[Xen-changelog] [xen-unstable] x86 time: Read platform time before locally-extrapolated time during

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1209631544 -3600
# Node ID 013a47065e8c4e815e3b1aba0883341c19238e82
# Parent  483d006cc60765357dcdb66ab0fc43c955ecd8fe
x86 time: Read platform time before locally-extrapolated time during
calibration and frequency changes. This places the variable delay
(acquiring the platform_timer_lock) safely as the very first thing we
do, avoiding a variable delay /between/ computing the two timestamps.

Problem diagnosed by Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>

Signed-off-by: Keir Fraser <keir.fraser@xxxxxxxxxx>
---
 xen/arch/x86/time.c |   14 +++++++++-----
 1 files changed, 9 insertions(+), 5 deletions(-)

diff -r 483d006cc607 -r 013a47065e8c xen/arch/x86/time.c
--- a/xen/arch/x86/time.c       Fri Apr 25 13:46:27 2008 +0100
+++ b/xen/arch/x86/time.c       Thu May 01 09:45:44 2008 +0100
@@ -759,12 +759,13 @@ int cpu_frequency_change(u64 freq)
     }
 
     local_irq_disable();
-    rdtscll(curr_tsc);
-    t->local_tsc_stamp = curr_tsc;
+    /* Platform time /first/, as we may be delayed by platform_timer_lock. */
     t->stime_master_stamp = read_platform_stime();
     /* TSC-extrapolated time may be bogus after frequency change. */
     /*t->stime_local_stamp = get_s_time();*/
     t->stime_local_stamp = t->stime_master_stamp;
+    rdtscll(curr_tsc);
+    t->local_tsc_stamp = curr_tsc;
     set_time_scale(&t->tsc_scale, freq);
     local_irq_enable();
 
@@ -834,11 +835,14 @@ static void local_time_calibration(void 
     prev_local_stime  = t->stime_local_stamp;
     prev_master_stime = t->stime_master_stamp;
 
-    /* Disable IRQs to get 'instantaneous' current timestamps. */
+    /*
+     * Disable IRQs to get 'instantaneous' current timestamps. We read platform
+     * time first, as we may be delayed when acquiring platform_timer_lock.
+     */
     local_irq_disable();
+    curr_master_stime = read_platform_stime();
+    curr_local_stime  = get_s_time();
     rdtscll(curr_tsc);
-    curr_local_stime  = get_s_time();
-    curr_master_stime = read_platform_stime();
     local_irq_enable();
 
 #if 0

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