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 505e54ebd7c34c67309d11e7273a66647743b01c..22352fa5b10ec1e087d8e80b86f72e27c4945f2d 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;