File 61e0296a-x86-time-calibration-relative-counts.patch of Package xen

# Commit 467191641d2a2fd2e43b3ae7b80399f89d339980
# Date 2022-01-13 14:30:18 +0100
# Author Jan Beulich <jbeulich@suse.com>
# Committer Jan Beulich <jbeulich@suse.com>
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>

--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -378,7 +378,7 @@ 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;
     /*
      * 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
@@ -451,11 +451,8 @@ static int64_t __init init_hpet(struct p
 
     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;
@@ -493,8 +490,8 @@ 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;
@@ -502,13 +499,10 @@ static s64 __init init_pmtimer(struct pl
     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;