[PATCH v2 4/6] x86/time: streamline platform time init on plt_init()

[Joao Martins <joao.m.martins@oracle.com>]

And use to initialize platform time solely for clocksource=tsc,
as opposed to initializing platform overflow timer, which would
only fire in ~180 years (on 2.2 Ghz Broadwell processor).

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>

Changes since v1:
 - Remove initialisation to 0.
 - Remove assignment and return mix.
 - Fix the style on plt_overflow_period calculation.

Changes since RFC:
 - Move clocksource_is_tsc variable to this patch.
 - s/1000000000/SECONDS(1)
---
 xen/arch/x86/time.c | 44 ++++++++++++++++++++++++++++++++++++--------
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 2602dda..9cadfcb 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -434,6 +434,7 @@ uint64_t ns_to_acpi_pm_tick(uint64_t ns)
 /************************************************************
  * PLATFORM TIMER 4: TSC
  */
+static bool_t clocksource_is_tsc;
 static u64 tsc_freq;
 static unsigned long tsc_max_warp;
 static void tsc_check_reliability(void);
@@ -466,6 +467,8 @@ static int __init init_tsctimer(struct platform_timesource *pts)
     }
 
     pts->frequency = tsc_freq;
+    clocksource_is_tsc = tsc_reliable;
+
     return tsc_reliable;
 }
 
@@ -516,17 +519,31 @@ static s_time_t __read_platform_stime(u64 platform_time)
     return (stime_platform_stamp + scale_delta(diff, &plt_scale));
 }
 
+static void __plt_init(void)
+{
+    u64 count;
+
+    ASSERT(spin_is_locked(&platform_timer_lock));
+    count = plt_src.read_counter();
+    plt_stamp64 += (count - plt_stamp) & plt_mask;
+    plt_stamp = count;
+}
+
+static void plt_init(void)
+{
+    spin_lock_irq(&platform_timer_lock);
+    __plt_init();
+    spin_unlock_irq(&platform_timer_lock);
+}
+
 static void plt_overflow(void *unused)
 {
     int i;
-    u64 count;
     s_time_t now, plt_now, plt_wrap;
 
     spin_lock_irq(&platform_timer_lock);
 
-    count = plt_src.read_counter();
-    plt_stamp64 += (count - plt_stamp) & plt_mask;
-    plt_stamp = count;
+    __plt_init();
 
     now = NOW();
     plt_wrap = __read_platform_stime(plt_stamp64);
@@ -621,11 +638,22 @@ static int __init try_platform_timer(struct platform_timesource *pts)
 
     set_time_scale(&plt_scale, pts->frequency);
 
-    plt_overflow_period = scale_delta(
-        1ull << (pts->counter_bits - 1), &plt_scale);
-    init_timer(&plt_overflow_timer, plt_overflow, NULL, 0);
     plt_src = *pts;
-    plt_overflow(NULL);
+
+    if ( clocksource_is_tsc )
+    {
+        plt_init();
+    }
+    else
+    {
+        plt_overflow_period = scale_delta(
+            1ull << (pts->counter_bits - 1), &plt_scale);
+        init_timer(&plt_overflow_timer, plt_overflow, NULL, 0);
+        plt_overflow(NULL);
+
+        printk("Platform timer overflow period is %lu secs\n",
+               plt_overflow_period/SECONDS(1));
+    }
 
     platform_timer_stamp = plt_stamp64;
     stime_platform_stamp = NOW();
-- 
2.1.4


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