct platform_timesource *pts)
@@ -510,22 +541,12 @@ static u64 cf_check read_pmtimer_count(void)
 
 static s64 __init cf_check init_pmtimer(struct platform_timesource *pts)
 {
-    uint64_t start;
-    uint32_t count, target, mask, elapsed;
-
     if ( !pmtmr_ioport || (pmtmr_width != 24 && pmtmr_width != 32) )
         return 0;
 
     pts->counter_bits = pmtmr_width;
-    mask = 0xffffffff >> (32 - pmtmr_width);
 
-    count = inl(pmtmr_ioport);
-    start = rdtsc_ordered();
-    target = CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
-    while ( (elapsed = (inl(pmtmr_ioport) - count) & mask) < target )
-        continue;
-
-    return adjust_elapsed(rdtsc_ordered() - start, elapsed, target);
+    return calibrate_tsc(pts);
 }
 
 static struct platform_timesource __initdata_cf_clobber plt_pmtimer =