x86/APIC: calibrate against platform timer when possible

Use the original calibration against PIT only when the platform timer
is PIT. This implicitly excludes the "xen_guest" case from using the PIT
logic (init_pit() fails there, and as of 5e73b2594c ["x86/time: minor
adjustments to init_pit()"] using_pit also isn't being set too early
anymore), so the respective hack there can be dropped at the same time.
This also reduces calibration time from 100ms to 50ms, albeit this step
is being skipped as of 0731a56c7c ("x86/APIC: no need for timer
calibration when using TDT") anyway.

While re-indenting the PIT logic in calibrate_APIC_clock(), besides
adjusting style also switch around the 2nd TSC/TMCCT read pair, to match
the order of the 1st one, yielding more consistent deltas.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/arch/x86/apic.c

@@ -1183,18 +1183,20 @@ static void __init check_deadline_errata(void)
            "please update microcode to version %#x (or later)\n", rev);
 }
 
-static void __init wait_tick_pvh(void)
+uint32_t __init apic_tmcct_read(void)
 {
-    u64 lapse_ns = 1000000000ULL / HZ;
-    s_time_t start, curr_time;
+    if ( x2apic_enabled )
+    {
+        /*
+         * Have a barrier here just like in rdtsc_ordered() as it's
+         * unclear whether this non-serializing RDMSR also can be
+         * executed speculatively (like RDTSC can).
+         */
+        alternative("lfence", "mfence", X86_FEATURE_MFENCE_RDTSC);
+        return apic_rdmsr(APIC_TMCCT);
+    }
 
-    start = NOW();
-
-    /* Won't wrap around */
-    do {
-        cpu_relax();
-        curr_time = NOW();
-    } while ( curr_time - start < lapse_ns );
+    return apic_mem_read(APIC_TMCCT);
 }
 
 /*
@@ -1212,9 +1214,6 @@ static void __init wait_tick_pvh(void)
 
 static void __init calibrate_APIC_clock(void)
 {
-    unsigned long long t1, t2;
-    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 */
 #define LOOPS_FRAC 10U      /* measure for one tenth of a second */
@@ -1227,39 +1226,38 @@ static void __init calibrate_APIC_clock(void)
      */
     __setup_APIC_LVTT(0xffffffff);
 
-    if ( !xen_guest )
+    bus_freq = calibrate_apic_timer();
+    if ( !bus_freq )
+    {
+        unsigned int i, tt1, tt2;
+        unsigned long t1, t2;
+
+        ASSERT(!xen_guest);
+
         /*
-         * The timer chip counts down to zero. Let's wait
-         * for a wraparound to start exact measurement:
-         * (the current tick might have been already half done)
+         * The timer chip counts down to zero. Let's wait for a wraparound to
+         * start exact measurement (the current tick might have been already
+         * half done):
          */
         wait_8254_wraparound();
-    else
-        wait_tick_pvh();
 
-    /*
-     * We wrapped around just now. Let's start:
-     */
-    t1 = rdtsc_ordered();
-    tt1 = apic_read(APIC_TMCCT);
+        /* We wrapped around just now. Let's start: */
+        t1 = rdtsc_ordered();
+        tt1 = apic_read(APIC_TMCCT);
 
-    /*
-     * Let's wait HZ / LOOPS_FRAC ticks:
-     */
-    for (i = 0; i < HZ / LOOPS_FRAC; i++)
-        if ( !xen_guest )
+        /* Let's wait HZ / LOOPS_FRAC ticks: */
+        for ( i = 0; i < HZ / LOOPS_FRAC; ++i )
             wait_8254_wraparound();
-        else
-            wait_tick_pvh();
 
-    tt2 = apic_read(APIC_TMCCT);
-    t2 = rdtsc_ordered();
+        t2 = rdtsc_ordered();
+        tt2 = apic_read(APIC_TMCCT);
 
-    bus_freq = (tt1 - tt2) * APIC_DIVISOR * LOOPS_FRAC;
+        bus_freq = (tt1 - tt2) * APIC_DIVISOR * LOOPS_FRAC;
 
-    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, "..... CPU clock speed is %lu.%04lu MHz.\n",
+                    ((t2 - t1) * LOOPS_FRAC) / 1000000,
+                    (((t2 - t1) * LOOPS_FRAC) / 100) % 10000);
+    }
 
     apic_printk(APIC_VERBOSE, "..... host bus clock speed is %ld.%04ld MHz.\n",
                 bus_freq / 1000000, (bus_freq / 100) % 10000);

xen/arch/x86/include/asm/apic.h

@@ -192,6 +192,9 @@ extern void record_boot_APIC_mode(void);
 extern enum apic_mode current_local_apic_mode(void);
 extern void check_for_unexpected_msi(unsigned int vector);
 
+uint64_t calibrate_apic_timer(void);
+uint32_t apic_tmcct_read(void);
+
 extern void check_nmi_watchdog(void);
 
 extern unsigned int nmi_watchdog;

xen/arch/x86/time.c

@@ -26,6 +26,7 @@
 #include <xen/symbols.h>
 #include <xen/keyhandler.h>
 #include <xen/guest_access.h>
+#include <asm/apic.h>
 #include <asm/io.h>
 #include <asm/iocap.h>
 #include <asm/msr.h>
@@ -1018,6 +1019,70 @@ static u64 __init init_platform_timer(void)
     return rc;
 }
 
+static uint64_t __init read_pt_and_tmcct(uint32_t *tmcct)
+{
+    uint32_t tmcct_prev = *tmcct = apic_tmcct_read(), tmcct_min = ~0;
+    uint64_t best = best;
+    unsigned int i;
+
+    for ( i = 0; ; ++i )
+    {
+        uint64_t pt = plt_src.read_counter();
+        uint32_t tmcct_cur = apic_tmcct_read();
+        uint32_t tmcct_delta = tmcct_prev - tmcct_cur;
+
+        if ( tmcct_delta < tmcct_min )
+        {
+            tmcct_min = tmcct_delta;
+            *tmcct = tmcct_cur;
+            best = pt;
+        }
+        else if ( i > 2 )
+            break;
+
+        tmcct_prev = tmcct_cur;
+    }
+
+    return best;
+}
+
+uint64_t __init calibrate_apic_timer(void)
+{
+    uint32_t start, end;
+    uint64_t count = read_pt_and_tmcct(&start), elapsed;
+    uint64_t target = CALIBRATE_VALUE(plt_src.frequency), actual;
+    uint64_t mask = (uint64_t)~0 >> (64 - plt_src.counter_bits);
+
+    /*
+     * PIT cannot be used here as it requires the timer interrupt to maintain
+     * its 32-bit software counter, yet here we run with IRQs disabled.
+     */
+    if ( using_pit )
+        return 0;
+
+    while ( ((plt_src.read_counter() - count) & mask) < target )
+        continue;
+
+    actual = (read_pt_and_tmcct(&end) - count) & mask;
+    elapsed = start - end;
+
+    if ( likely(actual > target) )
+    {
+        /*
+         * See the comment in calibrate_tsc(). But first scale down values
+         * to actually fit muldiv64()'s input range.
+         */
+        while ( unlikely(actual > (uint32_t)actual) )
+        {
+            actual >>= 1;
+            target >>= 1;
+        }
+        elapsed = muldiv64(elapsed, target, actual);
+    }
+
+    return elapsed * CALIBRATE_FRAC;
+}
+
 u64 stime2tsc(s_time_t stime)
 {
     struct cpu_time *t;