[Xen-changelog] [xen master] x86/time: adjust local system time initialization

[patchbot@xxxxxxx]

commit bb49fd3092a84ce151f5528794c0e612eeb4961a
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Wed Aug 3 14:39:31 2016 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Aug 3 14:39:31 2016 +0200

    x86/time: adjust local system time initialization
    
    Using the bare return value from read_platform_stime() is not suitable
    when local_time_calibration() is going to use its fast path: Divergence
    of several dozen microseconds between NOW() return values on different
    CPUs results when platform and local time don't stay in close sync.
    
    Latch local and platform time on the CPU initiating AP bringup, such
    that the AP can use these values to seed its stime_local_stamp with as
    little of an error as possible. The boot CPU, otoh, can simply
    calculate the correct initial value (other CPUs could do so too with
    even greater accuracy than the approach being introduced, but that can
    work only if all CPUs' TSCs start ticking at the same time, which
    generally can't be assumed to be the case on multi-socket systems).
    
    This slightly defers init_percpu_time() (moved ahead by commit
    dd2658f966 ["x86/time: initialise time earlier during
    start_secondary()"]) in order to reduce as much as possible the gap
    between populating the stamps and consuming them.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Tested-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
    Tested-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/smpboot.c     |  6 ++++--
 xen/arch/x86/time.c        | 33 ++++++++++++++++++++++++++++++++-
 xen/include/asm-x86/time.h |  1 +
 3 files changed, 37 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 1c02681..ae99c94 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -328,12 +328,12 @@ void start_secondary(void *unused)
 
     percpu_traps_init();
 
-    init_percpu_time();
-
     cpu_init();
 
     smp_callin();
 
+    init_percpu_time();
+
     setup_secondary_APIC_clock();
 
     /*
@@ -996,6 +996,8 @@ int __cpu_up(unsigned int cpu)
     if ( (ret = do_boot_cpu(apicid, cpu)) != 0 )
         return ret;
 
+    time_latch_stamps();
+
     set_cpu_state(CPU_STATE_ONLINE);
     while ( !cpu_online(cpu) )
     {
diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
index 9707e76..7ab3cfe 100644
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -1328,21 +1328,52 @@ static void time_calibration(void *unused)
                      &r, 1);
 }
 
+static struct {
+    s_time_t local_stime, master_stime;
+} ap_bringup_ref;
+
+void time_latch_stamps(void)
+{
+    unsigned long flags;
+    u64 tsc;
+
+    local_irq_save(flags);
+    ap_bringup_ref.master_stime = read_platform_stime();
+    tsc = rdtsc();
+    local_irq_restore(flags);
+
+    ap_bringup_ref.local_stime = get_s_time_fixed(tsc);
+}
+
 void init_percpu_time(void)
 {
     struct cpu_time *t = &this_cpu(cpu_time);
     unsigned long flags;
+    u64 tsc;
     s_time_t now;
 
     /* Initial estimate for TSC rate. */
     t->tsc_scale = per_cpu(cpu_time, 0).tsc_scale;
 
     local_irq_save(flags);
-    t->local_tsc_stamp = rdtsc();
     now = read_platform_stime();
+    tsc = rdtsc();
     local_irq_restore(flags);
 
     t->stime_master_stamp = now;
+    /*
+     * To avoid a discontinuity (TSC and platform clock can't be expected
+     * to be in perfect sync), initialization here needs to match up with
+     * local_time_calibration()'s decision whether to use its fast path.
+     */
+    if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
+    {
+        if ( system_state < SYS_STATE_smp_boot )
+            now = get_s_time_fixed(tsc);
+        else
+            now += ap_bringup_ref.local_stime - ap_bringup_ref.master_stime;
+    }
+    t->local_tsc_stamp    = tsc;
     t->stime_local_stamp  = now;
 }
 
diff --git a/xen/include/asm-x86/time.h b/xen/include/asm-x86/time.h
index 39d6bf3..0a7cb38 100644
--- a/xen/include/asm-x86/time.h
+++ b/xen/include/asm-x86/time.h
@@ -40,6 +40,7 @@ int time_suspend(void);
 int time_resume(void);
 
 void init_percpu_time(void);
+void time_latch_stamps(void);
 
 struct ioreq;
 int hwdom_pit_access(struct ioreq *ioreq);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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