[Xen-changelog] [xen master] x86/APIC: reduce rounding errors in calculations

[patchbot@xxxxxxx]

commit 740ffb7fc7b9d1afa23bb38910913279c28aa991
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Mar 16 17:32:41 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Mar 16 17:32:41 2020 +0100

    x86/APIC: reduce rounding errors in calculations
    
    Dividing by HZ/10 just to subsequently multiply by HZ again in all uses
    of the respective variable is pretty pointlessly introducing rounding
    (really: truncation) errors. While transforming the respective
    expressions it became apparent that "result" would be left unused except
    for its use as function return value. As the sole caller of the function
    doesn't look at the returned value, simply convert the function to have
    "void" return type.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/apic.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index ab30de939c..cde67cd87e 100644
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1204,14 +1204,14 @@ static void wait_tick_pvh(void)
  * APIC irq that way.
  */
 
-static int __init calibrate_APIC_clock(void)
+static void __init calibrate_APIC_clock(void)
 {
     unsigned long long t1, t2;
-    unsigned long tt1, tt2, result;
+    unsigned long tt1, tt2;
     unsigned int i;
     unsigned long bus_freq; /* KAF: pointer-size avoids compile warns. */
     unsigned int bus_cycle; /* length of one bus cycle in pico-seconds */
-    const unsigned int LOOPS = HZ/10;
+#define LOOPS_FRAC 10U      /* measure for one tenth of a second */
 
     apic_printk(APIC_VERBOSE, "calibrating APIC timer ...\n");
 
@@ -1238,9 +1238,9 @@ static int __init calibrate_APIC_clock(void)
     tt1 = apic_read(APIC_TMCCT);
 
     /*
-     * Let's wait LOOPS ticks:
+     * Let's wait HZ / LOOPS_FRAC ticks:
      */
-    for (i = 0; i < LOOPS; i++)
+    for (i = 0; i < HZ / LOOPS_FRAC; i++)
         if ( !xen_guest )
             wait_8254_wraparound();
         else
@@ -1249,17 +1249,16 @@ static int __init calibrate_APIC_clock(void)
     tt2 = apic_read(APIC_TMCCT);
     t2 = rdtsc_ordered();
 
-    result = (tt1-tt2)*APIC_DIVISOR/LOOPS;
+    bus_freq = (tt1 - tt2) * APIC_DIVISOR * LOOPS_FRAC;
 
-    apic_printk(APIC_VERBOSE, "..... CPU clock speed is %ld.%04ld MHz.\n",
-                ((long)(t2 - t1) / LOOPS) / (1000000 / HZ),
-                ((long)(t2 - t1) / LOOPS) % (1000000 / HZ));
+    apic_printk(APIC_VERBOSE, "..... CPU clock speed is %lu.%04lu MHz.\n",
+                ((unsigned long)(t2 - t1) * LOOPS_FRAC) / 1000000,
+                (((unsigned long)(t2 - t1) * LOOPS_FRAC) / 100) % 10000);
 
     apic_printk(APIC_VERBOSE, "..... host bus clock speed is %ld.%04ld MHz.\n",
-                result / (1000000 / HZ), result % (1000000 / HZ));
+                bus_freq / 1000000, (bus_freq / 100) % 10000);
 
     /* set up multipliers for accurate timer code */
-    bus_freq   = result*HZ;
     bus_cycle  = 1000000000000UL / bus_freq; /* in pico seconds */
     bus_cycle += (1000000000000UL % bus_freq) * 2 > bus_freq;
     bus_scale  = (1000*262144)/bus_cycle;
@@ -1269,7 +1268,7 @@ static int __init calibrate_APIC_clock(void)
     /* reset APIC to zero timeout value */
     __setup_APIC_LVTT(0);
 
-    return result;
+#undef LOOPS_FRAC
 }
 
 void __init setup_boot_APIC_clock(void)
--
generated by git-patchbot for /home/xen/git/xen.git#master

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