[PATCH v3 3/6] x86/time: streamline platform time init on plt_update()

[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>
---
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>

Changes since v2:
 - Remove pointless intializer and replace it with the
 platform_time init return.
 - s/plt_init/plt_update/g
---
 xen/arch/x86/time.c | 37 +++++++++++++++++++++++++++++++------
 1 file changed, 31 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index b2a11a8..a03127e 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -526,17 +526,31 @@ static s_time_t __read_platform_stime(u64 platform_time)
     return (stime_platform_stamp + scale_delta(diff, &plt_scale));
 }
 
+static void __plt_update(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_update(void)
+{
+    spin_lock_irq(&platform_timer_lock);
+    __plt_update();
+    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_update();
 
     now = NOW();
     plt_wrap = __read_platform_stime(plt_stamp64);
@@ -630,10 +644,21 @@ static s64 __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);
     plt_src = *pts;
 
+    if ( pts == &plt_tsc )
+    {
+        plt_update();
+    }
+    else
+    {
+        plt_overflow_period = scale_delta(
+            1ull << (pts->counter_bits - 1), &plt_scale);
+
+        printk(XENLOG_INFO "Platform timer overflow period is %lu msecs\n",
+               plt_overflow_period / MILLISECS(1));
+    }
+
     return rc;
 }
 
-- 
2.1.4


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