x86/time: use relative counts in calibration loops

Looping until reaching/exceeding a certain value is error prone: If the
target value is close enough to the wrapping point, the loop may not
terminate at all. Switch to using delta values, which then allows to
fold the two loops each into just one.

Fixes: 93340297802b ("x86/time: calibrate TSC against platform timer")
Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
master commit: 467191641d2a2fd2e43b3ae7b80399f89d339980
master date: 2022-01-13 14:30:18 +0100

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 6bc1fd11d6d1c3cebce0b535c4dd4dbf78e841a0..719dc38c7b6aae153c8bfd12e3277948fd7682c6 100644 (file)
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -376,7 +376,7 @@ static u64 read_hpet_count(void)
 static int64_t __init init_hpet(struct platform_timesource *pts)
 {
     uint64_t hpet_rate, start;
-    uint32_t count, target;
+    uint32_t count, target, elapsed;
 
     if ( hpet_address && strcmp(opt_clocksource, pts->id) &&
          cpuidle_using_deep_cstate() )
@@ -414,11 +414,8 @@ static int64_t __init init_hpet(struct platform_timesource *pts)
 
     count = hpet_read32(HPET_COUNTER);
     start = rdtsc_ordered();
-    target = count + CALIBRATE_VALUE(hpet_rate);
-    if ( target < count )
-        while ( hpet_read32(HPET_COUNTER) >= count )
-            continue;
-    while ( hpet_read32(HPET_COUNTER) < target )
+    target = CALIBRATE_VALUE(hpet_rate);
+    while ( (elapsed = hpet_read32(HPET_COUNTER) - count) < target )
         continue;
 
     return (rdtsc_ordered() - start) * CALIBRATE_FRAC;
@@ -456,8 +453,8 @@ static u64 read_pmtimer_count(void)
 
 static s64 __init init_pmtimer(struct platform_timesource *pts)
 {
-    u64 start;
-    u32 count, target, mask;
+    uint64_t start;
+    uint32_t count, target, mask, elapsed;
 
     if ( !pmtmr_ioport || (pmtmr_width != 24 && pmtmr_width != 32) )
         return 0;
@@ -465,13 +462,10 @@ static s64 __init init_pmtimer(struct platform_timesource *pts)
     pts->counter_bits = pmtmr_width;
     mask = 0xffffffff >> (32 - pmtmr_width);
 
-    count = inl(pmtmr_ioport) & mask;
+    count = inl(pmtmr_ioport);
     start = rdtsc_ordered();
-    target = count + CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
-    if ( target < count )
-        while ( (inl(pmtmr_ioport) & mask) >= count )
-            continue;
-    while ( (inl(pmtmr_ioport) & mask) < target )
+    target = CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
+    while ( (elapsed = (inl(pmtmr_ioport) - count) & mask) < target )
         continue;
 
     return (rdtsc_ordered() - start) * CALIBRATE_FRAC;