[PATCH RFC] x86/time: set AP's TSC scale estimate earlier

[Jan Beulich <jbeulich@xxxxxxxx>]

NOW() (in particular) can be used ahead of init_percpu_time(). As the
initial scale value set is merely the BSP's, we can as well set it before
actually launching the AP. Don't introduce yet another notifier function
though; do this from smpboot.c's.

Setting the scale alone, however, doesn't work, so the entire struct
cpu_time is copied.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC: Copying the entire struct won't work very well when tsc_adjust[] is
     in use (and values there differ between sockets).

This in particular eliminates an anomaly with log messages issued early
while APs are coming up, when "boot" console timestamps are in use.

--- a/xen/arch/x86/include/asm/time.h
+++ b/xen/arch/x86/include/asm/time.h
@@ -21,6 +21,7 @@ mktime (unsigned int year, unsigned int
 int time_suspend(void);
 int time_resume(void);
 
+void preinit_percpu_time(unsigned int cpu);
 void init_percpu_time(void);
 void time_latch_stamps(void);
 
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -1139,6 +1139,7 @@ static int cf_check cpu_smpboot_callback
     {
     case CPU_UP_PREPARE:
         rc = cpu_smpboot_alloc(cpu);
+        preinit_percpu_time(cpu);
         break;
     case CPU_UP_CANCELED:
     case CPU_DEAD:
--- a/xen/arch/x86/time.c
+++ b/xen/arch/x86/time.c
@@ -2346,6 +2346,12 @@ void time_latch_stamps(void)
     ap_bringup_ref.local_stime = get_s_time_fixed(ap_bringup_ref.local_tsc);
 }
 
+void preinit_percpu_time(unsigned int cpu)
+{
+    /* Initial estimate for TSC rate etc. */
+    per_cpu(cpu_time, cpu) = this_cpu(cpu_time);
+}
+
 void init_percpu_time(void)
 {
     struct cpu_time *t = &this_cpu(cpu_time);
@@ -2353,9 +2359,6 @@ void init_percpu_time(void)
     u64 tsc;
     s_time_t now;
 
-    /* Initial estimate for TSC rate. */
-    t->tsc_scale = per_cpu(cpu_time, 0).tsc_scale;
-
     if ( tsc_adjust )
     {
         unsigned int socket = cpu_to_socket(smp_processor_id());