x86/APIC: adjust types and comments in calibrate_APIC_clock()

First and foremost the comment talking about potential underflow being
taken care of by using signed long type variables was true only on
32-bit, which we've not been supporting for quite some time. Drop the
comment and change all involved types to unsigned. Take the opportunity
and also replace bus_cycle's fixed width type.

Additionally there's no point using an "arbitrary (but long enough)
timeout" here. Just use the maximum possible value; Linux does so too,
just as an additional data point.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

diff --git a/xen/arch/x86/apic.c b/xen/arch/x86/apic.c
index a3617814569bc94a1f348505a4af95be9fcc362d..dbea826d48713d1b768ecbd2065a60c24d14b19f 100644 (file)
--- a/xen/arch/x86/apic.c
+++ b/xen/arch/x86/apic.c
@@ -1207,21 +1207,19 @@ static void wait_tick_pvh(void)
 static int __init calibrate_APIC_clock(void)
 {
     unsigned long long t1, t2;
-    long tt1, tt2;
-    long result;
-    int i;
+    unsigned long tt1, tt2, result;
+    unsigned int i;
     unsigned long bus_freq; /* KAF: pointer-size avoids compile warns. */
-    u32 bus_cycle;          /* length of one bus cycle in pico-seconds */
-    const int LOOPS = HZ/10;
+    unsigned int bus_cycle; /* length of one bus cycle in pico-seconds */
+    const unsigned int LOOPS = HZ/10;
 
     apic_printk(APIC_VERBOSE, "calibrating APIC timer ...\n");
 
     /*
-     * Put whatever arbitrary (but long enough) timeout
-     * value into the APIC clock, we just want to get the
-     * counter running for calibration.
+     * Setup the APIC counter to maximum. There is no way the lapic
+     * can underflow in the 100ms detection time frame.
      */
-    __setup_APIC_LVTT(1000000000);
+    __setup_APIC_LVTT(0xffffffff);
 
     if ( !xen_guest )
         /*
@@ -1251,14 +1249,6 @@ static int __init calibrate_APIC_clock(void)
     tt2 = apic_read(APIC_TMCCT);
     t2 = rdtsc_ordered();
 
-    /*
-     * The APIC bus clock counter is 32 bits only, it
-     * might have overflown, but note that we use signed
-     * longs, thus no extra care needed.
-     *
-     * underflown to be exact, as the timer counts down ;)
-     */
-
     result = (tt1-tt2)*APIC_DIVISOR/LOOPS;
 
     apic_printk(APIC_VERBOSE, "..... CPU clock speed is %ld.%04ld MHz.\n",